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The Simplest XBee Network

SAMSUNGAs I continue my investigation of the XBee radio, I’m impressed by the functionality compressed into the the small package, but I have been frustrated by one fact.  It has been a hard road to understand the device and to make it do something useful.  There is a confusing mass of commands and options that can be used.  To make things more difficult for me, it is my nature to study my subject at depth, and understand it well, before I commit myself to a project.  The XBee radios are proving to be a deep subject.  I have been struggling to get a simple 802.15.4 network up and working, at least one that is sufficiently complex to be useful for my needs.

I stumbled into the realization that I don’t have to master 802.15.4 and a large set of XBee commands to make a very simple but potentially useful network.  It’s a very basic observation about all radio devices like the XBee.  You see, at its core the XBee radio is  a modem.  It encodes digital information, transmits its digits via electromagnetism, within a specific frequency band, to be received by another XBee, and  then converted back to digital information.  The use of packet data, the 802.15.4 protocol, and all the AT commands are layers on top of the XBee’s  modem capability.  The modem, the core communications sub-component, is a serial communications device with a Universal Asynchronous Receiver/Transmitter (UART) as examined in an earlier blog.  So why not we just treat the XBee radio as a simple serial communication device?  Drop the idea of packetized data and 802.15.4 protocol and just do raw serial communications.

While this simplification is seductive, it does come at a price. Data is packetized and transmitted using a protocol for very good reasons.  In data transmission you must consider the fact that data could get corrupted, you need to share the communication channel with others,  data streams may be long (if not endless) and need to be properly sequenced, communications is main between specific devices as apposed to just broadcasting, and many other concerns.  You give up much of this by doing raw serial communications but you gain simplicity.

What I plan to do here is list some simple, proof of concept programs that I used to create a network with a Arduino and Raspberry Pi (RPi)  using XBee radios.  You could simply add additional devices, using the same code, and it will become a fully interconnected network (i.e. where every device can talk to every other devices directly).  While inferior to a 802.15.4 network on many levels, its quick to get operational and easy to debug.  Also keep in mind that this is built on the XBee radio,  but you could do this with most any radio which supports serial communications.

Architecture

I’ll be using a Arduino and a RPis for for my network, each with a XBee radio thought which they can communicate. I’ll establish terminal interface into each device so I can enter text for the device to transmit and the terminal will also show what messages where revived. All  terminals will be within windows on my PC using PuTTY, Xterm, or the Arduino’s serial monitor screen.

Initializing the XBee Radios

First step is to make sure all the XBee radios that will be part of your network are properly configured.  Specifically, you need to make sure the PAN ID and Channel ID of the XBee radio’s are identical.  To accomplish this, I used the XBeeTerm.py utility I posted in my earlier blog titled Configuration Utilities for XBee Radios.  I’m going to setup my network with two XBee radios (but you can use as many as you wish) and I used the configuration file below on both radios:

# To remove comments, white spaces, and blank lines, use the following:
#		sed '/^#/d; s/\([^$]\)#.*/\1/' Modem-Device.txt | sed 's/[ \t]*$//' > modemd.txt
# Run this script to configure the XBee radio using the following:
#		./XBeeTerm.py modemd.txt
#
baudrate 9600		# (XBeeTerm command) set the baudrate used to comm. with the XBee
serial /dev/ttyUSB0	# (XBeeTerm command) serial device which has the XBee radio
+++ 			# (XBee command) enter AT command mode on the XBee
ATRE			# (XBee command) restore XBee to factory settings
ATID B000		# (XBee command) Set the PAN ID to eight byte hex (all XBee's must have this same value)
ATCH 0E			# (XBee command) set the Channel ID to a four byte hex (all XBee's must have same value)
ATPL 0			# (XBee command) power level at which the RF module transmits (0 lowest / 4 highest)
ATWR			# (XBee command) write all the changes to the XBee non-volatile memory
ATFR			# (XBee command) reboot XBee radio
exit			# (XBeeTerm command) exit python shell

Arduino Configuration

The sketch on the Arduino is very simple. Called XBeeModem.ino and is listed blow:

/*
    The XBee devise should be connected to the Arduino Uno in the following way:
        XBee RX is connected to Arduino TX pin 3
        XBee TX is connected to Ardunio RX pin 2
        XBee +5V and Ground pins connect to the same on the Arduino
 */

#define RXPIN 2
#define TXPIN 3
#define BAUDRATE 9600

#include

SoftwareSerial XBeeSerial =  SoftwareSerial(RXPIN, TXPIN);

void setup()
{
    pinMode(13, OUTPUT);

    // Set the data rate for the hardware Serial port
    // and post a message stating so on the Arduino's Serial Monitor.
    Serial.begin(BAUDRATE);
    Serial.println("Arduino #1 up and running.");

    // Set the data rate for the SoftwareSerial port and
    // send a message out stating so via the XBee to the other devices.
    XBeeSerial.begin(BAUDRATE);
    XBeeSerial.println("Arduino #1 up and running.");
}

void loop()
{
    char c;

    // Read data arriving from the XBee and send to Arduino Serial Monitor.
    if (XBeeSerial.available()) {
        Serial.print((char)XBeeSerial.read());
    }

    // Capture data typed at the Arduino Serial Monitor, echo the data to the Serial Monitor,
    // and send that data via the XBee.
    if (Serial.available()) {
        c = (char)Serial.read();
        Serial.print(c);
        XBeeSerial.print(c);
    }

    delay(100);
}

Raspberry Pi Configuration

The Python program on the RPi is also very simple, except for one point.  Linux I/O reads will block if there are no characters to read.  You must “turn-off” blocking.  The program is called XBeeModem.py and is listed blow:

#!/usr/bin/env python

"""XBeeModem.py bypasses the XBee's 802.15.4 capabilities and simply uses it modem for communications

    You don't have to master 802.15.4 and a large set of XBee commands
    to make a very simple but potentially useful network.  At its core,
    the XBee radio is  a modem and you can use it directly for simple serial communications.

    Reference Materials:
        Non-blocking read from stdin in python - http://repolinux.wordpress.com/2012/10/09/non-blocking-read-from-stdin-in-python/
        Non-blocking read on a subprocess.PIPE in python - http://stackoverflow.com/questions/375427/non-blocking-read-on-a-subprocess-pipe-in-python

    Originally Created By:
        Jeff Irland (jeff.irland@gmail.com) in March 2013
"""

# imported modules
import os                   # portable way of using operating system dependent functionality
import sys                  # provides access to some variables used or maintained by the interpreter
import time                 # provides various time-related functions
import fcntl                # performs file control and I/O control on file descriptors
import serial               # encapsulates the access for the serial port
from pretty import switchColor, printc  # provides colored text for xterm & VT100 type terminals using ANSI escape sequences

# text colors to be used during terminal sessions
ERROR_TEXT = 'bright red'
CMD_INPUT_TEXT = 'normal'
CMD_OUTPUT_TEXT = 'bright yellow'
TERM_OUTPUT_TEXT = 'purple'
TERM_INPUT_TEXT = 'bright purple'

if __name__ == '__main__':
    serial = serial.Serial()
    serial.port = '/dev/ttyUSB0'
    serial.baudrate = 9600
    serial.timeout = 1
    serial.writeTimeout = 1
    serial.open()

    # make stdin a non-blocking file
    fcntl.fcntl(sys.stdin, fcntl.F_SETFL, os.O_NONBLOCK)

    # post startup message to other XBee's and at stdout
    serial.writelines("RPi #1 is up and running.\r\n")
    print "RPi #1 is up and running."

    switchColor(CMD_OUTPUT_TEXT)
    print "Entering loop to read and print messages (Ctrl-C to abort)..."

    try:
        while True:
            # read a line from XBee and convert it from b'xxx\r\n' to xxx and print at stdout
            switchColor(TERM_OUTPUT_TEXT)
            line = serial.readline().decode('utf-8')
            if line:
                print line

            # read data from the keyboard (i.e. stdin) and send via the XBee modem
            switchColor(TERM_INPUT_TEXT)
            try:
                line = sys.stdin.readline()
                serial.writelines(line)
            except IOError:
                time.sleep(0.1)
                continue

    except KeyboardInterrupt:
        printc("\n*** Ctrl-C keyboard interrupt ***", ERROR_TEXT)
        serial.writelines("RPi #1 is going down.\r\n")

    finally:
        switchColor(CMD_INPUT_TEXT)

Closing

It doesn’t get much simpler than this. With a little work, you might make something useful out of this technique but its very limited in the types of problems that it could handle. Never the less, it was a good diversion for me to clear my mind. Now back to the XBee’s core capabilites, the 802.15.4 protocol, and the other minutia!

Configuration Utilities for XBee Radios

My ultimate aim is to wirelessly network several Arduino based platforms with a centralized Raspberry Pi controller. There is much for me to learn to get this operational, not the least of which is the radio device I plan to use, the Xbee.  To get up to speed on the Xbee, I found the tutorials at AdafruitSparkfun, and Parallax helpful.   More detailed references are listed at the end of this post, but the very first challenge is to configure the XBee radios for operation.  This post provides insight on how this can be done, and my main mission, create a few simple utilities that make that job easy.

Xbee Radios

I purchased two XBee Series 1 Module (Freescale 802.15.4 firmware) from Adafruit.  These are manufactured by Digi and are low-power module with wire antenna  (XB24-AWI-001).  They have a 250 kbps RF data rates and operate at 2.4 GHz.  These radios use the IEEE 802.15.4 networking protocol and can perform point-to-multi-point or peer-to-peer networking , but as configured here, they do not mesh.  The Digi models that handle meshing are Digimesh, ZNet2.5 and Zigbee (ZB).  Digimesh is a version of firmware that runs on Series 1 hardware. So, if you choose to, you can upgrade these modules to Digimesh firmware to get meshing.

Xbee Adapter Board

Along with the XBee radios, I purchased adapter boards designed to make it easier to work with the radios. The adopter provides on-board 3.3V regulator power from a 5 volt source, voltage level shifting circuitry so you can connect  5V circuitry to the XBee, commonly used pins are brought out along the edge (making it easy to breadboard), and engineered to be interface via FTDI cable to a computer via USB.  The image and the text below describe the pin-out for the Adafruit  XBee Adapter:

Adafruit Xbee Adapter Pinout

    1. 3V pin – This is either an input power pin (if 5V is not provided) or an output from the 250mA regulator if 5V is provided
    2. DTR – “Data terminal ready”  is a flow control pin used to tell the XBee that the microcontroller or computer host is ready to communicate.
    3. RST – “Reset”  pin can be used to reset the XBee.  By default it is pulled high by the 10K resistor under the module. To reset, pull this pin low.’
    4. Ground – common ground for power and signal
    5. CTS – “Clear to Send” this is a flow control pin that can be used to determine if there is data in the XBee input buffer ready to be read
    6. 5V – This is the power input pin into the 3.3V regulator. Provide up to 6V that will be linearly converted into 3.3V
    7. RX – “Receive Data” is the XBee’s serial recieve pin. Serial data is sent on this pin into the XBee to be transmitted wirelessly
    8. TX – “Transmit Data” is the XBee’s serial transmit pin. Serial data is sent on this pin out of the XBee, after it has been transmitted wirelessly from another module
    9. RTS – “Ready to Send” is a flow control pin that can be used to tell the XBee to signal that the computer or microcontroller needs a break from reading serial data.
    10. see pin #1

The DTR, RTS, RST and RX pins (going into the XBee) pass through a level converter chip that brings the levels to 3.3V. Adafruit claims you can use pretty much anywhere between 2.7 to 5.5V data to communicate with the XBee. The breakout pins on the bottom of the board are not level shifted and you should try to keep data going directly into the XBee pin under 3.3V

Xbee radio installed in a XBee Adapter with the USB FTDI TTL-232 Cable attached

XBee Initial Configuration and Testing

You need a way to communicate withe the Xbee, via it adapter,  to set it up.  This can be done via Adafruit’s  USB FTDI TTL-232 Cable, and the Digi X-CTU serial terminal program.  By the way, the X-CTU user guide describes the many more things it can do beyond the configuration shown here.

    1. Plug in the USB FTDI TTL-232 Cable into a PC USB port.  If drivers are not installed automatically (it didn’t for me), follow the steps at the FTDI site.
    2. Download the X-CTU, double click on the executable file, and follow the instructions to install the program.
    3. Now connect the USB FTDI TTL-232 Cable to the Xbee Adapter as shown in the picture to the right and insert the USB end of the cable to you PC.  Start the X-CTU.
    4. To connect, configure and upgrading the Xbee, follow the Adafruit instructions for the Xbee Adapter board. Note that if you follow the instructions (I didn’t – I kept it at 9600 baud), the modem’s serial interface is now set to 19,200 baud, not the default 9600 used by X-CTU.  Remember this next time you use X-CTU with this Xbee.
    5. If your instructed by X-CTU to reset the Xbee, you can do this by shorting the reset pin, RST pin,  to ground.

The configuration can be touchy, it can go badly, or not at all.  In my case, I seem to have one Xbee Adapter that can reliably perform a firmware upgrade but the other one took some time due to a lose fitting between the Adaptor and  Xbee.  If you run into configuration problems, check out these sites: Using XCTU to Invoke the BootloaderThe Unofficial XBee FAQ,  How to recover from a failed firmware upgrade.

Quickly Getting the Xbee’s Communicating

The next step for me was just do a basic test of getting two XBee device communicating with each other. This is just a sanity test to see evidence of communication between the devices. Basically, I just followed the instructions provided by Adafruit.
simple test

    1. Using the X-CTU, set the PAN ID to the same value on the two Xbee’s.
    2. Select an Ardunio that has been programmed to send repeated brief messages to its serial port.  I used the standard LED Blinking sketch but put in some write statements in the loop.
    3. Using an Arduino and breadboard, connect +5V and ground to provide power. Make sure the XBee’s LED is blinking.
    4. Connect the RX line (input) of the XBee to the TX line (output) of the Arduino. Connect the RX line (input) of the Arduino to the TX line (output) of the Xbee. Plug the Arduino into your PC’s serial port.
    5. Now take the second Xbee and connect the  USB FTDI TTL-232 Cable to the Xbee and the PC.  The cable is doing nothing but appling power to the Xbee.
    6. Now you should see the receive LED periodically light on the USB FTDI TTL-232 Cable tethered Xbee.
    7. You now got proof that the two Xbee’s are communicating.  The Arduino connected Xbee is sending data to its serial port and the USB FTDI TTL-232 Cable tethered Xbee is receiving it.

Above you’ll find a picture of the configuration, and below is the Arduino sketch I used.

/*
 *  Xbee Test via Blink LED
 *
 *Turns on an LED on for one second, then off for one second, repeatedly.
 *Also increase brigthness of analog LED.
 *
 *The circuit:
 * LED1 connected from digital pin 13 to ground.
 * LED2 connected from analog pin 9 to ground.
 * Note: On most Arduino boards, there is already an LED on the board
 * connected to pin 13, so you don't need any extra components for this example.

 *Created 1 June 2005
 *By David Cuartielles
 *http://arduino.cc/en/Tutorial/Blink
 *based on an orginal by H. Barragan for the Wiring i/o board
 *Modified by Jeff Irland in December 2012
 */

int ledPin1 =  13;    // LED connected to digital pin 13
int ledPin2 =  9;     // LED connected to analog pin 9
int brightness = 0;

// The setup() method runs once, when the sketch starts
void setup()   {
    Serial.begin(9600);
    pinMode(ledPin1, OUTPUT);     // initialize the digital pin as an output
    Serial.println("Arduino done with setup()");
}

// the loop() method runs over and over again,
// as long as the Arduino has power
void loop()
{
    digitalWrite(ledPin1, HIGH);   // set the LED on
    Serial.println("LED set HIGH.");
    delay(1000);                   // wait for a second

    digitalWrite(ledPin1, LOW);    // set the LED off
    Serial.println("LED set LOW.");
    delay(1000);                   // wait for a second

    brightness = brightness + 5;
    analogWrite(ledPin2, brightness);
    Serial.println("LED brightness increased.");
}

Installing XBee Python Tools for the RPi

While the MS Windows based Digi X-CTU tool is just fine, I want to use the RPi’s and Python to access the XBee serial communication API, and its advanced features, for one or more XBee devices.  I prefer simple utilities, that can be scripted within the Linux shell.  Call me a Linux snob if you wish, but I don’t care for MS Windows!

In my post “Selecting XBee Radios and Supporting Software Tools“, I referenced a Python package that could be used to create my utilitiesd, call python-xbee, and I will be using it here. It claims to provides a semi-complete implementation of the XBee binary API protocol and allows a developer to send and receive the information they desire without dealing with the raw communication details. It also claims the  library is compatible with both XBee 802.15.4 (Series 1) and XBee ZigBee (Series 2) modules, normal and PRO.

First, we need to load some additional required Python Packages, that being pySerial and Nose. pySerial extends python’s capabilities to include interacting with a serial port and Nose is a package providing a very easy way to build tests, based on the Python class unittest.  (Don’t let this all scare you away, these are necessary but your not going to use them directly).  To load these package:

sudo pip install pySerial
sudo pip install nose

Download the python-xbee tools from Google Code or Python Org and place them into the RPi’s $HOME/src.  The README file provides installation instructions.  It states that the following command automatically test and install the package for you:

sudo python setup.py install

There is a simple to use RPi platform tool that I have modified for my needs, that is a XBee serial command shell for interacting with XBee radios.  It performs the core functions of the official configuration tool, X-CTU, which only runs on Windows. (There happens to be a cross-platform version of X-CTU called moltosenso Network Manager but I don’t need all this horse power.)  I’ll use this X-CTU-alternative to configure the individual XBee radios.  With the X-CTU, you can update firmware, etc. but most of the time you need the program to do simple configuration tasks. You could use Linux’s minicom, but I prefer a simpler tool which can be scripted so I can configure several XBee radios identically.  I found much of what I wanted in an existing Python XBee tools for configuration.  I made some modification/improvements, I call it the XBeeTerm, and its listed below:


#!/usr/bin/env python

"""XBeeTerm.py is a XBee serial command shell for interacting with XBee radios

	This command interpretors establishes communications with XBee radios so that AT Commands can be sent to the XBee.
	The interpretors output is color coded to help distinguish user input, from XBee radio output, and from
	interpretors output. This command-line interpretor uses Python modual Cmd, and therefore, inherit bash-like history-list
	editing (e.g. Control-P or up-arrow scrolls back to the last command, Control-N or down-arrow forward to the next one,
	Control-F or right-arrow moves the cursor to the right non-destructively, Control-B or left-arrow moves the cursor
	to the left non-destructively, etc.).

	XBeeTerm is not a replacement for the Digi X-CTU program but a utility program for the Linux envirnment.  You can pipe
	scripts of XBee configration commands, making it easy to multiple radios.  Also, XBeeTerm wait for the arrival of a
	XBee data packet, print the XBee frame, and wait for the next packet, much like a packet sniffer.

	XBeeTerm Commands:
		baudrate <rate>		set the baud rate at which you will communicate with the XBee radio
		serial <device>		set the serial device that the XBee radio is attached
		watch				wait for the arrival of a XBee data packet, print it, wait for the next
		shell or !			pause the interpreter and invoke command in Linux shell
		exit or EOF			exit the XBeeTerm
		help or ?			prints out short discription of the commands (similar to the above)

	Just like the Digi X-CTU program, the syntax for the AT commands are:
		AT+ASCII_Command+Space+Optional_Parameter+Carriage_Return
		Example: ATDL 1F<CR>

	Example Session:
		baudrate 9600			# (XBeeTerm command) set the baudrate used to comm. with the XBee
		serial /dev/ttyUSB0		# (XBeeTerm command) serial device which has the XBee radio
		+++ 					# (XBee command) enter AT command mode on the XBee
		ATRE					# (XBee command) restore XBee to factory settings
		ATAP 2					# (XBee command) enable API mode with escaped control characters
		ATCE 0					# (XBee command) make this XBee radio an end device
		ATMY AAA1				# (XBee command) set the address of this radio to eight byte hex
		ATID B000				# (XBee command) Set the PAN ID to eight byte hex
		ATCH 0E					# (XBee command) set the Channel ID to a four byte hex
		ATPL 0					# (XBee command) power level at which the RF module transmits
		ATWR					# (XBee command) write all the changes to the XBee non-volatile memory
		ATFR					# (XBee command) reboot XBee radio
		exit					# (XBeeTerm command) exit python shell

	Referance Materials:
		XBee 802.15.4 (Series 1) Module Product Manual (section 3: RF Module Configuration)
			ftp://ftp1.digi.com/support/documentation/90000982_A.pdf
		python-xbee Documentation: Release 2.0.0, Paul Malmsten, December 29, 2010
			http://python-xbee.googlecode.com/files/XBee-2.0.0-Documentation.pdf
		cmd - Support for line-oriented command interpreters
			http://docs.python.org/2/library/cmd.html
		cmd - Create line-oriented command processors
			http://bip.weizmann.ac.il/course/python/PyMOTW/PyMOTW/docs/cmd/index.html
		Easy command-line applications with cmd and cmd2
			http://pyvideo.org/video/306/pycon-2010--easy-command-line-applications-with-c

	Orginally Created By:
		Amit Snyderman (amit@amitsnyderman.com), on/about August 2012, and taken from
		https://github.com/sensestage/xbee-tools

	Modified By:
		Jeff Irland (jeff.irland@gmail.com) in January 2013
"""

# imported modules
import os						# portable way of using operating system dependent functionality
import sys						# provides access to some variables used or maintained by the interpreter
import time						# provides various time-related functions
import cmd						# provides a simple framework for writing line-oriented command interpreters
import serial					# encapsulates the access for the serial port
import argparse					# provides easy to write and user-friendly command-line interfaces
from xbee import XBee			# implementation of the XBee serial communication API
from pretty import switchColor	# colored text in Python using ANSI Escape Sequences

# authorship information
__author__ = "Jeff Irland"
__copyright__ =	"Copyright 2013"
__credits__ = "Amit Snyderman, Marije Baalman, Paul Malmsten"
__license__ = "GNU General Public License"
__version__ = "0.1"
__maintainer__ = "Jeff Irland"
__email__ = "jeff.irland@gmail.com"
__status__ = "Development"
__python__ = "Version 2.7.3"

# text colors to be used during terminal sessions
CMD_INPUT_TEXT = 'normal'
CMD_OUTPUT_TEXT = 'bright yellow'
XBEE_OUTPUT_TEXT = 'bright red'
SHELL_OUTPUT_TEXT = 'bright cyan'
WATCH_OUTPUT_TEXT = 'bright green'

class ArgsParser():
	"""Within this object class you should load all the command-line switches, parameters, and arguments to operate this utility"""
	def __init__(self):
		self.parser = argparse.ArgumentParser(description="This command interpretors establishes communications with XBee radios so that AT Commands can be sent to the XBee.  It can be used to configure or query the XBee radio", epilog="This utility is primarily intended to change the AT Command parameter values, but could be used to query for the parameter values.")
		self.optSwitches()
		self.reqSwitches()
		self.optParameters()
		self.reqParameters()
		self.optArguments()
		self.reqArguments()
	def optSwitches(self):
		"""optonal switches for the command-line"""
		self.parser.add_argument("--version", action="version", version=__version__, help="print version number on stdout and exit")
		self.parser.add_argument("-v", "--verbose", action="count", help="produce verbose output for debugging")
	def reqSwitches(self):
		"""required switches for the command-line"""
		pass
	def optParameters(self):
		"""optonal parameters for the command-line"""
		pass
	def reqParameters(self):
		"""required parameters for the command-line"""
		pass
	def optArguments(self):
		"""optonal arguments for the command-line"""
		self.parser.add_argument(nargs="*", action="store", dest="inputs", default=None, help="XBeeTerm script file with AT Commands to be executed")
	def reqArguments(self):
		"""required arguments for the command-line"""
		pass
	def args(self):
		"""return a object containing the command-line switches, parameters, and arguments"""
		return self.parser.parse_args()

class XBeeShell(cmd.Cmd):
	def __init__(self, inputFile=None):
		"""Called when the objects instance is created"""
		cmd.Cmd.__init__(self)
		self.serial = serial.Serial()
		if inputFile is None:
			self.intro = "Command-Line Interpreter for Configuring XBee Radios"
			self.prompt = "xbee% "
		else:
			self.intro = "Configuring XBee Radios via command file"
			self.prompt = ""			# Do not show a prompt after each command read
			sys.stdin = inputFile

	def default(self, p):
		"""Command is assumed to be an AT Commands for the XBee radio"""
		if not self.serial.isOpen():
			print "You must set a serial port first."
		else:
			if p == '+++':
				self.serial.write('+++')
				time.sleep(2)
			else:
				self.serial.write('%s\r' % p)
				time.sleep(0.5)

			output = ''
			while self.serial.inWaiting():
				output += self.serial.read()
			if output == '' :
				print 'XBee timed out, so reissue "+++". (Or maybe XBee doesn\'t understand "%s".)' % p
			else:
				switchColor(XBEE_OUTPUT_TEXT)
				print output.replace('\r', '\n').rstrip()

	def emptyline(self):
		"""method called when an empty line is entered in response to the prompt"""
		return None		# do not repeat the last nonempty command entered

	def precmd(self, p):
		"""executed just before the command line line is interpreted"""
		switchColor(CMD_OUTPUT_TEXT)
		return cmd.Cmd.precmd(self, p)

	def postcmd(self, stop, p):
		"""executed just after a command dispatch is finished"""
		switchColor(CMD_INPUT_TEXT)
		return cmd.Cmd.postcmd(self, stop, p)

	def do_baudrate(self, p):
		"""Set the baud rate used to communicate with the XBee"""
		self.serial.baudrate = p
		print 'baudrate set to %s' % self.serial.baudrate

	def do_serial(self, p):
		"""Linux serial device path to the XBee radio (e.g. /dev/ttyUSB0)"""
		try:
			self.serial.port = p
			self.serial.open()
			print 'Successfully opened serial port %s' % p
		except Exception, e:
			print 'Unable to open serial port %s' % p

	def do_shell(self, p):
		"""Pause the interpreter and invoke command in Linux shell"""
		print "running shell command: ", p
		switchColor(SHELL_OUTPUT_TEXT)
		print os.popen(p).read()

	def do_watch(self, p):
		"""Wait for the arrival of a XBee data packet, print it when it arrives, wait for the next"""
		if not self.serial.isOpen():
			print "You must set a serial port first."
		else:
			print "Entering watch mode..."
			switchColor(WATCH_OUTPUT_TEXT)
			while 1:
				packet = xbee.find_packet(self.serial)
				if packet:
					xb = xbee(packet)
					print xb

	def do_exit(self, p):
		"""Exits from the XBee serial terminal"""
		self.serial.close()
		print "Exiting", os.path.basename(__file__)
		return True

	def do_EOF(self, p):
		"""EOF (end-of-file) or Ctrl-D will return True and drops out of the interpreter"""
		self.serial.close()
		print "Exiting", os.path.basename(__file__)
		return True

	def help_help(self) :
		"""Print help messages for command arguments"""
		print 'help\t\t', self.help_help.__doc__
		print 'shell <cmd>\t', self.do_shell.__doc__
		print 'EOF or Ctrl-D\t', self.do_EOF.__doc__
		print 'exit\t\t', self.do_exit.__doc__
		print 'watch\t\t', self.do_watch.__doc__
		print 'serial <dev>\t', self.do_serial.__doc__
		print 'baudrate <rate>', self.do_baudrate.__doc__

# Enter into XBee command-line processor
if __name__ == '__main__':
	# parse the command-line for switches, parameters, and arguments
	parser = ArgsParser()							# create parser object for the command-line
	args = parser.args()							# get list of command line arguments, parameters, and switches

	if args.verbose > 0:							# print what is on the command-line
		print os.path.basename(__file__), "command-line arguments =", args.__dict__

	# process the command-line arguments (i.e. script file) and start the command shell
	if len(args.inputs) == 0:	# there is no script file
		shell= XBeeShell()
		shell.cmdloop()
	else:							# there is a script file on the command-line
		if len(args.inputs) > 1:
			print os.path.basename(__file__), "will process only the first command-line argument."
		if os.path.exists(args.inputs[0]) :
			inputFile = open(args.inputs[0], 'rt')
			shell = XBeeShell(inputFile)
			shell.cmdloop()
		else:
			print 'File "%s" doesn\'t exist. Program terminated.' % args.inputs[0]

The XBeeTerm.py module imports functions from the pretty.py package, specifically to colorize the output for xterm on the Raspberry Pi.  This package is provided here:

#!/usr/bin/env python

"""pretty.py will color text for Xterm/VT100 type terminals using ANSI Escape Sequences

	A library that provides a Python print, stdout, and string wrapper that makes colored terminal
	text easier to use(e.g. without having to mess around with ANSI escape sequences).

	Referance Materials:
		Colored text in Python using ANSI Escape Sequences
			http://nezzen.net/2008/06/23/colored-text-in-python-using-ansi-escape-sequences/

	Orginally Created By:
		Copyright (C) 2008 Brian Nez <thedude at bri1 dot com>

	Modified By:
		Jeff Irland (jeff.irland@gmail.com) in January 2013
"""

# imported modules
import sys

# authorship information
__author__ = "Jeff Irland"
__copyright__ =	"Copyright 2013"
__credits__ = "Brian Nez"
__license__ = "GNU General Public License"
__version__ = "1.0"
__maintainer__ = "Jeff Irland"
__email__ = "jeff.irland@gmail.com"
__status__ = "Production"

# Dictionary of ANSI escape sequences for coloring text
colorCodes = {
	'black':	'0;30',		'bright gray':	'0;37',
	'blue':		'0;34',		'white':		'1;37',
	'green':	'0;32',		'bright blue':	'1;34',
	'cyan':		'0;36',		'bright green':	'1;32',
	'red':		'0;31',		'bright cyan':	'1;36',
	'purple':	'0;35',		'bright red':	'1;31',
	'yellow':	'0;33',		'bright purple':'1;35',
	'dark gray':'1;30',		'bright yellow':'1;33',
	'normal':	'0'
}

def printc(text, color):
	"""Print in color"""
	print "\033["+colorCodes[color]+"m"+text+"\033[0m"

def writec(text, color):
	"""Write to stdout in color"""
	sys.stdout.write("\033["+colorCodes[color]+"m"+text+"\033[0m")

def switchColor(color):
	"""Switch terminal color"""
	sys.stdout.write("\033["+colorCodes[color]+"m")

def stringc(text, color):
	"""Return a string with ANSI escape sequences to color text"""
	return "\033["+colorCodes[color]+"m"+text+"\033[0m"

# Simple test routine to validate thing are working correctly
if __name__ == '__main__':
	printc("Welcome to the pretty.py test routine!", 'white')

	printc("I will now try to print a line of text in each color using \"writec()\"", 'white')
	for color in colorCodes.keys():
		writec("Hello, world!", color)
		print "\t", color

	printc("\n\nI will now try to print a line of text in each color using \"switchColor()\"", 'white')
	for color in colorCodes.keys():
		switchColor(color)
		print 'Hello World #2!'

	printc("\n\nI will now try to print a line of text in each color using \"printc()\"", 'white')
	for color in colorCodes.keys():
		printc('Hello World #3!', color)

	printc("\n\nI will now try to print a line of text in each color using \"stringc()\"", 'white')
	for color in colorCodes.keys():
		print stringc('Hello World #4!', color)

Identifying the RPi USB device used by the XBee

Since the python-xbee library wants to talk to the via a Linux serial devices, I’m using the USB FTDI TTL-232 Cable (FTDI is the USB chip manufacturer) used in the XBee configuration step done earlier.  I connected the cable to the RPi USB port  and then we need to find the serial tty the cable is associated with.  To do this, it takes a bit of detective work. Run the commands:

lsusb
dmesg | grep Manufacturer
dmesg | grep FTDI

A better command might be (but I’m not sure it will work every time):

dmesg | grep -i usb | grep -i tty

The interpretation of the output tells us the cable is attached to serial device /dev/ttyUSB0.  See the output below.

best use of dmesg

Another possibility is to use udevadm to gather information about specific devices but I never figured out exactly how to use it to answer my question.  Python also has a package called PyUSB that might provide some help, but also here you’ll still need the vendor and product identification information.

Chances are that when you plug the cable into the same USB port the next time, it will default to the same tty but there is no certainty.  To assign a permanent tty name to the device, and never do any of this again, check out Persistent names for usb-serial devices.

Configuring the XBee Radios for API Mode

The configuration and testing of the XBee’s done earlier was done in AT Command mode (Transparent Mode). In AT mode, everything sent to the RX line of the XBee radio will be sent out via the antenna, and all the incoming data from antenna will go to the XBee’s TX line.  This is why we could check the sanity of the XBee radios in the earlier section, XBee Initial Configuration and Testing using a simple Arduino sketch.  We sent junk to the XBee and it transmitted it!

Now we’ll configure two XBee radios (with a Coordinator and a single End Device) to form a network using API Mode.  In API Mode, XBee won’t send out anything until it received the correct form of commands from the serial interface.  The XBee AT Command Set (page 27), specifically the  ATAP 2 command, allows you to configure the XBee radio for API Mode.   So why API Mode, consider the following:

    • When sending a packet, the transmitting radio receives an ACK, indicating the packet was successfully delivered. The transmitting radio will resend the packet if it does not receive an ACK.
    • Receive packets (RX), contain the source address of transmitting radio
    • You can configure a remote radio with the Remote AT feature
    • Easily address multiple radios and send broadcast TX packets
    • Receive I/O data from 1 or more remote XBees
    • Obtain RSSI (signal strength) of an RX packet
    • Packets include a checksum for data integrity

The XBeeTerm utility will  easily configure the XBee radios for API mode and set the appropriate network parameters.  To get a deeper appreciation of configuring the XBee radios, see the References at the end.  For here, I’ll just run through the steps using the XBeeTerm.py tool and the configuration commands used, documented in file scripts.

Coordinator Configuration File: Config-Coordinator.txt

# To remove comments, white spaces, and blank lines, use the following:
#		sed '/^#/d; s/\([^$]\)#.*/\1/' Config-Coordinator.txt | sed 's/[ \t]*$//' > coord.txt
# Run this script to configure the XBee radio using the following:
#		python XBeeTerm.py coord.txt
#
baudrate 9600			# (XBeeTerm command) set the baudrate used to comm. with the XBee
serial /dev/ttyUSB0		# (XBeeTerm command) serial device which has the XBee radio
+++ 					# (XBee command) enter AT command mode on the XBee
ATRE					# (XBee command) restore XBee to factory settings
ATAP 2					# (XBee command) enable API mode with escaped control characters
ATCE 1					# (XBee command) make this XBee radio the network coordinator
ATMY AAA0				# (XBee command) set the address of this radio to eight byte hex (must be unique)
ATID B000				# (XBee command) Set the PAN ID to eight byte hex (all XBee's must have this same value)
ATCH 0E					# (XBee command) set the Channel ID to a four byte hex (all XBee's must have same value)
ATPL 0					# (XBee command) power level at which the RF module transmits (0 lowest / 4 highest)
ATWR					# (XBee command) write all the changes to the XBee non-volatile memory
ATFR					# (XBee command) reboot XBee radio
exit					# (XBeeTerm command) exit python shell

End Device Configuration File: Config-End-Device.txt

# To remove comments, white spaces, and blank lines, use the following:
#		sed '/^#/d; s/\([^$]\)#.*/\1/' Config-End-Device.txt | sed 's/[ \t]*$//' > endd.txt
# Run this script to configure the XBee radio using the following:
#		python XBeeTerm.py endd.txt
#
baudrate 9600			# (XBeeTerm command) set the baudrate used to comm. with the XBee
serial /dev/ttyUSB0		# (XBeeTerm command) serial device which has the XBee radio
+++ 					# (XBee command) enter AT command mode on the XBee
ATRE					# (XBee command) restore XBee to factory settings
ATAP 2					# (XBee command) enable API mode with escaped control characters
ATCE 0					# (XBee command) make this XBee radio an end device
ATMY AAA1				# (XBee command) set the address of this radio to eight byte hex (must be unique)
ATID B000				# (XBee command) Set the PAN ID to eight byte hex (all XBee's must have this same value)
ATCH 0E					# (XBee command) set the Channel ID to a four byte hex (all XBee's must have same value)
ATPL 0					# (XBee command) power level at which the RF module transmits (0 lowest / 4 highest)
ATWR					# (XBee command) write all the changes to the XBee non-volatile memory
ATFR					# (XBee command) reboot XBee radio
exit					# (XBeeTerm command) exit python shell

As they stand right now, these files could not be processed by XBeeTerm.py because of the comments (included to make the contents understandable).  To clean this up, the command sed '/^#/d; s/\([^$]\)#.*/\1/'will remove all shell type comments from a file and sed 's/[ \t]*$//'will remove unneeded white space.  Putting this all together and you can use this to prepare the above files for XBeeTerm.py:

sed '/^#/d; s/\([^$]\)#.*/\1/' Config-Coordinator.txt | sed 's/[ \t]*$//' > coord.txt
sed '/^#/d; s/\([^$]\)#.*/\1/' Config-End-Device.txt | sed 's/[ \t]*$//' > endd.txt

Now execute the following python XBeeTerm.py coord.txt and you get the output below:

XBeeTerm Script

The yellow text is responses back from the XBee serial terminal and the red text is from the XBee radio itself.  Since all the red text is “OK”, all the commands took and the XBee radio is now configured as a Coordinator.  Now repeat this for the End Device XBee radio.

In this example, I have one End Device but what if you have multiple devices, do you need a Config-End-Device.txt file for each end device?  The only change within the configuration file is the radio’s address, which is established via the ATMY command.  Here is a trick to avoid the need for multiple files.  First, configure all your End Devices using the configuration file.  Then, for each radio, modify the ATMY use the following:

echo -e "baudrate 9600\nserial /dev/ttyUSB0\n+++\nATMY AAA1\nATWR\nATFR\nexit" | python XBeeTerm.py

but for each End Device radio, increment the ATMY address by one (e.g. AAA2, AAA3, …).

Querying XBee for Configuration

Now that we believe the XBee radios are properly configured, lets verify that by query the radios.  You could use XBeeTerm to perform this function by including only the AT Command without the parameter but I wanted a more informative tool. For this, I have created another utility that can take a list of AT Commands as arguments and query the XBee radio for the AT’s parameter value.  This utility, call XBeeQuery.py, is listed below:


#! /usr/bin/python

"""
	This utility will query a XBee radio for some of it's AT Command parameters and print their values, as well as
	optional discriptive information.  It has a set of default AT Commands or the use can provide the desired set
	of AT Commands on the command-line.

	The dictionay of AT Command descriptive information is limited but can be easily expanded.  The descriptive
	information was taken from the first referance given below. Also note that if this utility appears to hang,
	it is almost certainly waiting on a response from the XBee. To continue processing the AT Command list, use Ctrl-C.

	Reference Materials:
		XBee/XBee-PRO OEM RF Modules: Product Manual v1.xCx - 802.15.4 Protocol
			ftp://ftp1.digi.com/support/documentation/90000982_A.pdf
		python-xbee Documentation: Release 2.0.0, Paul Malmsten, December 29, 2010
			http://python-xbee.googlecode.com/files/XBee-2.0.0-Documentation.pdf
		Parser for command-line options, arguments and sub-commands
			http://docs.python.org/2/library/argparse.html#module-argparse

"""

# imported modules
import os						# portable way of using operating system dependent functionality
import sys						# provides access to some variables used or maintained by the Python interpreter
import serial					# encapsulates the access for the serial port
import argparse					# provides easy to write and user-friendly command-line interfaces
from xbee import XBee			# implementation of the XBee serial communication API
from pretty import stringc		# provides colored text for xterm and VT100 type terminals using ANSI Escape Sequences

# authorship information
__author__ = "Jeff Irland"
__copyright__ =	"Copyright 2013"
__credits__ = "Paul Malmsten"
__license__ = "GNU General Public License"
__version__ = "0.1"
__maintainer__ = "Jeff Irland"
__email__ = "jeff.irland@gmail.com"
__status__ = "Development"
__python__ = "Version 2.7.3"

# text colors to be used during terminal sessions
NORMAL_TEXT = 'normal'
CMD_TEXT = 'bright red'
NAME_TEXT = 'bright yellow'
CAT_TEXT = 'bright yellow'
DESC_TEXT = 'normal'
RANGE_TEXT = 'bright cyan'
DEFAULT_TEXT = 'bright green'

class ArgsParser():
	"""Within this object class you should load all the command-line switches, parameters, and arguments to operate this utility"""
	def __init__(self):
		self.parser = argparse.ArgumentParser(description="This utility will query a XBee radio for some of it's AT Command parameters and print their values.  It has a set of default AT Commands or the use can provide the desired set of AT Commands on the command-line.", epilog="This utility is for query only and will not change the AT Command parameter values.")
		self.optSwitches()
		self.reqSwitches()
		self.optParameters()
		self.reqParameters()
		self.optArguments()
		self.reqArguments()
	def optSwitches(self):
		"""optonal switches for the command-line"""
		self.parser.add_argument("--version", action="version", version=__version__, help="print version number on stdout and exit")
		self.parser.add_argument("-v", "--verbose", action="count", help="produce verbose output for debugging")
		self.parser.add_argument("-n", "--name", required=False, action="store_true", help="print the name (i.e. short description) of the XBee AT Command")
		self.parser.add_argument("-d", "--description", required=False, action="store_true", help="print the full description of the XBee AT Command")
	def reqSwitches(self):
		"""required switches for the command-line"""
		pass
	def optParameters(self):
		"""optonal parameters for the command-line"""
		self.parser.add_argument("-b", "--baudrate", required=False, action="store", metavar="RATE", type=int, default=9600, help="baud rate used to communicate with the XBee radio")
		self.parser.add_argument("-p", "--device", required=False, action="store", metavar="DEV", type=str, default='/dev/ttyUSB0', help="open this serial port or device to communicate with the XBee radio")
	def reqParameters(self):
		"""required parameters for the command-line"""
		pass
	def optArguments(self):
		"""optonal arguments for the command-line"""
		self.parser.add_argument(nargs="*", action="store", dest="inputs", help="AT Commands to be queried")
	def reqArguments(self):
		"""required arguments for the command-line"""
		pass
	def args(self):
		"""return a object containing the command-line switches, parameters, and arguments"""
		return self.parser.parse_args()

class ATDict():
	"""Within this object class you should load a dictionary of { "AT Command" : [ "Name", "Category", "Description", "Parameter Range", "Default Value" ] }"""
	# Networking & Security, RF Interfacing, Sleep Modes (NonBeacon), Serial Interfacing, I/O Settings, Diagnostics, AT Command Options
	def __init__(self):
		self.commands = {
		"CH" : [ "Channel", "Networking", "Set/Read the channel number used for transmitting and receiving data between RF modules.", "0x0B - 0x1A", "0x0C" ],
		"ID" : [ "PAN ID", "Networking", "Set/Read the PAN (Personal Area Network) ID. Use 0xFFFF to broadcast messages to all PANs.", "0 - 0xFFFF" , "0x3332" ],
		"DH" : [ "Destination Address High", "Networking", "Set/Read the upper 32 bits of the 64-bit destination address. When combined with DL, it defines the destination address used for transmission. To transmit using a 16-bit address, set DH parameter to zero and DL less than 0xFFFF. 0x000000000000FFFF is the broadcast address for the PAN.", "0 - 0xFFFFFFFF", "0" ],
		"DL" : [ "Destination Address Low", "Networking", "Set/Read the lower 32 bits of the 64-bit destination address. When combined with DH, DL defines the destination address used for transmission. To transmit using a 16-bit address, set DH parameter to zero and DL less than 0xFFFF. 0x000000000000FFFF is the broadcast address for the PAN.", "0 - 0xFFFFFFFF", "0" ],
		"MY" : [ "16-bit Source Address", "Networking", "Set/Read the RF module 16-bit source address. Set MY =0xFFFF to disable reception of packets with 16-bit addresses. 64-bit source address(serial number) and broadcast address (0x000000000000FFFF) is always enabled.", "0 - 0xFFFF", "0" ],
		"SH" : [ "Serial Number High", "Networking", "Read high 32 bits of the RF module's unique IEEE 64-bit address. 64-bit source address is always enabled.", "0 - 0xFFFFFFFF [read-only]", "Factory-set" ],
		"SL" : [ "Serial Number Low", "Networking", "Read low 32 bits of the RF module's unique IEEE 64-bit address. 64-bit source address is always enabled.", "0 - 0xFFFFFFFF [read-only]", "Factory-set" ],
		"RR" : [ "XBee Retries", "Networking", "Set/Read the maximum number of retries the module will execute in addition to the 3 retries provided by the 802.15.4 MAC. For each XBee retry, the 802.15.4 MAC can execute up to 3 retries.", "0 - 6", "0" ],
		"RN" : [ "Random Delay Slots", "Networking", "Set/Read the minimum value of the back-off exponent in the CSMA-CA algorithm that is used for collision avoidance. If RN = 0, collision avoidance is disabled during the first iteration of the algorithm (802.15.4 - macMinBE).", "0 - 3 [exponent]", "0" ],
		"MM" : [ "MAC Mode", "Networking", "Set/Read MAC Mode value. MAC Mode enables/disables the use of a Digi header in the 802.15.4 RF packet. When Modes 0 or 3 are enabled(MM=0,3), duplicate packet detection is enabled as well as certain AT commands.", "0 = Digi Mode, 1 = 802.15.4 (no ACKs), 2 = 802.15.4 (with ACKs), 3 = Digi Mode (no ACKs)", "0" ],
		"NI" : [ "Node Identifier", "Networking", "Stores a string identifier. The register only accepts printable ASCII data. A string can not start with a space. Carriage return ends command. Command will automatically end when maximum bytes for the string have been entered. This string is returned as part of the ND (Node Discover) command. This identifier is also used with the DN (Destination Node) command.", "20-character ASCII string", "-" ],
		"NT" : [ "Node Discover Time", "Networking", "Set/Read the amount of time a node will wait for responses from other nodes when using the ND (Node Discover) command.", "0x01 - 0xFC [x 100 ms]", "0x19" ],
		"CE" : [ "Coordinator Enable", "Networking", "Set/Read the coordinator setting. A value of 0 makes it an End Device but a value of 1 makes it a Coordinator.", "0 = End Device, 1 = Coordinator", "0" ],
		"SC" : [ "Scan Channels", "Networking", "Set/Read list of channels to scan for all Active and Energy Scans as a bitfield. This affects scans initiated in command mode (AS, ED) and during End Device Association and Coordinator startup", "0 - 0xFFFF [bitfield](bits 0, 14, 15 not allowed on the XBee-PRO)", "0x1FFE (all XBee-PRO Channels)" ],
		#"SD" : [ "Scan Duration", "Networking", "Set/Read the scan duration exponent.  For End Device - Duration of Active Scan during Association.  For Coordinator - If ‘ReassignPANID’ option is set on Coordinator [refer to A2 parameter],  SD determines the length of time the Coordinator will scan channels to locate existing PANs. If ‘ReassignChannel’ option is set, SD determines how long the Coordinator will perform an Energy Scan to determine which channel it will operate on.  ‘Scan Time’ is measured as (# of channels to scan] * (2 ^ SD) * 15.36ms). The number of channels to scan is set by the SC command. The XBee can scan up to 16 channels (SC = 0xFFFF).", "0-0x0F [exponent]", "4" ],
		"A1" : [ "End Device Association", "Networking", "Set/Read End Device association options. bit 0 - ReassignPanID (0 - Will only associate with Coordinator operating on PAN ID that matches module ID / 1 - May associate with Coordinator operating on any PAN ID), bit 1 - ReassignChannel(0 - Will only associate with Coordinator operating on matching CH Channel setting / 1 - May associate with Coordinator operating on any Channel), bit 2 - AutoAssociate (0 - Device will not attempt Association / 1 - Device attempts Association until success Note: This bit is used only for Non-Beacon systems. End Devices in Beacon-enabled system must always associate to a Coordinator), bit 3 - PollCoordOnPinWake (0 - Pin Wake will not poll the Coordinator for indirect (pending) data / 1 - Pin Wake will send Poll Request to Coordinator to extract any pending data), bits 4 - 7 are reserved.", "0 - 0x0F [bitfield]", "0" ],
		#"XX" : [ "", "", "", "", "" ],
		#"XX" : [ "", "", "", "", "" ],
		}
	def dictionary(self):
		"""return the whole dictionary of command"""
		return self.commands
	def command(self, cmd):
		"""return the colorized AT Command"""
		return stringc(cmd, CMD_TEXT)
	def name(self, cmd):
		"""return the colorized name of the AT Command"""
		return stringc(self.commands[cmd][0], NAME_TEXT)
	def category(self, cmd):
		"""return the colorized category of the AT Command"""
		return stringc(self.commands[cmd][1], CAT_TEXT)
	def description(self, cmd):
		"""return the colorized description of the AT Command"""
		return stringc(self.commands[cmd][2], DESC_TEXT)
	def range(self, cmd):
		"""return the colorized range of the AT Command"""
		return stringc(self.commands[cmd][3], RANGE_TEXT)

	def default(self, cmd):
		"""return the colorized devault value of the AT Command"""
		stringc(self.commands[cmd][3], DEFAULT_TEXT)

class FrameID():
	"""XBee frame IDs must be in the range 0x01 to 0xFF (0x00 is a special case).
	This object allows you to cycles through these numbers."""
	def __init__(self):
		self.frameID = 0
	def inc(self):
		if self.frameID == 255:
			self.frameID = 0
		self.frameID += 1
		return self.frameID

def byte2hex(byteStr):
	"""Convert a byte string to it's hex string representation"""
	hex = []
	for aChar in byteStr:
		hex.append("%02X " % ord(aChar))
	return ''.join(hex).strip()

if __name__ == '__main__':
	# parse the command-line for switches, parameters, and arguments
	parser = ArgsParser()							# create parser object for the command-line
	args = parser.args()							# get list of command line arguments, parameters, and switches
	if args.verbose > 0:							# print what is on the command-line
		print os.path.basename(__file__), "command-line arguments =", args.__dict__

	# create required objects
	frameID = FrameID()								# create a cycling frame ID sequence number to be used in XBee frames
	at = ATDict()									# create a dictionary of XBee AT Commands definitions
	ser = serial.Serial(args.device, args.baudrate)	# Open the serial port that has the XBee radio
	xbee = XBee(ser)								# Create XBee object to communicate with the XBee radio

	# if user provided desired AT Commands, use them for query, otherwise use the whole dictionary
	if len(args.inputs) == 0:
		args.inputs = at.dictionary()

	# for the command list provided, query the XBee radio AT Command's parameters
	for cmd in args.inputs:
		xbee.send('at', frame_id=chr(frameID.inc()), command=cmd)
		try:
			response = xbee.wait_read_frame()
			if args.name:
				print at.command(cmd) + " = " + byte2hex(response['parameter']) + "  " + at.name(cmd)
			else:
				print at.command(cmd) + " = " + byte2hex(response['parameter'])
			if args.description:
				print at.description(cmd) + "\n"
		except KeyboardInterrupt:
			print "*** AT Command \"" + at.command(cmd) + "\" interrupted. ***"
			if args.description:
				print " "

	ser.close()

Here is a sample output for the XBeeQuery utility:

XBeeQuery Script

References

Configuring the XBee Radios

General Documentation

Selecting XBee Radios and Supporting Software Tools

DigiMesh_networking

My ultimate aim is to wirelessly network several Arduino based platforms with a centralized Raspberry Pi controller or gateway.  There is much for me to learn to get this operational, not the least of which is the selection of the radio device platform I plan to use. After reviewing other devices, I have settled on the XBee, in part because of its popularity, its mesh capabilities, and it power management.  To get up to speed on the Xbee, I found the tutorials at AdafruitSparkfun, and Parallax helpful.   Some additional good references are listed at the end of this post.

As I did in the post Raspberry Pi Serial Communication: What, Why, and a Touch of How, I have a desire (obsessive need?) to do some extensive researching before diving into implementing a project.  What is listed below are my research findings.

XBee Series 1 vs. Series 2

Digi’s XBee website gives you a confusing set of options for selecting radios but after reviewing multiple sources, it boils down to the XBee Series 1 vs. Series 2 for the DIY type applications I would do.

    • XBee Series 1 Module (Freescale technology with 802.15.4 firmware) have a 250 kbps RF data rates and operate at 2.4 GHz.  These radios use the IEEE 802.15.4 networking protocol and can perform point-to-multi-point networking.  You can also do peer-to-peer networking (a form of mesh network) but this will require a firmware  upgrade called DigiMesh designed specifically for the Series 1 hardware.
    • XBee Series 2 Module (Ember/Silicon Labs technology with ZigBee firmware) are similar to the Series 1 in many respects but use the ZigBee standard, and  therefore, have the potential for interoperability with devices made by different vendors.

Mesh networking (topology) is a type of networking where each node must not only capture and disseminate its own data, but also may serve as a relay for other nodes, that is, it must collaborate to propagate the data in the network.  This gives the network self configuringself healingdynamic routing, and other capabilities.  Wireless mesh networks can be implemented with various wireless technology including 802.11802.15802.16, cellular technologies or combinations of more than one type.  The mesh enabling capability for these technologies is the routing protocol being used. There are many routing protocols being used by mesh networks today for many different types of products, but I will concern ourselves with just the XBee Series 1 and Series 2 products.

The Zigbee Alliance is a group of more than 300 companies who is responsible for publishing and maintaining the Zigbee specification. In the ZigBee network topology,  there are different node types in the network (Coordinator, Router, End Device). DigiMesh is a proprietary peer-to-peer networking using a simplified topology (no need to define and organize coordinators, routers or end-nodes).  Digi has a white paper that does a nice comparison of the two types of meshing products.

To further clarify the similarity/difference between Series 1 & 2, see the table below:

Feature

XBee Series 1 / 802.15.4

XBee Series 2 / ZigBee

Ramifications

Price

~ $23

~ $26

Price shouldn’t be a driver but If you are looking for a simple point-to-point configuration, you should go with the Series 1. The Series 2 requires considerable setup and configuration.

Transmit Power Output 

Indoor/Urban Range 

Line-of-Sight Range

1 mW (0dbm)

up to 100 ft.

up to 300 ft.

2 mW (+3dbm)

up to 133 ft.

up to 400 ft.

The additional power of the Series 2 give you a ~25% increase in range.

Firmware

Comes standard with 802.15.4 firmware for point-point or star topology. Requires DigiMesh firmware to mesh.

does not offer any 802.15.4-only firmware; it is always running the XBee ZB ZigBee mesh firmware

DigiMesh, while proprietary, appears to have less overhead and easier to configure

Network Topologies

Point-to-Point, Star, and Mesh (with DigiMesh firmware)

Point-to-Point, Star, and Mesh

With a closer reading of the specs, you’ll find that the Series 1 with DigiMesh has a peer-to-peer topology but the Series 2 is hierarchical. I believe peer-to-peer is superior.

Routing Protocol

Ad hoc On-Demand Distance Vector (AODV) Routing + Hierarchical Tree Routing as last resort

Ad hoc On-Demand Distance Vector (AODV) Routing

I suspect there are more differences here but couldn’t uncover them.

RF Data Rate

250 Kbps

250 Kbps

RF data rate doesn’t have much practical meaning

Practical Maximum Throughput

~ 80kbps

ZigBee is significantly slower than the 802.15.4

ZugBee is a full OSI stack, and as a result, has significant overhead.

Power-Down Current

10 uA

1 uA

Series 2 was built for low power consumption.

After pondering this all for a bit, I believe the choose boils down to two questions:

    • Is the interoperability of ZigBee important to you?
    • What are the benefits of ZigBee vs. DigiMesh?

The answer to the first question is “no”.  While I find ZigBee’s interoperability seductive, the practical matter is that I just don’t see that many applications that I could envision integrating into a project.  Maybe some day, but not within my planning horizon.  As to the second question, I don’t see any advantages to ZigBee’s imposed topology of coordinators, routers, and end-nodes.  I believe the homogeneous types of nodes will give my applications the flexibility I may need. Of course, there are many other things one might want to consider, but I think this analysis is sufficient for my needs.  I’m going with the XBee Series 1 Module and I’ll install DigiMesh firmware when it comes time to build meshed networks.

AT Mode vs. API Mode

XBee modules support two modes of operation – AT mode and API mode.  In API mode, you communicate with the radio by sending and receiving packets. In AT (transparent) mode, the XBee radio simply relays serial data to the receiving XBee, as identified by the DH+DL address.  Series 1 radios support both AT and API modes with a single firmware version, allowing you switch between the modes with the  X-CTU software.

To create simple point-to-point links, XBee works nicely in AT mode without much coding. However, if your goal is to build a network consisting of more than two devices, AT mode becomes too difficult to bear. You will spend almost all the time switching in and out of AT mode, wasting time and draining batteries in the process. On the other hand, in API mode commands and data travel in specially formatted frames and no switching is necessary. Another advantage of API mode is that serial speed on transmitters doesn’t have to match – one can be configured for 115,200bps, another for 2400bps, third left with default 9600bps. There is another nice feature called remote command; you can remotely request the state of XBee module pins, for example, or change an output pin level.

It’s clear that I’ll want to work in API mode, but judging from the examples of XBee API mode code, it sure would be nice to have a library package that has designed in some basic utilities that I can leverage.  That is the next topic.

Supporting Software

One of the first packages I discovered was the Xbee Network Protocol (XNP).  I was impressed by the volume and quality of the documentation.  Never the less, I passed on it for two reasons.  First, it isn’t as mature as other packages I discovered (and not widely used), and most importantly, it appears to be implementing mesh networking in software.  Ether the author didn’t recognize that the Series 1 modules can mesh via the DigiMesh firmware (not a surprise since many websites wrongfully report that the Series 1 do not mesh) or he just wants to roll-his-own (what better way to learn about mesh routing protocols).

xbee-arduino is a C++ Arduino library for communicating with XBees in API mode, with support for both Series 1 and Series 2.  Judging from the documentation,  it appears that it could be ported to Raspberry Pi but it would be far easier to use something targeted for the RPi (see below).  With the latest beta software, the XBee’s serial communications can be handle via SoftwareSerial, freeing up the Arduino USB for debugging.  It also appears to that the author is experimenting  if not already supporting DigiMesh. The package is well documented, actively maintained, and equally important, appears to be popular.

Another possibility for the Raspberry Pi is the python-xbee. This Python package is not as well documented as the xbee-arduino but does appear to be actively used and supported. The fact that its on the Python Organization’s web site as a listed package gives it some additional credibility. See this and this with respect to DigiMesh support.

libxbee is another C++ library but targeted at Linux and Windows. Fewer users and the author states “development is coming to an end” may mean this platform isn’t as strong as the others.

While not a very rigorous analysis, I believe I’ll place my bets on xbee-arduino for Arduino development and python-xbee for Raspberry Pi development.  Using Python is intriguing in part because it appears to be the preferred software language for the RPi.  But what if you have some C++ code, a popular language for Linux, and that you want to use some existing libraries? There are tools that could make this happen. You could use Python’s C/C++ extension modules. Also, there is the Simplified Wrapper and Interface Generator (SWIG), which is a software development tool that connects programs written in C/C++ with a variety of high-level programming languages. SWIG is used with different types of target languages including common scripting languages such as Perl, PHP, Python, Tcl and Ruby.

References

An unspoken consideration in my analysis is documentation availability/quality, example implementations  for learning, and the availability of software libraries I could potential use. Here are some of the more interesting things I uncovered.

Tutorials (ZigBee but can be generalized to the XBee)

XBee Documentation

Example Implementations

    • Collection of XBee Projects – Digi’s collection of XBee projects done by the Hacker/DIY community
    • XBee Examples & Guides – Digi’s website giving you step-by-step instructions for simple projects
    • Playing Xbee: Part 1, 2, 3, 4 – author does a nice “teaching tour” on his introduction to XBee

Other Sources

Raspberry Pi Serial Communication: What, Why, and a Touch of How

I started exploring how to get a LCD display operational with a Raspberry Pi (RPi).  I checked out two hardware configurations: the bare bone LCD 16×2 display driven by the HD44780 chip set, which takes 6 wires to operate,  and another configuration that uses a Adafruit shield that requires only 2 wires.  It quickly became clear to me that I knew very little about how the LCD display works (maybe not an important topic for me) and I didn’t fully understands the RPi’s serial communications capabilities (something I must fully understand).  Adafruit does give some nice tutorial that  provide instructions on how to get both configurations work on the RPi, but I don’t like blindly follow tutorials without having a deeper understanding of my options and the underlining hardware.   So what are the serial communication options supported by the Raspberry Pi, under what situations would you use them, and how do you use them?

So I did my research, and for the moment, I’m not going to concern myself about LCD displays but I will dive neck deep into understanding RPi serial communications.  I’ll return to the LCD display topic later.

The Raspberry Pi Board

As our first step, lets take a quick scan of the interfaces we have on RPi for moving data in and out.  The picture below labels the most prominent components on the RPi board.

raspberry-pi

Of course, the “lead actor” on the RPi board is the Broadcom BCM2835, System on a Chip (SoC).  Broadcom bills this chip as a “multimedia applications processor for advanced mobile and embedded applications that require the highest levels of multimedia performance”.  Maybe more importantly for the DIY/Hacker community is that all the firmware running on the chip is now open source.

There is another big chip, that being the SMSC LAN9512/LAN9512i USB  Hub & Ethernet Controller.  This is the major work horse in getting data in/out of the RPi for USB peripheral devices and IP networking.

Now note that most of the board’s other large components are for data I/O.  Specifically, the HDMI, RCA Video, and Audio Out (3.5mm jack) are output only (at least for general purpose data communication perspective), and therefore, not part of our discussion.  On the other hand, the USB and Ethernet ports, are powerful  and widely supported serial communication devices, but I will only lightly touch the USB. I want to focus on serial communication with simple/bare-bone devices, like an LCD 16×2 display, which generally are not interface via something as sophisticated at USB or Ethernet.  But the USB can be used to some for some types of device interfacing, so I will cover that as a final topic.

There are three other board components that concern themselves with data communications but will not be covered in this post:

  • JTAG Header: The JTAG Header is used as a debug port. Embedded systems developer relies on debuggers communicating with chips via the JTAG to perform operations like single stepping and break-pointing.  In time, I suspect people will find ways to exploit these pins for both good & evil, but for now, this is for the RPi hardware development community to use.
  • CSI Camera Connector: The Camera Serial Interface (CSI)  specification is a standard interface between a camera and a host processor for mobile device applications.  This will be where you’ll connect cameras and video devices to the RPi.
  • DSI Display Connector: The Display Serial Interface (DSI) is a specification  aimed at reducing the cost of display sub-systems in a mobile device. This is commonly targeted at LCD and similar display technologies. RPi may be able to interface to some of these.  Some graphical LCD/OLED displays might be attached to it.  DSI would seem like a good candidate for implementing my LCD display but there is a problem.  It appears the DSI isn’t supported at this time.

This leaves one remaining board input/output component standing, that be the General Purpose Input/Output (GPIO).  On some electronics boards, GPIO pins have no special purpose defined, and some pins go unused by default.  The RPi developer has identified a handful of digital control lines and provided services on them for you to use.  By having these available can save you the hassle of having to arrange additional circuitry to provide them or implement functionality in software.

Raspberry Pi’s GPIO as a Data Bus
The first thing to get your head around, is how is data moved around the RPi, or in any general purpose computer.  In the most general sense in electronics, a bus or data bus is used to move data words of any type from one place to another. Computing is based on data words made up of collections of data bits. These “words” can contain as few as four data bits and often much larger.

The task of a bus designer is to devise circuitry that passes these data words from one circuit to another.  These words can be communicated serially  (i.e. serial communications) or in parallel.

  • Serial Bus: The least expensive method in terms of wire cost is to send the bits one at a time over a single pair of wires. This is called serial data transmission.  Data words start as sets of bits that exist in parallel. In order to ship these words on a serial basis they must be converted to a serial stream of bits at the transmit end and then reconverted to a parallel word at the receive end. The common name for the circuitry that does this conversion is a SerDes circuit which stands for serializer/deserializer. Integrated circuits are more expensive when they have more pins. To reduce the number of pins in a package, many ICs use a serial bus to transfer data when speed is not important. Some examples of such low-cost serial buses include Serial Peripheral Interface (SPI)Inter-Integrated Circuit (I²C)UNI/O, and 1-Wire.
  • Parallel Bus: At some point, it is more cost effective to add a wire for each bit in the word and send it in parallel on a data bus. Parallel buses have a limited data rate and distance at which they can be reliably run (more so than a serial bus).  Some widely used parallel bus standards are Parallel Bus Interface (PBI), Peripheral Component Interface (PCI), Small Computer Systems Interface (SCSI), the VMEbus used in instrumentation, the Rambus interface used in memories, and others.

As the name implies, GPIO pins can be configured through software  to provide some specific function or purpose within the hardware device design.  The GPIO pins connect directly into the core of the processor, and the Raspberry Pi developers implemented several alternate functions for the GPIO pins.  Several are desirable because of the multiple standards and types of devices you may wish to interface.  On boot-up,  the RPi board GPIO is in alternate function state “ALT0” and will support I2C, SPI, and UART.  This is shown below:

Raspberry Pi (Rev 1) GPIO Pin Out

It can be confusing to call the RPi’s whole 26 pin array GPIO and also some specific pins GPIO.   In reality, all the GPIO pins can be reconfigured to provide alternate functions. 

While PCM isn’t a topic for this posting, in the figure above, you’ll also see references to PCM on pins 18 & 21.  PCM stands for Pulse-code modulation and is s a method used to digitally represent analog signals.  It is often used to control light intensity or motors.  RPi’s native PCM capabilities are not well documented but it appears that people have has some success.  Generally, to do anything useful, you need multiple PCM channels so people have resorted to adding hardware or software to get the desired functionality from the RPi.  My guess is these RPi pins don’t have much of a future.

I2C, SPI, UART … Say what?

Much has been lightly covered so far, including terms like I2C, SPI, and UART.  So what is the significance?  Well, I2C, SPI, and UART are the heart of our quest to understand RPi’s serial communications capability. Via their exposure on the GPIO pins, these capabilities are what can be used to integrate things like LCD displays to the RPi.  Now lets dive deeper into each one of them.

Universal Asynchronous Receiver/Transmitter (UART)

The Universal Asynchronous Receiver/Transmitter (UART) takes bytes of data and transmits the individual bits in a sequential fashion. The device changes processor’s parallel information to serial data which can be sent on a communication line. A second UART (maybe on another processor)  can be used to receive the serial information. The UART performs all the tasks, timing, parity checking, etc. needed for the communication. The only extra devices attached are line driver chips capable of transforming the TTL level signals (0/5 volts) to line voltages (on RS-232 line this could be as +/- 25 volts) and vice versa.

Each UART contains a shift register, which is the fundamental method of conversion between serial and parallel forms. Serial transmission of digital information (bits) through a single wire or other medium is much more cost effective than parallel transmission through multiple wires.  The UARTs transmit/receive one bit at a time at a specified data rate (i.e. 9600bps, 115,200bps, etc.). This method of serial communication is sometimes referred to as TTL serial communications.

Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver. Instead, the sender and receiver must agree on timing parameters in advance and special bits are added to each word which are used to synchronize the sending and receiving units. When a word is given to the UART for Asynchronous transmissions, a bit called the “Start Bit” is added to the beginning of each word that is to be transmitted. The Start Bit is used to alert the receiver that a word of data is about to be sent, and to force the clock in the receiver into synchronization with the clock in the transmitter.

For a further description of synchronous and asynchronous line communications, check out this tutorial.

Raspberry Pi’s Mini-UART

Warning: Misleading information ahead – See Warren Gay’s comments.
The Raspberry Pi actually has two UARTs. One UART  is part of the internal ARM architecture of the Broadcom BCM2835 chip, in the core of the Raspberry Pi and not accessible externally.  The other UART is sometimes called the RPi’s “Serial Port” (even thou the USB supports serial communications, and therefore a serial port).  The serial port being reference here is serviced by a UART, sometime refereed to as the  “Mini-UART” since it doesn’t appear to be very rich in functionality.  It is basically be used as a console port for access to the Raspberry Pi.  The serial console is a convenient way to interact with the Raspberry Pi for debugging or your network is down and it is the destination of console messages (including boot-up messages).  From the Raspberry Pi pinout and the eLinux wiki, I can see that the serial port (aka Mini-UART) on the Pi is on GPIO Pin 14 (TX) and GPIO Pin 15 (RX):

Mini-UART

Since the GPIO pins give access to the Mini UART, you can establish a serial console, which can be used to log in to the Pi, and many other things.  However, normal console device communicate with -12V (logical “1″) and +12V (logical “0″) RS-232, which may just fry something in the 3.3V Pi. Even “TTL level” serial at 5V runs the same risk.  See this tutorial for one example on how to build 3.3V to RS-232 levels converter with a MAX3232CPE and a few passive components.

You can reconfigure the RPi so that the Mini UART isn’t acting as a serial console and use it for outer purposes (e.g. communicate with an attached Arduino or Xbee).  Using the Raspberry Pi’s serial port requires some Linux reconfiguration and the abandonment of the serial console, and potentially some level conversion, but it could be useful. The Mini-UART pins to provide access to Linux’s /dev/ttyAMA0 serial port.  To be able to use the serial port to connect and talk to other devices, the serial port console login needs to be disabled and the post “Raspberry Pi and the Serial Port” shows you how.

Again, keep in mind that RX and TX lines are available on the GPIOs but operate at 3.3 volts. You’ll need a board or cable to level convert 3.3 volt UART signals to connect with other devices (e.g. RS-232, USB).

Serial Peripheral Interface Bus (SPI) — aka 4-Wire Serial Bus

The Serial Peripheral Interface Bus or SPI (pronounced as either S-P-I or spy) bus is a synchronous serial data link standard, named by Motorola, that operates in full duplex mode.  SPI is much simpler than I2C. Master and slave are linked by three data wires, usually called MISO, (Master in, Slave out), MOSI (Master out, Slave in), the SCLK clock line (sometimes called M-CLK), and an optional SS (Slave Select; sometimes known as the Chip Select or CS line or Chip Enable or CE line) is the slave select or chip select line.  Its optional only if you have one slave, otherwise one or more SS lines are provided.  The Raspberry Pi has two Slave Select lines: CE0 and CE1.

SPI Diagram and Sescription

Usually the transfer sequence consist of driving the SS line low, sending X number of clock signals with the proper polarity and phase, then driving the SS line high to end the communication. As the clock signals are generated, data is transferred in both directions, therefore in a “transmit only” system the received bytes have to be discarded and in a “receive only” system a dummy byte has to be transmitted.

Many SPI-enabled ICs and Microcontrollers can cope with data rates of over 10MHz, so transfer is much faster than with I2C. Since it is synchronous communications, it is not limited to 8-bit words so you can send any message sizes with arbitrary content and purpose. The SPI interface does not require pull-up resistors, which translates to lower power consumption. The downside is that SPI normally has no addressing capability; instead, devices are selected by means of a SS signal which the master can use to enable one slave out of several connected to the SPI bus. If more than one slave exists, one chip select line is required per device, which can use precious GPIO lines on the Master.

Inter-Integrated Circuit (I2C) — aka 2-Wire Serial Bus

Inter-Integrated Circuit or I2C (pronounced as either I-squared-C or I-2-C) is generically referred to as a “two-wire interface”.  It’s a multi-master serial single-ended computer bus invented by Philips that is used to attach low-speed peripherals to a motherboard, embedded system, cellphone, or other electronic device.

I2C can be used to connect up to 127 nodes via a bus has two data wires, called SCL and SDA.  SCL is the clock line. It is used to synchronize all data transfers over the I2C bus. SDA is the data line. Of course, there is a third wire being ground. There may also be a 5 volt wire to distribute power to the devices. Both SCL and SDA lines are “open drain” drivers. What this means is that the chip can drive its output low, but it cannot drive it high.  For the line to be able to go high you must provide pull-up resistors to the 5v supply. There should be a resistor from the SCL line to the 5v line and another from the SDA line to the 5v line. The value of the resistors is not critical.  Anything from 1800 ohms to 47K ohms used (1.8K, 47K and 10K are common values). You only need one set of pull-up resistors for the whole I2C bus, not for each device, as illustrated below:

I2C Diagram and Description

In theory the I2C bus can support multiple masters, but most micro-controllers can’t. A master is usually a microcontroller, although it doesn’t have to be. Slaves can be ICs or microcontrollers.  When the master wishes to communicate with a slave it sends a series of pulses down the SDA and SCL lines. The data that is sent includes an address that identifies the slave with which the master needs to interact. Addresses take 7 bits out of a data byte; the remaining bit specifies whether the master wishes to read (get data from a slave) or write (send data to a slave).

Some devices have an address that is entirely fixed by the manufacturer; others can be configured to take one of a range of possible addresses. When a micro-controller is used as a slave it is normally possible to configure its address by software, and for that address to take on any of the 127 possible values.  The address byte may be followed by one or more byes of data, which may go from master to slave or from slave to master.

When data is being sent on the SDA line, clock pulses are sent on the SCL line to keep master and slave synchronised. Since the data is sent one bit at a time, the data transfer rate is one eighth of the clock rate. The original standard specified a standard clock rate of 100KHz, and most I2C chips and micro-controllers can support this. Later updates to the standard introduced a fast speed of 400KHz and a high speed of 1.7 or 3.4 MHz.  The Arduino and Raspberry Pi can support standard and fast speeds.

The fast rate corresponds to a data transfer rate of 50K bytes/sec which is too slow for some control applications. One option in that case is to use SPI instead of I2C.

1-Wire — aka 1-Wire Serial Bus

On a 1-Wire bus (sometime refered to as a  “MicroLan”), a single master device communicates with one or more 1-Wire slave devices over a single data line, which can also be used to provide power to the slave devices. Devices drawing power from the 1-wire bus are said to be operating inparasitic power mode.  When operating in parasite power mode, only two wires are required: one data wire, and ground. At the master, a 4.7k pull-up resistor must be connected to the 1-wire bus. With an external supply, three wires are required: the bus wire, ground, and power. The 4.7k pull-up resistor is still required on the bus wire.

1-Wire Diagram and Description

Each 1-Wire device contains a unique 64-bit  code, consisting of an 8-bit family code, a 48-bit serial number, and an 8-bit CRC. Before sending a command to a slave device, the master must first select that device using its code.

How do you use I2C, SPI, UART, or 1-Wire on the Raspberry Pi?

Now that we know the what & why for serial communications options on the Raspberry Pi, how do we use them?  This topic deserves technical details and examples but this post has already run too long.  I’m likely to do some specific implementation in the future, but for now I’ll reference some sources of information on the web.

First, lets be clear about the RPi software distribution I’m using, since not all will be supporting all these serial communications options.  I’m using Adafruit’s Occidentalis distribution (based on “Wheezy”) which comes with hardware SPI, I2C, and 1-wire support. In the Occidentalis distribution, Adafruit has included in the Linux kernel the needed drivers. SPI and I2C has been implement on the GPIO pins as outline above.  RPi doesn’t have a predetermined GPIO pin assignment for 1-Wire, but Adafruit choose GPIO pin 4 for 1-Wire.  Note that this unassigned GPCLK0 (General Purpose Clock Voltage) function.

Given you have the Occidentalis distribution, you can check on the installation of I2C, SPI, and 1-Wire via the following:

    • To validate I2C, connect any I2C device to power, ground, SDA and SCL. Then run  sudo i2cdetect -y 0 to detect which addresses are on the bus. i2cdetect is a program to scan an I2C bus for devices.  Also, you can list the I2C device drivers via the command ls /dev/*i2c*.  This illustrates that RPi supports two I2C buses, 0 and 1.
    • To validate SPI, the command ls /dev/*spi* will list two SPI devices, one each for the 0 & 1 SS lines.  To go further, use the  spidev_test.c tool described in Getting SPI working on the Raspberry Pi.
    • Occidentalis implementation of 1-Wire isn’t done via a kernel installed driver but bitbanged.  Adafruit states that the implementation is  “flakier than SPI or I2C” and they don’t have any tutorials.  Maybe 1-Wire should be shelved for now.

There are some good RPi SPI & I2C tutorials on Adafruits.  There are others on personal blogs but generally they are scares right now.

Attach Peripherals to the Raspberry Pi

Now that we know the in’s & out’s of serial communications with the RPi, what can be done to make the physical aspects of interfacing the board easier.  Dealing with the GPIO pins on the board can be a pain.  One solution is the Pi Cobbler which plug the GPIO pins into a solderless breadboard via a ribbon cable.  My personal favorite is the Pi Crust.  In this case, the confusing layout of  GPIO pins are much more clearly organized and supplied with more useful female headers.  A whole development environment will be supplied via PiFace Digital and Gertboard … when they become available.

The USB Serial Ports

As promised, the final topic is the USB port. The USB can be used for some types of device interfacing, particularly if you make it look like a simple serial port to the device.  The conventional serial port (not the newer USB port) is a very old I/O (Input/Output) port. It’s slow compared to newer Universal Serial Bus (USB) serial devices, but conventional serial ports are still in use and many devices you’ll hook to your RPi will want to use them.  Until around 2006, most new desktop PC’s had one but has been largely replaced by the USB. Conventional serial ports are still widely used in embedded systems, but the RPi choose to use the USB.  Never the less, it is possible to put a conventional serial port device on the USB bus by using a Serial to USB Adapter hardware or cables.  This could be necessary for some of the hardware hacking you’ll do with an RPi.  (For example, I’ll be posting a project where I’ll use the RPi’s USB to talk to a XBee radio.)

USB does synchronous communications (synchronous means that bytes are sent out at a constant rate one after another in step with a clock signal tick) and transmits in special packets like a network. Conventional serial ports are typically asynchronous (i.e. “not synchronous”). Just like a network, USB can have several devices physically attached to it, including serial ports. Each device on it gets a time-slice of exclusive use for a short time. A device can also be guaranteed the use of the bus at fixed intervals. One device can monopolize it if no other device wants to use it.

Under Linux, each and every hardware device, including USB ports, are treated as a file and call a device file.  A device file allows a user to access hardware devices, but shields the users from the technical details about the hardware. (This is unlike what we’ll see for the RPi GPIO interfaces where hardware level technical details must be address directly.) Under Linux, a conventional serial port will typically have a device file such as /dev/ttyS0, /dev/ttyS1, etc. but the USB serial ports will appear as /dev/ttyUSB0, /dev/ttyUSB1, etc.  When your device is plugged in, Linux assigns the filename as it sees fit and isn’t always predicable (it doesn’t have to be this way). If you need to know what device file your serial device is connected too (and your software often needs to know), using a combination of the commands lsusb, dmesg, and grep, plus some basic insight, will often do the trick.

The lsusb command can show you the hubs connected to your system, but you won’t necessarily see any entries in /dev until you plug something into it, and what that entry may be will be dependent upon the type of device you are plugging in.  The system doesn’t recognize new USB devices right away. It can take from a couple of seconds to as much as a minute.  If I plug my Serial to USB Adapter cable, using the lsusb command I can identify the cables .  Now using dmesg, and grepping for the “Manufacturer”, I can get the “FTDI”.  Now I grep again for “FTDI” to get the device file name “ttyUSB0“.  This is all illustrated below:

lsusb and dmesg

Speech Synthesis on the Raspberry Pi

Now that I can get sound out of my Raspberry Pi (RPi), the next logical step for me is speech synthesis … Right?  I foresee my RPi being used as a controller/gateway for other devices (e.g. RPi or Arduino).  In that capacity, I want the RPi to provide status via email, SMS, web updates, and so why not speech?  Therefore, I’m looking for a good text-to-speech tool that will work nicely with my RPi.

The two dominate free speech synthesis tools for Linux are eSpeak and Festival (which has a light-weight version called Flite). Both tools appear very popular, well supported, and produce quality voices.  I sensed that Festival is more feature reach and configurable, so I went with it.

To install Festival and Flite (which doesn’t require Festival to be installed), use the following command:

sudo apt-get install festival
sudo apt-get install flite

Festival

To test out the installation, check out Festival’s man page, and execute the following:

echo "Why are you in front of the computer?" | festival --tts
date '+%A, %B %e, %Y' | festival --tts
festival --tts Gettysburg_Address.txt

Text, WAV, Mp3 Utilities

Festival also supplies a tool for converting text files into WAV files.  This tool, called text2wave can be executes as follows:

text2wave -o HAL.wav HAL.txt
aplay HAL.wav

MP3 files can be 5 to 10 times smaller than WAV files, so it might be nice to convert a WAV file to MP3.  You can do this via a tool called lame.

lame HAL.wav HAL.mp3

Flite

Flite (festival-lite) is a small, fast run-time synthesis engine developed using Festival for small embedded machines. Taking a famous quote from HAL, the computer in the movie “2001: A Space Odyssey

flite "Look Dave, I can see you're really upset about this. I honestly think you ought to sit down calmly, take a stress pill, and think things over."

Depending how the software was built for the package, you find that flite (and festival) has multiple voices. To find what voices where built in, use the command

flite -lv

To play a specific voice from the list, use the -voice parameter in the command

flite -voice kal "I'm now speaking kal's voice. By the way, please call me Dr. Hawking."

In my case, the default voice appears to be “kal”, which sounds somewhat like Stephen Hawking.  “slt” appears to be a female version of the “kal” voice.

flite_time is a talking clock that can speak things like “The time is now, exactly two, in the afternoon.”

flite_time 14:00
flite_time `date +%H:%M`

Documentation

The documentation for Festival and Flite isn’t all that great but there does seem to be multiple sources.  Here is what I found most useful:

Getting Audio Out Working on the Raspberry Pi

I want to deliver sound from my Raspberry Pi’s (RPi) Audio Output 3.5mm jack.  I’ll need to get audio drivers working on Audio Out, and to test it, I’ll need some sound files and players.  I’m choosing the Advanced Linux Sound Architecture (ALSA) drivers because its widely supported and because ALSA not only provides audio but  Musical Instrument Digital Interface (MIDI) functionality to Linux.   I’ll also be using the popular command line MP3 players, mpg321 and the WAV player that comes with ALSA, aplay.

To get things going, I installed ALSA, a MP3 tools, and a WAV to MP3 conversion tool via the following commands:

sudo apt-get install alsa-utils
sudo apt-get install mpg321
sudo apt-get install lame

Enabling the Sound Module

Reboot the RP and when it comes back up, its time to load the Sound Drivers.  This will be done via loadable kernel module (LKM) which are object file  that contains code to extend the Linux kernel.  lsmod is a command on Linux systems which prints the contents of the /proc/modules file.  It shows which loadable kernel modules are currently loaded.  In my case, lsmod gives me:

The snd-bcm2835 module appears to be already installed. RPi has a Broadcom  BCM2835 system on a chip (SoC) which is a High Definition 1080p Embedded Multimedia Applications Processor.  snd-bcm2835 is the sound driver.  If  lsmod doesn’t list the snd-bcn2835 module, then it can be installed via the following command:

sudo modprobe snd-bcm2835

Enabling Audio Output

By default, the RPi audio output is set to automatically select the digital HDMI interface if its being used, otherwise the analog audio output. You can force it to use a specific interface via the sound mixer controls.  amixer allows command-line control of the mixer for the ALSA driver.

You can force the RPi to use a specific interface using the command amixer cset numid=3 N where the N parameter means the following: 0=auto, 1=analog, 2=hdmi.  Therefore, to force the Raspberry Pi to use the analog output:

amixer cset numid=3 1

Sound Check

With this done, you should be ready for a simple test.  Plug a speaker into the (RPi) Audio Output 3.5mm jack.  I used a simple battery powered iHM60 iHome speaker.  The jack will not deliver much power, so the speaker needs to be powered.

To test the RPi audio, you can play a WAV file (download this … excellent for user-error notification) with aplay, mpg321 for MP3 files, or use the speaker-test command if you don’t have a WAV/MP3 file.

aplay numnuts.wav
speaker-test -t sine -f 440 -c 2 -s 1
mpg321 "Mannish Boy.mp3"

More on the ALSA Sound Drivers and Utilities

While ALSA is a powerful tool, it documentation appears is very weak.  Also, it appears that the capabilities of ALSA drivers and utilities are very  dependent on the hardware used.  The best sources of documentation that I found include Advanced Linux Sound Architecture (ALSA) project homepage,  archlinux Advanced Linux Sound Architecture, and ALSA-sound-mini-HOWTO.

You can find useful information in the directory /proc, which is a “virtual” file system (meaning that it does not exist in real life, but merely is a mapping to various processes and tasks in your computer).

    • /proc/modules gives information about loaded kernel modules.  The command lsmod | grep snd will list modules relevant to the sound system.
    • You can check the existence of a soundcard by looking  at cat /proc/asound/cards.

The amixer command can provide useful information (sometimes):

    • You can look at the mixer settings by typing amixer without any arguments. This command lists the mixer settings of the various parts of the soundcard. The output from amixer can greatly differ from card to card. Unfortunately  you can’t find much documentation on how to interpret the out.
    • The RPi doesn’t have a “Master” control only “PCM”.  So commands like amixer set Master... will not work.  You must use amixer set PCM ...
    • You can mute /unmute the sound via these commands: amixer set PCM mute and amixer set PCM unmute
    • As of August 2012, there appears to be a known bug in RPi ALSA driver that ignores volume settings at the start of playback and always plays at max volume.  Therefore, commands like amixer set PCM 50% unmute will not set the volume to 50%, at least until this bug is fixed.  Maybe this isn’t really a bug but a limitation of the hardware because there is a workaround for this …. see below.

Volume Control

The RPi built-in sound chips don’t have a “master volume” control, and as a result, you must control the volume via software.  I guess the RPi views itself as a preamplifier (preamp) and volume controls would be supplied down stream.  ALSA provides a software volume control using the softvol plugin.

The /etc/asound.conf file is a configuration files for ALSA drivers (system-wide).  The main use of this configuration file is to add functionality. It allows you to create “virtual devices” that pre or post-process audio streams. Any properly written ALSA program can use these virtual devices as though they were normal devices.  My RPi  /etc/asound.conf file looks like this:

For most changes to /etc/asound.conf you will need to restart the sound server (ie. sudo /etc/init.d/alsa-utils restart) for the changes to take effect.

I attempted to implement the software volume controls outline in a softvol how-to that I found, but I couldn’t get it to work.  I did some additional digging, and I found a solution buried within a python script for a Adafruit project.  The following works for controlling the volume (in this case, reducing the volume to 80% of maximum):

amixer cset numid=1 -- 80%

Note that you can use this command to change the volume while sound is being played an its effect takes place immediately.  Also, I noticed that once the volume has been adjusted, its effect remains even after a reboot.

WAV and MP3 Conversion

The MP3 player mpg321 can convert MP3 files to WAV files but the WAV player, aplay, can not do a conversion.  To make a MP3 file from a WAV file, you’ll need the tool lame.

    • To convert from WAV to MP3: lame input.wav output.mp3
    • To convert from MP3 to WAV:  mpg321 -w output.wav input.mp3

Bottomeline

While you can get ALSA working on the Raspberry Pi, it appears only partly supported, maybe buggy, and poorly documented.  If you just want to simply get sound out of the device (like I do), you’ll be fine.  But if you have some desire to do some sound processing with ALSA, your likely to be very frustrated.

Epilogue

This specific post has gotten about 25% of all the viewings of my blog. I’m not sure why this is the case but I speculate that there are many people tying to make RPi into a Media Player and looking for answers to their technical problems.

At this point in time, others have done some additional postings and they are more instructive than my post. You should check out:

Dropbox for the Raspberry Pi (sort of)

I’m presently using Dropbox as a service for quick & easy movement of files between multiple PCs I use.  Its copy & paste operation is very intuitive on a Windows PC. I would like the same on the Raspberry Pi (RPi), particularly for moving files from my PC to the RPi.  I wanted the same utility on my RPi, but at the same time, I want the Linux command line paradigm supported and not be force to run X Windows on the RPi.

I did some searching and found things like Dropbox’s Linux distribution (which I wasn’t confident would work “out of the box” for the RPi’s ARM architecture, but source is provided),  GoodSync or Owncloud (which wouldn’t access my existing Dropbox files but instead create an alternative), python or  bash shell based up/down loaders (appears to behave like a simplified FTP tool), or the Secure SHell File System (SSHFS) based approach (the PC’s Dropbox directory is mounted on the RPi).

While the Dropbox’s Linux distribution is likely the ultimate way to go, I didn’t want to commit myself to the effort it would potentially require.  I settled on the SSHFS based approach.  I’m running my RPi headless and access it via my PC using Cygwin/X and Secure Shell (ssh).  With the SSHFS approach, I could make the Dropbox directory available for mounting at boot-up or mount it at will.  I only envision using the RPi-based Dropbox when I’m doing development and I will be doing that from my PC.  So this SSHFS approach seems fine for the way that I operate.

The SSHFS approach means files will not really be replicated on the RPi, like Dropbox does.  The files will reside within my PC’s Dropbox folder (and replicated on my other PCs via the Dropbox service) but accessible by the RPi via the SSHFS file mount.  This means I can’t have any applications I run on the RPi depend on files located in its Dropbox directory since it may not be always mounted.  I’m OK with this limitation, and in fact is consistent with the ad-hoc purpose I have for the Dropbox directory.

Installation Required on the PC

For me, nothing needs to be done here.  I already have Dropbox running on my PC, and via Cygwin/X, I have the foundations required for the host side of the SSHFS solution.  If you need help with this, signup for Dropbox here and find out how I’m using Cygwin/X here.

Installation Required on the RPi

On the client side of the solution, you’ll need to install SSHFS and FUSE. FUSE is the user-space filesystem framework and is the foundation on which SSHFS resides. FUSE allows user-space software, SSH in this case, to present a virtual file system interface to the user; something generally only done by the Linux kernel.  SSHFS connects to the remote system and provide the look and feel of a regular file system interface for remote files. On the RPi, install SSHFS via the command:

sudo apt-get install sshfs 

FUSE appear to be already installed on the RPi or maybe comes with SSHFS. Next you need to add required users to the FUSE usergroup.  In my case, that is the user pi.  You can see the existing groups pi is part of via the command groups pi.  You can validate that the FUSE user group has been created by using the command cat /etc/group | grep fuse.  To add pi to the FUSE user group, use the command:

sudo gpasswd -a pi fuse

The fuse group lets you limit which users are allowed to use FUSE-based file systems, in my case the Dropbox. This is important because FUSE installs setuid programs, which always carry security implications.

Configuring the Dropbox File System

Now its time to make your Dropbox directory on the RPi, and mount it to the PC’s instance of Dropbox. On the RPi, use this command to create the Dropbox:

mkdir ~/Dropbox

The next thing to do is to make sure that you can connect to the PC via ssh.  When I installed Cygwin, my focus was on using it as an X Server and making ssh connections from the PC to RPi.  I never tried the inverse (connect from the RPi to the PC) and that is what SSHFS is effectively doing.  So check for two things:

    • Is the ssh server running on the PC?  You can check for its status via the command cygrunsrv -Q sshd. In my case it was running, so its fine.
    • Is the port used by the ssh server on the PC open? You’ll need to open SSH port 22 for ssh services to work.  You can check its status by attempting to use it.  In my case, this was ssh Jeff@HomePC.home.  If this command appears to hang or time out (as it did for me), the port is likely blocked.  You’ll need to go to your Windows Firewall and open port 22.

There is another Cygwin sublimity that has to be taken into consideration.  When using the Cygwin, Windows drive letters are mapped to a special directory.  In my case, the Dropbox directory appears to Cygwin to have the following path: /cygdrive/c/Users/Jeff/Dropbox.

With this all addressed, reboot the RPi, and then you can now fire up you RPi Dropbox via:

sshfs Jeff@HomePC.home:/cygdrive/c/Users/Jeff/Dropbox ~/Dropbox

After you supply the PC’s password, you should now be able to access the Dropbox directory on the PC.  If you wish, you can remove the file system connection to the PC via the command:

fusermount -u ~/Dropbox

This connection will stay established as long as you don’t do the fusermount -u or reboot the RPi.  If you wish to mount the file system upon boot-up, and avoid executing the sshfs when you log-in, you can follow the procedure outline in the article that initially inspired me: Dropbox on Raspberry Pi via SSHFS

Something to Keep in Mind

While for the most part, moving between Windows/DOS and the Linux file systems isn’t a problem, there is one thing to remember. Windows-based text editors put one set of special characters at the end of lines (i.e. carriage return and line break = ‘\r\n’), while Unix/Linux puts other characters (i.e. line break = ‘\n’).  This odd anomaly is normally harmless, but some applications on a Linux cannot understand these characters and can cause Linux to not respond correctly.

The best example of Linux behaving badly (and the only one I know) is the execution of “shebang” or the “#!…” at the top of a bash, python, perl, etc. script.  If you edit these files in DOS, then move them to Linux, shebang will stop working.  Editing them under DOS is the origin of the problem, since a DOS based text editor will inject the extra carriage return character at the end of the text line.

To fix this problem, you can quickly convert an ASCII text file from DOS format (carriage return and line break) to the Unix format (line break), you can use the tool dos2unix.  Run this utility on the effected file and shebang should work once again.

Epilogue

At its foundation, SSH functions as a protocol for authenticating and encrypting remote shell sessions.  SSH can be thought of as much more than just a secure shell.  Using SSH’s foundation, SSHFS creates a new capability.  To learn more, check out the link SSH: More than secure shell.

Key sources I consulted to write this include:

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