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Custom Arduino Commands

At the core of the servo assembly is an Arduino Uno. By default the Arduino runs firmware created by SourceBots providing simple access to the attached servo shield, compatible ultrasound sensors and to the GPIO pins directly.

You can also extend the firmware on the Arduino to support more features, for example if you have other types of sensor which require higher precision timing than can be achieved through the Python API.


As your new firmware will still need to be able to talk to the Python code running on the Raspberry Pi, you'll need to base it on SourceBots default firmware. You can get a copy of the source code from its GitHub repo at Note that you will also need the Adafruit PWM Servo Driver Library which is included in that repo as a submodule.

We recommend using Arduino Studio to edit your firmware as it provides direct support for building the code into the firmware binary and downloading it onto the Arduino.

Adding a command

In order for the code running on your Pi to be able to interact with the functionality you're adding to the Arduino, you'll need to define a new command.

Adding a command requires two changes. First you must define the function which will implement the operations of that command and then you register that command under the name you'll use to call it from the Python code.

Implementing the command

You'll need to define a function to implement your command. The function should be placed in the firmware.cpp file just above the definition of the commands array (likely about line 217).

The function definition needs to look like this:

static CommandResponse my_function(int requestID, String argument) {
  // implementation here
  return OK;

You should choose a suitable name for your function (above it's called my_function), though you'll need to stick to letters, numbers and underscore (_). The rest of the function signature (the line starting static CommandResponse above) needs to be exactly as above.

You command will need to adhere to some conventions to be able to communicate with the Pi, though you can otherwise do pretty much anything you normally would on an Arduino.

The following example command shows how to handle arguments from the Pi as well as how to return errors and other values:

static CommandResponse echo(int requestID, String argument) {

  if (argument.length() == 0) {
    // If something went wrong, you should return a `COMMAND_ERROR` with a
    // message about the failure. That message will be fed back to Python where
    // an exception will be raised.
    return COMMAND_ERROR("nothing to echo");

  while (argument.length() > 0) {
    // You can use `pop_option` to extract individual parameters from the
    // `argument` string. They're sent space-separated, so be sure not to
    // include any spaces within your argument values!
    // You don't need to check the length of `argument` before calling
    // `pop_option` -- if there isn't an option to be had you'll get back an
    // empty string ("").
    String next_arg = pop_option(argument);

    // You can write values to be read by the code on the Pi using the
    // `serialWrite` function. The first two arguments need to be exactly as
    // they are here; the third argument needs to be a `String` containing the
    // value you want to send back to the Pi.
    serialWrite(requestID, '>', next_arg);

  // If everything went fine, you should return `OK`.
  return OK;

For other examples of how to define commands (and the helper functions you can use), you can have a look at the other commands the Arduino already supports. In particular the version, led and servo functions are a good starting point.

Registering the command

To register this command we need to modify the list of commands stored in the commands array. To do this we add a new line containing a CommandHandler for the command.

static const CommandHandler commands[] = {
  // ... lots of existing commands ... (make sure you leave them in place!)
  CommandHandler('E', &echo),

The first argument is a char containing the name you'll use in Python to call this command. It can contain exactly one letter or number. It is case-sensitive.

The second argument is the function you defined for your command, with an ampersand (&) just before the name.

Calling commands

To call a command from Python you should call the direct_command method on a Servo instance. You can send strings and numbers:

r.servo_board.direct_command('E', 'first argument', 2, 'third argument')

This will return a list of the responses from the command (anything which was sent back using the serialWrite function):

>>> r.servo_board.direct_command('E', 'first argument', 2, 'third argument')
['first argument', '2', 'third argument']

Note that the responses will all be strings, so you'll need to explicitly convert them back to numbers if you want to use them as numbers:

>>> a, b = r.servo_board.direct_command('E', 6, 2)
>>> print(int(a) / int(b))
>>> print(a / b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for /: 'str' and 'str'


If an error is encountered while handling a command an exception will be raised. The exceptions will contain information about the error so that you can change your code to fix it.


A CommandError indicates that the servo assembly experienced an error in processing the command and sent back an error message. The error message will be that returned by the command on the Arduino as a COMMAND_ERROR:

>>> r.servo_board.direct_command('echo')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
CommandError: Error: nothing to echo

You'll also see a CommandError if you try to call a command which doesn't exist:

>>> r.servo_board.direct_command('X')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
CommandError: Error, unknown command: X

This error will generally indicate that something is wrong in your Python code.


An InvalidResponse indicates that the Python API which handles the actual communication to the Arduino received an invalid response from the Arduino. The error message will be the actual text received from the Arduino, allowing you to easily see the cause of the problem.

>>> r.servo_board.direct_command('Q')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
InvalidResponse: b'received data'

This error will generally indicate that one of the messages being sent back from the Arduino is badly formatted. Check to ensure that you're using serialWrite correctly for all your messages back to the Pi.

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