8.12. Example I²C code for Linux in Python

The example code below uses a Python library named smbus2 to communicate with a Simple Motor Controller G2 via I²C. This example works on Linux with either Python 2 or Python 3.

If you are using a Raspberry Pi, please see the notes about setting up I²C for the Raspberry Pi in Section 8.11.

To install the smbus2 library, you will need to run either pip install smbus2 or pip3 install smbus2 depending on what version of Python you want to use and what Linux distribution you are using. On Raspbian, pip is for Python 2 and pip3 is for Python 3.

# Uses the smbus2 library to send and receive data from a
# Simple Motor Controller G2.
# Works on Linux with either Python 2 or Python 3.
#
# NOTE: The SMC's input mode must be "Serial/USB".
# NOTE: You might nee to change the 'SMBus(3)' line below to specify the
#   correct I2C bus device.
# NOTE: You might need to change the 'address = 13' line below to match
#   the device number of your Simple Motor Controller.

from smbus2 import SMBus, i2c_msg

class SmcG2I2C(object):
  def __init__(self, bus, address):
    self.bus = bus
    self.address = address

  # Sends the Exit Safe Start command, which is required to drive the motor.
  def exit_safe_start(self):
    write = i2c_msg.write(self.address, [0x83])
    self.bus.i2c_rdwr(write)

  # Sets the SMC's target speed (-3200 to 3200).
  def set_target_speed(self, speed):
    cmd = 0x85  # Motor forward
    if speed < 0:
      cmd = 0x86  # Motor reverse
      speed = -speed
    buffer = [cmd, speed & 0x1F, speed >> 5 & 0x7F]
    write = i2c_msg.write(self.address, buffer)
    self.bus.i2c_rdwr(write)

  # Gets the specified variable as an unsigned value.
  def get_variable(self, id):
    write = i2c_msg.write(self.address, [0xA1, id])
    read = i2c_msg.read(self.address, 2)
    self.bus.i2c_rdwr(write, read)
    b = list(read)
    return b[0] + 256 * b[1]

  # Gets the specified variable as a signed value.
  def get_variable_signed(self, id):
    value = self.get_variable(id)
    if value >= 0x8000:
      value -= 0x10000
    return value

  # Gets the target speed (-3200 to 3200).
  def get_target_speed(self):
    return self.get_variable_signed(20)

  # Gets a number where each bit represents a different error, and the
  # bit is 1 if the error is currently active.
  # See the user's guide for definitions of the different error bits.
  def get_error_status(self):
    return self.get_variable(0)

# Open a handle to "/dev/i2c-3", representing the I2C bus.
bus = SMBus(3)

# Select the I2C address of the Simple Motor Controller (the device number).
address = 13

smc = SmcG2I2C(bus, address)

smc.exit_safe_start()

error_status = smc.get_error_status()
print("Error status: 0x{:04X}".format(error_status))

target_speed = smc.get_target_speed()
print("Target speed is {}.".format(target_speed))

new_speed = 3200 if target_speed <= 0 else -3200
print("Setting target speed to {}.\n".format(new_speed));
smc.set_target_speed(new_speed)