This page describes experiments in communicating between a Raspberry Pi (as a master) and an Arduino (as a slave) over I2C. The Raspberry Pi is programmed in Python, the Arduino using a sketch (the Arduino's own language).

I based this experiment on an article found on The Robotics Back-End (see References below), but have modified it to use Python rather than C on the Raspberry Pi, since that's the main language I've been using to write the operating system of the KR01 robot. Some of the ideas I found online tended to lock up the I2C bus or cause other kinds of problems. When I stumbled onto an example using the WiringPi library things seemed to even out.

WiringPi is a C++ library for communicating with a Raspberry Pi. It's been ported to Python as the WiringPi-Python library.

WiringPi (both C++ and Python versions) has an I2C class to provide communication over I2C. The basic API is as follows:

class I2C(object):
  def setupInterface(self,*args):
        return wiringPiI2CSetupInterface(*args)
  def setup(self,*args):
    return wiringPiI2CSetup(*args)
  def read(self,*args):
    return wiringPiI2CRead(*args)
  def readReg8(self,*args):
    return wiringPiI2CReadReg8(*args)
  def readReg16(self,*args):
    return wiringPiI2CReadReg16(*args)
  def write(self,*args):
    return wiringPiI2CWrite(*args)
  def writeReg8(self,*args):
    return wiringPiI2CWriteReg8(*args)
  def writeReg16(self,*args):
    return wiringPiI2CWriteReg16(*args)

In particular, we can expect to be using the wiringPiI2CRead() and wiringPiI2CWrite() methods.

For documentation, see The I2C Reference Library.

Raspberry Pi Installation#

WiringPi for Python can be installed via:

 sudo pip3 install wiringpi

Arduino Installation#

You'll need to install an Arduino sketch on your Arduino using the Arduino IDE. As an example, I modified the original source to blink the Arduino's built-in LEDs when either a transmit (TX) or receive (RX) event occurs.

#include <Wire.h>
#define SLAVE_ADDRESS 0x08
byte data_to_echo = 0;

// this slave code is meant to work on I2C address 0x08
// with rpi_arduino_wiringpi_i2c.cpp as described at:
// https://roboticsbackend.com/raspberry-pi-master-arduino-slave-i2c-communication-with-wiringpi/

void setup()
{
  Wire.begin(SLAVE_ADDRESS);
  Wire.onReceive(receiveData);
  Wire.onRequest(sendData);
  ready_blink();
}

void loop() {

}

void receiveData(int bytecount)
{
  for (int i = 0; i < bytecount; i++) {
    data_to_echo = Wire.read();
  }
  blink_rx();
}

void sendData()
{
  Wire.write(data_to_echo);
  blink_tx();
}

// status displays .....................................................

void ready_blink() {
  int i = 0;
  while ( i < 800 ) {
    blink_builtin();
    delay(i);
    i = i + 50;
  }
}

void blink_builtin() {
  digitalWrite(LED_BUILTIN, HIGH);      // turn the LED on (HIGH is the voltage level)
  delay(100);                           // wait 100ms
  digitalWrite(LED_BUILTIN, LOW);       // turn the LED off by making the voltage LOW
}

void blink_tx() {
  digitalWrite(LED_BUILTIN_TX, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(100);                           // wait 100ms
  digitalWrite(LED_BUILTIN_TX, LOW);    // turn the LED off by making the voltage LOW
}

void blink_rx() {
  digitalWrite(LED_BUILTIN_RX, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(100);                           // wait 100ms
  digitalWrite(LED_BUILTIN_RX, LOW);    // turn the LED off by making the voltage LOW
}

References#

• WiringPi (C++ library)
• WiringPi-Python on github
• Raspberry Pi (master) Arduino (slave) I2C communication with WiringPi
• From Raspi.TV an article on WiringPi2 (really only the first is relevant here):
  • How to use WiringPi2 for Python on the Raspberry Pi in Raspbian part 1
  • How to use WiringPi2 for Python with pull-ups or pull-downs and PWM – pt 2
  • Using the MCP23017 port expander with WiringPi2 to give you 16 new GPIO ports – part 3
• i2c SLAVE on Adafruit's CircuitPython forum
• Raspberry Pi - Arduino Serial Communication