Problems with I2C communication

Question

Hi I am attempting to test I2C comms between an pi model 4B and teensy 4.1 (running code uploaded from the arduino IDE via usb). While I can read from the slave with a python script if the arduino code has a Wire.write call in setup(), the event handler for onReceive doesn't trigger which makes me suspect that either data is not being written from the pi side correctly or I am not formatting the arduino side correctly.

I am new to I2C and working with arduino/c so am not sure where I am going wrong but hope if I can sort this out it will help me build from this to the functionality I need. Any help appreciated.

Python script on master pi:

from smbus2 import SMBus, i2c_msg


DEVICE_BUS = 1
DEVICE_ADDR_TEST = 0x8

bus = SMBus(DEVICE_BUS)

write = i2c_msg.write(DEVICE_ADDR_TEST, [40,50])
read = i2c_msg.read(DEVICE_ADDR_TEST, 80)

bus.i2c_rdwr(write, read)

print(read)

code on Arduino IDE:

#include <Wire.h>
  
 
void setup() {
  Wire.begin(0x8);               
  Wire.onReceive(receiveEvent); #this doesn't work
  Wire.write("foo"); #this does
}

void receiveEvent(int howMany) {
  while (Wire.available()) {
    Wire.write("foo");
    char c = Wire.read();
    printf("received: %c", c);
  }
}

void loop() {
  delay(100);
}

Answer 1

Your Question involves 4 components

1. Pi Configuration (unspecified)
2. PythonCode
3. Connection between Pi & other device (unspecified)
4. Arduino code

Do not assume anyone on this site knows what a "teensy 4.1" is (although I doubt this is relevant).

Most of us would attempt to debug this systematically.
I would start with i2cdetect or other i2c_tools.

Frankly using I²C between devices is a poor choice (although I have done this).
For a simple message exchange serial would be more suitable (and easier to debug).

For what is is worth the Pi code looks OK (as it appears to have been copied verbatim from the smbus2 website) but is totally unrelated to the Arduino software.

Answer 2

Ok so from reading around more I realised that I misunderstood how onReceive related to I2C structure.

onReceive is purely for handling being written data, in this case from the master and will not allow the slave to write back in this scope even if the master has a subsequent read request.

What I was missing was an onRequest event handler:

#include <Wire.h>
  
 
void setup() {
  Wire.begin(0x8);
  Serial.begin(9600);                
  Wire.onReceive(receiveEvent);
  Wire.onRequest(requestEvent);
}

void receiveEvent(int howMany) {
  while (Wire.available()) {
    char c = Wire.read();
    Serial.print(c);
  }
}

void requestEvent() {
  Wire.write("foo");
}

void loop() {
  delay(100);
}

Which is called in the event of a master requesting data from the slave, with this addition the code is now working as intended and I should be able to build from here to get the functionality I want.