I2C with Arduino Mega as Master & Raspberry Pi as Slave

So I haven't gotten any replies, but to keep folks updated here's the latest in my research... It seems that perhaps the PI's GPIO02 and GPIO03 pins commonly used for I2C for master mode are NOT the ones you use for slave mode(?). I found this forum post that suggests using the pigpio library along with pins GPIO18 (SDA) and GPIO19 (SCL) in I2C mode. Here's the diagram they have posted (using the UNO rather than a Mega, but Pi setup would be similar).

I'd assume I could also use a logic converter as before rather than connecting to the Arduino's 3.3v. I also noticed the pullup resistors. The RPI has internal pull-up resistors (as described in the DroneBot blog) for the standard I2C lines (used for master mode). I'm assuming this is NOT the case for other GPIO pins. I'd need to determine what resistance of pullup resistors to use (looks like their pic uses 47k I believe). I'm doing my homework on pullup/pulldown resistors now.

Once I do I think I'm going to try recreating this code from another forum post.

On the Pi3:

#!/usr/bin/env python

import time
import pigpio

I2C_ADDR=9

def i2c(id, tick):
   global pi

   s, b, d = pi.bsc_i2c(I2C_ADDR)

   if b:

     print(d[:-1])

pi = pigpio.pi()

if not pi.connected:
    exit()

# Respond to BSC slave activity

e = pi.event_callback(pigpio.EVENT_BSC, i2c)

pi.bsc_i2c(I2C_ADDR) # Configure BSC as I2C slave

time.sleep(600)

e.cancel()

pi.bsc_i2c(0) # Disable BSC peripheral

pi.stop()

On the Arduino Mega:

// This example code is in the public domain.


#include 

void setup()
{
   Wire.begin(); // join i2c bus as master
}

char str[17];

int x = 0;

void loop()
{
   sprintf(str, "Message %7d\n", x);
   if (++x > 9999999) x=0;

   Wire.beginTransmission(9); // transmit to device #9
   Wire.write(str);           // sends 16 bytes
   Wire.endTransmission();    // stop transmitting

   delay(50);
}

Am I making this too difficult? Seems like there should be an easier way, but maybe not. Cheers!

Ok, so I got it (partly) working!

I basically hooked everything up as described for the GPIO 18 & GPIO 19 with the logic converter and no pullup resistors. 

I removed all the other slave devices out of the equation too. I made one slight adjustment to the python code to decode the string being sent over. Basically it was literally printing out

bytearray(b'Message 1')
bytearray(b'Message 2')
...

I changed it from

print(d[:-1])

to

print(d[:-1].decode())

which cleared it up. So I proved that I can get messages being sent from the Arduino Mega (master) to the Pi3 (slave) using I2C. Next step will be to integrate it now with my previous code and other I2C slave devices (to do something that I actually want it to do - lol).

Wish me luck!

@racerx 

Well done so far and ... Good luck 🙂