How do I read a byte from a Slave I2C Device while handling a request from the Master?

Question

I have a Raspberry Pi Zero W acting as a Master in communication with an Arduino Pro Mini acting as a Slave. I would like for the Master to send commands to the Slave. However, when I try using commands such as bus.write_byte_data or bus.write_byte from the Master, the Slave only ever seems to receive the value 255. Here's the code:

Master (in Python):

import time
import smbus

i2c_ch = 1
bus = smbus.SMBus(i2c_ch)

i2c_address = 20
bus.write_byte_data(i2c_address, 113,111)
val = bus.read_i2c_block_data(i2c_address,12)
bus.write_byte(i2c_address, 123)
print(val)

And here's the Slave's requestEvent() (in Arduino C):

void requestEvent()
{
  byte command = Wire.read();
  Serial.println(command);
  command = Wire.read();
  Serial.println(command);
  command = Wire.read();
  Serial.println(command);
...
}

What would be an approach whereby the Slave could receive bytes when being commanded by the Master?

Answer 1

You're probably trying to use a handler created by Wire.onRequest instead of one created by Wire.onReceive. An onReceive handler will do what you want:

Wire.onReceive(receieveEvent);
Wire.onRequest(requestEvent);
...
void receieveEvent()
{
  Serial.println("received some data");
  while(0 < Wire.available()) // loop through all but the last
  {
    byte command = Wire.read();
    Serial.println(command);
  }
} 

PS: LOL you have the same name as me!