Wire.endTransmission() hangs, but only, if wire.begin(); is called with an address (MKR WiFi 1010)

Question

This code runs without issues, scoped and seen from serial output:

#include <Wire.h>

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Wire.begin();
  Wire.onReceive(receiveEvent);

  while (!Serial);
}

void loop()
{
  Serial.println("1");
  Wire.beginTransmission(8);
  Serial.println("2");
  Wire.write(0x1);
  Serial.println("3");
  delay(100);
  Wire.endTransmission();
  delay(100);
  Serial.println("4");
  delay(500);
}

void receiveEvent(int howmany)
{
  delay(100);
}

This one hangs on Wire.endtransmission():

#include <Wire.h>

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Wire.begin(10);
  Wire.onReceive(receiveEvent);

  while (!Serial);
}

void loop()
{
  Serial.println("1");
  Wire.beginTransmission(8);
  Serial.println("2");
  Wire.write(0x1);
  Serial.println("3");
  delay(100);
  Wire.endTransmission();
  delay(100);
  Serial.println("4");
  delay(500);
}

void receiveEvent(int howmany)
{
  delay(100);
}

The problem starts as soon as I assign an address in Wire.begin(). As I understand, a slave is also allowed to send data and temporarily becoming a master when addressing another device.

Answer 1

I want to push data from the slaves to the master immediately when they receive said data.

So that means, that each slaves becomes master directly after it has received data from the MKR. This is still a multi-master setup, even while you are avoiding collisions through a fixed protocol. The Wire library does not support multi-master.

But I also don't understand, why you would need this. Your protocol does this: The MKR sends data to a slave (triggering it and maybe also sending some commands). Then the slave should send data back to the master. Then the transmission ends. This is normally done in I2C with just one master. The master does a master write (via Wire.endTransmission() and sends some command data to the slave. The slave will prepare the data, that the master requested, in it's buffer. Then the master will start a master read (via Wire.requestFrom()) which reads the data from the slave.

So you don't need a multi-master setup for this. It does exactly, what you described, but with only one master.

Answer 2

While the wire library supports multi-master for AVR, it doesn't for ATSAMD.

Temporary promotion to master is disabled when given a slave address, so the program will hang on wire.endTransmission().