HAL I2C always sends 0xFF as first byte when responding to master using HAL_I2C_Slave_Transmit

[BAdam.1]

I have an STM32G4 using the HAL function HAL_I2C_Slave_Transmit to respond with some data when requested by a master on the I2C bus. In the image below you can see the device sends 0xFF followed by the actual data it was meant to send (the data I am expecting is 0x11 0x01 0x00).

If I instead use HAL_I2C_Slave_Transmit_IT then I get a worse result, as shown below:

I have tried this with the minimal test program below so I don't believe it's anything to do with my full program having a delay:

  /* USER CODE BEGIN 2 */
  typedef struct {
      char id;
      char payload[2];
  } I2C_RSP_TO_MASTER;
  I2C_RSP_TO_MASTER  I2C_response;
 
  /* USER CODE END 2 */
 
  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
  while (1)
  {
    I2C_response.id = 0x11;
    I2C_response.payload[0] = 0x01;
    I2C_response.payload[1] = 0x00;
 
    HAL_I2C_Slave_Transmit(&hi2c2, (uint8_t*) &I2C_response, 3, 10);
 
    while (HAL_I2C_GetState(&hi2c2) != HAL_I2C_STATE_READY) {
      }
    /* USER CODE END WHILE */
 
    /* USER CODE BEGIN 3 */
  }
  /* USER CODE END 3 */

For my current testing the master is an Arduino Nano using the Arduino function:

Wire.requestFrom(ADDR, 4);

I am requesting 4 bytes because the initial byte is garbage (0xFF). The STM32 is clocked at 128MHz so I don't believe it is possible that it is running too slowly to respond to the first byte - I think it must be a problem with the HAL! Any advice?

[TDK]

What is the definition of I2C_response? Is there another variable before payload? Probably pointing to payload directly is better.

You should probably be using a larger timeout as well. 10ms is not much time. If you want it to be blocking, use HAL_MAX_DELAY

HAL_I2C_Slave_Transmit(&hi2c2, (uint8_t*) &I2C_response.payload, 3, HAL_MAX_DELAY);

[BAdam.1]

I've updated my post with code that includes the rest of the i2c_response definition. It is just those three bytes. Using HAL_MAX_DELAY does not seem to change the 0xFF behaviour.

[TDK]

> I've updated my post 

Well, you also changed the code. In the previous version, payload was 3 bytes.

[BAdam.1]

Yes, I tried different things. Previously the 'id' field was commented out and I tried putting all the data in the 'payload' fields by adding a byte there - but it made no difference. Running the code above has the same result as before.

[Piranha]

The code is nonsense. I don't see any code where slave would be responding to master. I see only code where slave is non-stop trying to "transmit" on it's own. Do you even understand why slave is called slave? That's because it cannot initiate a transfer!

[TDK]

> I see only code where slave is non-stop trying to "transmit" on it's own. 

The code waits for the master to initiate and then sends a response while the master continues to clock SCL. That's how it works.