STM32H7 SAI weird behaviour on receiving data

[SeyyedMohammad]

As you can see the CH2(blue) is triggers 8 time asynchronous to transmission CH1(yellow). Why this happen since fifo threshold is empty?

The code:

 while (1)

 {

 for(uint8_t i=0;i<7;i++)

 {

 while(hsai_BlockA2.State != HAL_SAI_STATE_READY);

 HAL_SAI_Transmit_IT(&hsai_BlockA2, (uint8_t*)&noisebuf[0], 1);

 }

 for(uint8_t i=0;i<20;i++)

 {

 while(hsai_BlockB2.State != HAL_SAI_STATE_READY);

 HAL_SAI_Receive_IT(&hsai_BlockB2, (uint8_t*)&adcbuf[0], 1);

 }

 HAL_Delay(5);

 }

static void MX_SAI2_Init(void)

{

 hsai_BlockA2.Instance = SAI2_Block_A;

 hsai_BlockA2.Init.AudioMode = SAI_MODEMASTER_TX;

 hsai_BlockA2.Init.Synchro = SAI_ASYNCHRONOUS;

 hsai_BlockA2.Init.OutputDrive = SAI_OUTPUTDRIVE_DISABLE;

 hsai_BlockA2.Init.NoDivider = SAI_MASTERDIVIDER_ENABLE;

 hsai_BlockA2.Init.FIFOThreshold = SAI_FIFOTHRESHOLD_EMPTY;

 hsai_BlockA2.Init.AudioFrequency = SAI_AUDIO_FREQUENCY_8K;

 hsai_BlockA2.Init.SynchroExt = SAI_SYNCEXT_DISABLE;

 hsai_BlockA2.Init.MonoStereoMode = SAI_MONOMODE;

 hsai_BlockA2.Init.CompandingMode = SAI_NOCOMPANDING;

 hsai_BlockA2.Init.TriState = SAI_OUTPUT_RELEASED;

 if (HAL_SAI_InitProtocol(&hsai_BlockA2, SAI_I2S_STANDARD, SAI_PROTOCOL_DATASIZE_16BITEXTENDED, 2) != HAL_OK)

 {

  Error_Handler();

 }

 hsai_BlockB2.Instance = SAI2_Block_B;

 hsai_BlockB2.Init.AudioMode = SAI_MODESLAVE_RX;

 hsai_BlockB2.Init.Synchro = SAI_SYNCHRONOUS;

 hsai_BlockB2.Init.OutputDrive = SAI_OUTPUTDRIVE_DISABLE;

 hsai_BlockB2.Init.FIFOThreshold = SAI_FIFOTHRESHOLD_EMPTY;

 hsai_BlockB2.Init.SynchroExt = SAI_SYNCEXT_DISABLE;

 hsai_BlockB2.Init.MonoStereoMode = SAI_MONOMODE;

 hsai_BlockB2.Init.CompandingMode = SAI_NOCOMPANDING;

 hsai_BlockB2.Init.TriState = SAI_OUTPUT_RELEASED;

 if (HAL_SAI_InitProtocol(&hsai_BlockB2, SAI_I2S_STANDARD, SAI_PROTOCOL_DATASIZE_16BITEXTENDED, 2) != HAL_OK)

 {

  Error_Handler();

 }

}

void HAL_SAI_RxCpltCallback(SAI_HandleTypeDef *hsai)

{

HAL_GPIO_TogglePin(test1_GPIO_Port, test1_Pin);

}

void HAL_SAI_TxCpltCallback(SAI_HandleTypeDef *hsai)

{

HAL_GPIO_TogglePin(test_GPIO_Port, test_Pin);

}

By changing receiver FIFO threshold to Full the graph is now symmetrical, But what happened, could someone explain?

And by inserting

SET_BIT((&hsai_BlockB2)->Instance->CR2, SAI_xCR2_FFLUSH);

before Rx for loop the shapes also improves!