Interrupt SPI Slave Transmit Delayed

[EthanMankins]

I am using a Nucleo-H743ZI as a SPI slave device for the following coms method:

Master Pulls SS low, Delay 1ms.
Master Transmit/Receive 1 Byte (Wait for high busy line)
Slave RXP Interrupt-> Lower Busy Line
Slave Transmit/Receive 1 Byte
Slave Process Byte, Set next transmit byte
Slave Raise Busy Line
Master Transmit/Receive 1 Byte
Repeat untilldata is done
Master Raises SS

The Master is acting properly and the slave receive is correct; however, the slave transmit is two bytes behind. For now I have hard coded the transmit to send 5. All the data is received correctly by the slave in this time but the slave does not transmit 5 until the third byte is received.


SLAVE SETUP:

hspi1.Instance = SPI1;
  hspi1.Init.Mode = SPI_MODE_SLAVE;
  hspi1.Init.Direction = SPI_DIRECTION_2LINES;
  hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
  hspi1.Init.CLKPolarity = SPI_POLARITY_LOW;
  hspi1.Init.CLKPhase = SPI_PHASE_2EDGE;
  hspi1.Init.NSS = SPI_NSS_SOFT;
  hspi1.Init.FirstBit = SPI_FIRSTBIT_MSB;
  hspi1.Init.TIMode = SPI_TIMODE_DISABLE;
  hspi1.Init.CRCCalculation = SPI_CRCCALCULATION_DISABLE;
  hspi1.Init.CRCPolynomial = 0x0;
  hspi1.Init.NSSPMode = SPI_NSS_PULSE_DISABLE;
  hspi1.Init.NSSPolarity = SPI_NSS_POLARITY_LOW;
  hspi1.Init.FifoThreshold = SPI_FIFO_THRESHOLD_01DATA;
  hspi1.Init.TxCRCInitializationPattern = SPI_CRC_INITIALIZATION_ALL_ZERO_PATTERN;
  hspi1.Init.RxCRCInitializationPattern = SPI_CRC_INITIALIZATION_ALL_ZERO_PATTERN;
  hspi1.Init.MasterSSIdleness = SPI_MASTER_SS_IDLENESS_00CYCLE;
  hspi1.Init.MasterInterDataIdleness = SPI_MASTER_INTERDATA_IDLENESS_00CYCLE;
  hspi1.Init.MasterReceiverAutoSusp = SPI_MASTER_RX_AUTOSUSP_DISABLE;
  hspi1.Init.MasterKeepIOState = SPI_MASTER_KEEP_IO_STATE_ENABLE;
  hspi1.Init.IOSwap = SPI_IO_SWAP_DISABLE;
  if (HAL_SPI_Init(&hspi1) != HAL_OK)
  {
    Error_Handler();
  }

//MY INIT:
	HAL_GPIO_WritePin(nFPINT_GPIO_Port, nFPINT_Pin, GPIO_PIN_SET); //raise /busy line
	__HAL_SPI_DISABLE(&hspi1);
	hspi1.Instance->UDRDR = 0xA5;
	__HAL_SPI_CLEAR_OVRFLAG(&hspi1);
	__HAL_SPI_CLEAR_EOTFLAG(&hspi1);
	__HAL_SPI_CLEAR_FREFLAG(&hspi1);

	__HAL_SPI_DISABLE_IT(&hspi1, SPI_IT_TXP);
	__HAL_SPI_ENABLE_IT(&hspi1, SPI_IT_RXP);

	HAL_NVIC_EnableIRQ(SPI1_IRQn);

// /CS handler
void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin) {
	if (GPIO_Pin == GPIO_PIN_14) {
		if(HAL_GPIO_ReadPin(nCS_GPIO_Port, nCS_Pin) == GPIO_PIN_RESET){
			__HAL_SPI_DISABLE(&hspi1);
        	__HAL_SPI_ENABLE(&hspi1);
			txBuffer = 0;		//transmit 0 for first byte
		}
	}
	__HAL_GPIO_EXTI_CLEAR_IT(GPIO_Pin);
}



void spiISRSTM(SPI_HandleTypeDef *hspi) {
	HAL_GPIO_WritePin(nFPINT_GPIO_Port, nFPINT_Pin, GPIO_PIN_RESET);	//Lower /Busy

	if (hspi->Instance->SR & SPI_FLAG_TXP) {
		(*(__IO uint8_t *)&hspi->Instance->TXDR) = 5;			// transmit
	}

	//Receive if ready
	if (hspi->Instance->SR & SPI_FLAG_RXP) {
    	rxBuffer = (*(__IO uint8_t *)&hspi->Instance->RXDR);
    	ProcessSPI();										//Process SPI data
	}
	__HAL_SPI_CLEAR_UDRFLAG(hspi);
	__HAL_SPI_CLEAR_OVRFLAG(hspi);
	__HAL_SPI_CLEAR_EOTFLAG(hspi);
	__HAL_SPI_CLEAR_FREFLAG(hspi);

}