SPI Triggering GPIO

[EthanMankins]

SEE EDIT

I am using the STM32H7B3I-DK. I am trying to write my own simple SPI ISR to handle transmitting and receiving of just one byte I have gotten the slave working with my ISR (while master used HAL). I am now implementing on the master and having issues with the transmit. When I am in debug mode and transmit one byte then hit a breakpoint it works fine. However, when I just run normal after I start transmitting, my board says it starts receiving button presses. It is acting as if someone is randomly pressing all the buttons connected on the PC2 (input- pull down - active High) line. The weird thing is I don't have this issue if I use the HAL Transmit, only when I use my transmit. This occurs even if the slave is not on and the master is just sending bytes to a non powered device.

My SPI Setup uses SPI2 which PC2 could have the AF SPI2 MISO but I do not have it configured that way. Could the spi be interfering with the GPIO input internally? 

Any other suggestions to try?

SPI ISR:

volatile uint8_t rxBuffer[8];
volatile uint16_t rxIndex = 0;
volatile uint8_t txBuffer[8] = {1,2,3,4,5,6,7,8};
volatile uint16_t txIndex = 0;

[...]

void spiISR(SPI_HandleTypeDef *hspi) {

	//Receive if ready
	if (hspi->Instance->SR & SPI_FLAG_RXP) {
    	rxBuffer[rxIndex] = (*(__IO uint8_t *)&hspi->Instance->RXDR);
    	rxIndex = (rxIndex + 1) % 8;
	}

	if (hspi->Instance->SR & SPI_FLAG_TXP) {
		SET_BIT(hspi->Instance->CR1, SPI_CR1_CSTART);
		HAL_GPIO_WritePin(GPIOI, GPIO_PIN_0, GPIO_PIN_RESET);  			// Pull CS low
		(*(__IO uint8_t *)&hspi->Instance->TXDR) = txBuffer[txIndex];
		txIndex++;

		//if sent whole buffer, end Transmission
		if(txIndex == 8){
			HAL_GPIO_WritePin(GPIOI, GPIO_PIN_0, GPIO_PIN_SET);    // Pull CS high
			txIndex = 0;
			return;
		}
		SET_BIT(hspi->Instance->CR1, SPI_CR1_CSTART);

	}

	__HAL_SPI_CLEAR_OVRFLAG(hspi);
	__HAL_SPI_CLEAR_EOTFLAG(hspi);
	__HAL_SPI_CLEAR_FREFLAG(hspi);
}

EDIT:

I Seemed to fix the issue by changing my ISR slightly. I believe it was never leaving the ISR before but I am not sure how this was causing the Error. Any Explanations? 

void spiISR(SPI_HandleTypeDef *hspi) {

	//Receive if ready
	if (hspi->Instance->SR & SPI_FLAG_RXP) {
    	rxBuffer[rxIndex] = (*(__IO uint8_t *)&hspi->Instance->RXDR);
    	rxIndex = (rxIndex + 1) % 8;
	}

	if (hspi->Instance->SR & SPI_FLAG_TXP) {
		//if sent whole buffer, end Transmission
		if(txIndex == 8){
			__HAL_SPI_DISABLE_IT(&hspi2, SPI_IT_TXP);
			HAL_GPIO_WritePin(GPIOI, GPIO_PIN_0, GPIO_PIN_SET);    // Pull CS high
			return;
		}
		SET_BIT(hspi->Instance->CR1, SPI_CR1_CSTART);
		HAL_GPIO_WritePin(GPIOI, GPIO_PIN_0, GPIO_PIN_RESET);  			// Pull CS low
		(*(__IO uint8_t *)&hspi->Instance->TXDR) = txBuffer[txIndex];
		txIndex++;

		SET_BIT(hspi->Instance->CR1, SPI_CR1_CSTART);

	}

	__HAL_SPI_CLEAR_OVRFLAG(hspi);
	__HAL_SPI_CLEAR_EOTFLAG(hspi);
	__HAL_SPI_CLEAR_FREFLAG(hspi);
}