# STM32F303 code gets stuck in SysTick Exception Handler

I am working with a STM32F303 microcontroller with a Nucleo board and my code gets stuck in the SysTick_Handler routine of the startup_stm32f303xe.s file. (My code is based on an example Nucleo project, but rebuilt from scratch for learning purposes).

This post suggests that I need to set the Systick Interrupt to higher priority, which I did and doesn't fix the problem. This post suggests that the Systick Exception occurs when the system timer reaches 0... but I don't think I am using the system timer in my code.

In debug view with some breakpoints, I can see that the code executes to the two GPIO_TogglePin functions, after which it goes into the exception handler.

How can I prevent the program from causing the Systick Exception?

Here is my code:

int main(void) {

HAL_Init();
SystemClock_Config();
LED2_GPIO_CLK_ENABLE();
__HAL_RCC_GPIOA_CLK_ENABLE();

GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
GPIO_InitStruct.Pull = GPIO_PULLUP;
GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_HIGH;

GPIO_InitStruct.Pin = LED2_PIN;
HAL_GPIO_Init(LED2_GPIO_PORT, &GPIO_InitStruct);

GPIO_InitStruct.Pin = GPIO_PIN_8;
HAL_GPIO_Init(LED2_GPIO_PORT, &GPIO_InitStruct);

HAL_NVIC_SetPriority(SysTick_IRQn, 0, 0);
// Also tried HAL_InitTick(0);

while(1) {

HAL_GPIO_TogglePin(LED2_GPIO_PORT, LED2_PIN);
HAL_GPIO_TogglePin(GPIOA, GPIO_PIN_8);

// Project code
}
}

void SystemClock_Config(void) {
RCC_ClkInitTypeDef RCC_ClkInitStruct;
RCC_OscInitTypeDef RCC_OscInitStruct;

/* HSI Oscillator already ON after system reset, activate PLL with HSI as source */
RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_NONE;
RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI;
RCC_OscInitStruct.PLL.PREDIV = RCC_PREDIV_DIV2;
RCC_OscInitStruct.PLL.PLLMUL = RCC_PLL_MUL16;
RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK) {
Error_Handler();
}

/* Select PLL as system clock source and configure the HCLK, PCLK1 and PCLK2
clocks dividers */
RCC_ClkInitStruct.ClockType = (RCC_CLOCKTYPE_SYSCLK | RCC_CLOCKTYPE_HCLK
| RCC_CLOCKTYPE_PCLK1 | RCC_CLOCKTYPE_PCLK2);
RCC_ClkInitStruct.SYSCLKSource = RCC_SYSCLKSOURCE_PLLCLK;
RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV2;
RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;
if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_2) != HAL_OK) {
Error_Handler();
}

}


The ST HAL requires the systick for operation.
It increments the system tick you can use via HAL_GetTick(), it is incremented in the SysTick_Handler() via HAL_IncTick();.
It will be configured by HAL_SYSTICK_Config somewhere in the HAL.

You don't get stuck there. It just gets called every 1 ms.

If you do get stuck in the systick handler, there could be something wrong with the vector table. Eg: the address for SysTick_Handler at the position for something else.

Check the IABR registers when halted.
4.2.6 Interrupt Active Bit Registers.

• Yep, turns out I was missing the SysTick_Handler() implementation. Adding the Handler function with HAL_IncTick(); solves the "problem". Thanks! – a2xia Mar 20 '18 at 8:45
• HAL doesn't requre SysTick, it requires a a working HAL_GetTick() that can somehow figure out how many milliseconds did elapse since the program was started. HAL_InitTick() must be overridden if SysTick is not used. – followed Monica to Codidact Mar 20 '18 at 10:12