HAL_GetTick has accuracy problems

Question

I've tried doing my own millisecond timer on stm32f103r6t, I've used timer 2 with interrupt on period elapsed, then I increase the counter by one step. The clock frequency is 64mhz, (APB1&2 are 64Mhz as well), prescaller is at 127 and the period value is set to 500. I tested by toggling a pin on interrupt and I got a 1ms half-period on the oscilloscope (which is expected). The other test that I did was to compare it with __Hal_get_ticks() and send it to uart. It seems that __Hal_get_ticks() is faster, and their difference keeps increasing with time. I've posted the code bellow, although I do initialize more peripherals, I haven't used them yet.

long milliseconds=0; void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef* htim) { HAL_GPIO_TogglePin(GPIOC, GPIO_PIN_9); milliseconds++; }

int main(void)
{
  /* MCU Configuration--------------------------------------------------------*/

  /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
  HAL_Init();
  /* Configure the system clock */
  SystemClock_Config();
  /* Initialize all configured peripherals */
  MX_GPIO_Init();
  MX_ADC1_Init();
  MX_CAN_Init();
  MX_SPI1_Init();
  MX_TIM2_Init();
  MX_TIM3_Init();
  stm32_uart2_set_state(1);
  HAL_TIM_Base_Start_IT(&htim2);
//  MX_WWDG_Init();
  /* USER CODE BEGIN 2 */
  char string[100]={0};
  int index=0; 
  /* Infinite loop */
  while (1)
  {
      uint32_t hall_tick=HAL_GetTick();
      sprintf(string,"It:%lu\tHt:%lu\n\r",milliseconds,hall_tick);
      stm32_uart2_send_string(string, strlen(string));
  }
}

/**
  * @brief System Clock Configuration
  * @retval None
  */
void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};
  RCC_PeriphCLKInitTypeDef PeriphClkInit = {0};

  /** Initializes the RCC Oscillators according to the specified parameters
  * in the RCC_OscInitTypeDef structure.
  */
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
  RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSI_DIV2;
  RCC_OscInitStruct.PLL.PLLMUL = RCC_PLL_MUL16;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    Error_Handler();
  }

  /** Initializes the CPU, AHB and APB buses clocks
  */
  RCC_ClkInitStruct.ClockType = RCC_CLOCKTYPE_HCLK|RCC_CLOCKTYPE_SYSCLK
                              |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();
  }
  PeriphClkInit.PeriphClockSelection = RCC_PERIPHCLK_ADC;
  PeriphClkInit.AdcClockSelection = RCC_ADCPCLK2_DIV8;
  if (HAL_RCCEx_PeriphCLKConfig(&PeriphClkInit) != HAL_OK)
  {
    Error_Handler();
  }
}

static void MX_TIM2_Init(void)
{
  TIM_ClockConfigTypeDef sClockSourceConfig = {0};
  TIM_MasterConfigTypeDef sMasterConfig = {0};
  htim2.Instance = TIM2;
  htim2.Init.Prescaler = 127; //prescaller is zero-based (0 means clk/1)
  htim2.Init.CounterMode = TIM_COUNTERMODE_UP;
  htim2.Init.Period = 500;
  htim2.Init.ClockDivision = TIM_CLOCKDIVISION_DIV1;
  htim2.Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_ENABLE;
  if (HAL_TIM_Base_Init(&htim2) != HAL_OK)
  {
    Error_Handler();
  }
  sClockSourceConfig.ClockSource = TIM_CLOCKSOURCE_INTERNAL;
  if (HAL_TIM_ConfigClockSource(&htim2, &sClockSourceConfig) != HAL_OK)
  {
    Error_Handler();
  }
  sMasterConfig.MasterOutputTrigger = TIM_TRGO_RESET;
  sMasterConfig.MasterSlaveMode = TIM_MASTERSLAVEMODE_DISABLE;
  if (HAL_TIMEx_MasterConfigSynchronization(&htim2, &sMasterConfig) != HAL_OK)
  {
    Error_Handler();
  }
}

EDIT: This is the default init function for the hal counter, interrupt priority by default is 15, I've tried setting it to 0 but the results are the same. I've measured the perio of HAL_get_tick() and its 998us instead of 1ms

    _weak HAL_StatusTypeDef HAL_InitTick(uint32_t TickPriority)
{
  /* Configure the SysTick to have interrupt in 1ms time basis*/
  if (HAL_SYSTICK_Config(SystemCoreClock / (1000U / uwTickFreq)) > 0U)
  {
    return HAL_ERROR;
  }

  /* Configure the SysTick IRQ priority */
  if (TickPriority < (1UL << __NVIC_PRIO_BITS))
  {
    HAL_NVIC_SetPriority(SysTick_IRQn, TickPriority, 0U);
    uwTickPrio = TickPriority;
  }
  else
  {
    return HAL_ERROR;
  }

  /* Return function status */
  return HAL_OK;

}

Answer 1

I think you probably want to set your TIM2 period to 499. It is zero-based just like the pre-scaler. From the manual:

In upcounting mode, the counter counts from 0 to the auto-reload value(content of the TIMx_ARR register), then restarts from 0 and generates a counter overflow event.

You want to count from 0 to 499, then reset to zero.

This would explain why you are out by 2us per ms.