Frequency counter

1. Introduction

The frequency counter counts the number of cycles of an input signal over a specific time period and calculates the frequency based on the count. It is used to measure the frequency of periodic signals like square waves, sine waves, or pulses. The frequency counter can be implemented using timers in microcontrollers.

2. CubeMX Configuration

• Open CubeMX and generate basic code with:

  • microcontroller: stm32f407vgt6 or board: STM32F407VG-DISC1

  • project name: frequency_counter

  • Toolchain/IDE: Makefile

• Go to Pinout and Congiguration > Timers > TIM1 and select Clock Source: Internal Clock.

• Go to TIM2 and select Clock Source: ETR2.

• Go to TIM3 and select Channel1: PWM Generation CH1.

• Generate Code.

3. Code to Get Frequency

• Navigate to Core > Src and open main.c.

• Include header file.

  SWVUSB

  /* USER CODE BEGIN Includes */
  #include <stdio.h>
  /* USER CODE END Includes */
  

• Overwrite definition of _write as:

  SWVUSB

  /* USER CODE BEGIN 0 */
  int _write(int file, char *data, int len)
  {
    for (int i = 0; i < len; ++i)
    {
      ITM_SendChar(data[i]);
    }
    return len;
  }
  /* USER CODE END 0 */
  

• Create variable to store timer-1 period ellapsed count.

  /* USER CODE BEGIN PV */
  uint32_t htim1PeriodEllapsedCount = 0;
  /* USER CODE END PV */
  

• Increment the htim1PeriodEllapsedCount in HAL_TIM_PeriodElapsedCallback. Also add function to get microtick.

  /* USER CODE BEGIN 0 */
  // ...
  // ...
  
  void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
  {
    if (htim->Instance == htim1.Instance)
    {
      htim1PeriodEllapsedCount++;
    }
  }
  
  uint64_t GetMicros()
  {
    return (uint64_t)htim1PeriodEllapsedCount * htim1.Instance->ARR + (uint64_t)htim1.Instance->CNT;
  }
  /* USER CODE END 0 */
  

• Add timer init code for TIM1, TIM2 and TIM3.

  /* USER CODE BEGIN 2 */
  HAL_TIM_Base_Start_IT(&htim1);
  HAL_TIM_Base_Start(&htim2);
  HAL_TIM_PWM_Start(&htim3, TIM_CHANNEL_1);
  /* USER CODE END 2 */
  

• Add code to get frequency of input signal.

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
  uint64_t startTick;
  uint64_t endTick;
  uint32_t count;
  double freq;
  
  htim3.Instance->CCR1 = (uint32_t)(0.5f * htim3.Instance->ARR);
  
  while (1)
  {
    startTick = GetMicros(); // using microsecond to get precise frequency
    htim2.Instance->CNT = 0;
    HAL_Delay(100); // 100ms sampling time
    count = htim2.Instance->CNT;
    uint64_t endTick = GetMicros();
  
    freq = (double)count / ((endTick - startTick) / 1000000.0);
    printf("Frequency: %f Hz\n", (float)freq);
    /* USER CODE END WHILE */
  
    /* USER CODE BEGIN 3 */
  }
  /* USER CODE END 3 */
  

• Build and flash the code.

Warning

To print floating point, make sure you have added -u _printf_float to LDFLAGS in Makefile. Otherwise it does not get printed.

4. View Output

• Connect TIM3_CH1 pin to TIM2_ETR pin.

• Open STM32CubeProgrammer and view output on SWV. If you used USB, see on terminal or serial monitor.

• Now, change the frequency of PWM signal to 50Hz and again view the output. To know how change PWM frequency, see PWM tutorial.