​#include "main.h"
 
/* Private variables ---------------------------------------------------------*/
TIM_HandleTypeDef htim9; // Timer 9 handle
unsigned char Font[18] = {0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D, 0x7C, 0x07, 0x7F, 0x67, 0x77, 0x7C, 0x39, 0x5E, 0x79, 0x71, 0x08, 0x80};
int t_cnt = 0;
int m_cnt = 0;
 
/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
static void MX_TIM9_Init(void);
 
/* Private user code ---------------------------------------------------------*/
void HAL_TIM_PeriodElapsedCallback(TIM_HandleTypeDef *htim)
{
  if (htim == &htim9)
  {
    t_cnt++; // Increment the time counter
 
    // Check if about 5 seconds have passed (500 * 10 ms)
    if (t_cnt >= 25)
    {
      t_cnt = 0; // Reset the time counter
      m_cnt += 5; // Increase the number by 5
      if (m_cnt > 9999) // Reset to 0 after reaching 9999
      {
        m_cnt = 0;
      }
    }
  }
}
 
void dec_segment(unsigned int M)
{
  unsigned char M1000, M100, M10, M1;
 
  M1000 = M / 1000;
  M100 = (M % 1000) / 100;
  M10 = (M % 100) / 10;
  M1 = M % 10;
 
  HAL_GPIO_WritePin(GPIOC, 0xFFFF, RESET);
  HAL_GPIO_WritePin(GPIOC, Font[M1000] | 0x0e00, SET); // Display thousands digit
 
 
  HAL_GPIO_WritePin(GPIOC, 0xFFFF, RESET);
  HAL_GPIO_WritePin(GPIOC, Font[M100] | 0x0d00, SET); // Display hundreds digit
 
 
  HAL_GPIO_WritePin(GPIOC, 0xFFFF, RESET);
  HAL_GPIO_WritePin(GPIOC, Font[M10] | 0x0b00, SET); // Display tens digit
 
 
  HAL_GPIO_WritePin(GPIOC, 0xFFFF, RESET);
  HAL_GPIO_WritePin(GPIOC, Font[M1] | 0x0700, SET); // Display units digit
 
}
 
int main(void)
{
  HAL_Init();
  SystemClock_Config();
  MX_GPIO_Init();
  MX_TIM9_Init();
 
  HAL_TIM_Base_Start_IT(&htim9); // Start timer 9 interrupts
 
  while (1)
  {
    dec_segment(m_cnt); // Update the display
  }
}
 
/* System Clock Configuration */
void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};
 
  __HAL_RCC_PWR_CLK_ENABLE();
  __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE1);
  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSE;
  RCC_OscInitStruct.HSEState = RCC_HSE_ON;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_ON;
  RCC_OscInitStruct.PLL.PLLSource = RCC_PLLSOURCE_HSE;
  RCC_OscInitStruct.PLL.PLLM = 8;
  RCC_OscInitStruct.PLL.PLLN = 168;
  RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV2;
  RCC_OscInitStruct.PLL.PLLQ = 4;
  if (HAL_RCC_OscConfig(&RCC_OscInitStruct) != HAL_OK)
  {
    Error_Handler();
  }
 
  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_5) != HAL_OK)
  {
    Error_Handler();
  }
}
 
/* TIM9 init function */
void MX_TIM9_Init(void)
{
  TIM_MasterConfigTypeDef sMasterConfig = {0};
 
  htim9.Instance = TIM9;
  htim9.Init.Prescaler = 1680 - 1; // 10 ms period with 168 MHz clock
  htim9.Init.CounterMode = TIM_COUNTERMODE_UP;
  htim9.Init.Period = 10000 - 1;   // 10 ms period with 10 kHz clock
  htim9.Init.AutoReloadPreload = TIM_AUTORELOAD_PRELOAD_DISABLE;
  if (HAL_TIM_Base_Init(&htim9) != HAL_OK)
  {
    Error_Handler();
  }
  sMasterConfig.MasterOutputTrigger = TIM_TRGO_RESET;
  sMasterConfig.MasterSlaveMode = TIM_MASTERSLAVEMODE_DISABLE;
  if (HAL_TIMEx_MasterConfigSynchronization(&htim9, &sMasterConfig) != HAL_OK)
  {
    Error_Handler();
  }
}
 
/* GPIO init function */
void MX_GPIO_Init(void)
{
  GPIO_InitTypeDef GPIO_InitStruct = {0};
 
  __HAL_RCC_GPIOC_CLK_ENABLE();
 
  GPIO_InitStruct.Pin = GPIO_PIN_0 | GPIO_PIN_1 | GPIO_PIN_2 | GPIO_PIN_3 |
                        GPIO_PIN_4 | GPIO_PIN_5 | GPIO_PIN_6 | GPIO_PIN_7 |
                        GPIO_PIN_8 | GPIO_PIN_9 | GPIO_PIN_10 | GPIO_PIN_11;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOC, &GPIO_InitStruct);
}
 
void Error_Handler(void)
{
  __disable_irq();
  while (1)
  {
  }
}
타이머9를 10ms 주기 인터럽트가 발생하도록 하고, 약 5초가 되면 7-세그먼트에서 숫자가 5씩 증가하는 코드입니다 
위에 if (t_cnt >= 25)값을 25로 해야만 5초씩 증가합니다
이게 계산법이 5x100 해서 500에서 5로나누어 100을 4초 나누라던데 마지막에 왜 4로 나누는건가요?
항상 질문에 답해주셔서 감사합니다