/* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file           : main.c
  * @brief          : Main program body
  ******************************************************************************
  */
/* USER CODE END Header */
#include "main.h"
#include <string.h>

/* Private variables */
volatile uint16_t virtualSecDelay = 1000;
volatile uint32_t fastTicks = 0;
volatile uint16_t virtSecCnt = 0;
volatile uint8_t hh = 10, mm = 43, ss = 0;
volatile uint8_t digitToShow = 0;

static uint32_t lastBtn1Press = 0;
static uint32_t lastBtn2Press = 0;

/* Private function prototypes */
void SystemClock_Config(void);
static void MX_GPIO_Init(void);
static void shiftOut_8bit_noLatch(uint8_t data);
static void latchData(void);
static void beep(uint16_t ms);

/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
static const uint8_t seg7_CA[10] = {
  ~0x3F, ~0x06, ~0x5B, ~0x4F, ~0x66,
  ~0x6D, ~0x7D, ~0x07, ~0x7F, ~0x6F
};

static void shiftOut_8bit_noLatch(uint8_t data)
{
  for(int i = 0; i < 8; i++){
    HAL_GPIO_WritePin(DATA_GPIO_Port,  DATA_Pin,  (data & 0x80) ? GPIO_PIN_SET : GPIO_PIN_RESET);
    HAL_GPIO_WritePin(CLOCK_GPIO_Port, CLOCK_Pin, GPIO_PIN_SET);
    HAL_GPIO_WritePin(CLOCK_GPIO_Port, CLOCK_Pin, GPIO_PIN_RESET);
    data <<= 1;
  }
}

static void latchData(void)
{
  HAL_GPIO_WritePin(LATCH_GPIO_Port, LATCH_Pin, GPIO_PIN_SET);
  HAL_GPIO_WritePin(LATCH_GPIO_Port, LATCH_Pin, GPIO_PIN_RESET);
}

static void beep(uint16_t ms)
{
  HAL_GPIO_WritePin(BUZZER_GPIO_Port, BUZZER_Pin, GPIO_PIN_RESET);
  for (volatile uint32_t i = 0; i < ms * 1200; i++);
  HAL_GPIO_WritePin(BUZZER_GPIO_Port, BUZZER_Pin, GPIO_PIN_SET);
}

/**
  * @brief  The application entry point.
  */
int main(void)
{
  HAL_Init();
  SystemClock_Config();
  MX_GPIO_Init();

  while (1)
  {
    HAL_GPIO_WritePin(BUZZER_GPIO_Port, BUZZER_Pin, GPIO_PIN_SET);

    if (fastTicks >= virtualSecDelay) {
      fastTicks = 0;
      virtSecCnt++;
      ss++;
      if (ss >= 60) { ss = 0; mm++; }
      if (mm >= 60) { mm = 0; hh++; }
      if (hh >= 24)  hh = 0;

      if (virtSecCnt >= 3600) {
        virtSecCnt = 0;
        beep(100);
      }
    }

    uint32_t now = HAL_GetTick();

    if (HAL_GPIO_ReadPin(BUTTON1_GPIO_Port, BUTTON1_Pin) == GPIO_PIN_RESET) {
      if (now - lastBtn1Press > 200) {
        lastBtn1Press = now;
        if (virtualSecDelay > 120)
          virtualSecDelay -= 100;
        else
          virtualSecDelay = 20;
      }
    }

    if (HAL_GPIO_ReadPin(BUTTON2_GPIO_Port, BUTTON2_Pin) == GPIO_PIN_RESET) {
      if (now - lastBtn2Press > 200) {
        lastBtn2Press = now;
        if (virtualSecDelay < 1900)
          virtualSecDelay += 100;
        else
          virtualSecDelay = 2000;
      }
    }
  }
}

/**
  * @brief System Clock Configuration
  */
void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

  RCC_OscInitStruct.OscillatorType = RCC_OSCILLATORTYPE_HSI;
  RCC_OscInitStruct.HSIState = RCC_HSI_ON;
  RCC_OscInitStruct.HSICalibrationValue = RCC_HSICALIBRATION_DEFAULT;
  RCC_OscInitStruct.PLL.PLLState = RCC_PLL_NONE;
  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_HSI;
  RCC_ClkInitStruct.AHBCLKDivider = RCC_SYSCLK_DIV1;
  RCC_ClkInitStruct.APB1CLKDivider = RCC_HCLK_DIV1;
  RCC_ClkInitStruct.APB2CLKDivider = RCC_HCLK_DIV1;

  if (HAL_RCC_ClockConfig(&RCC_ClkInitStruct, FLASH_LATENCY_0) != HAL_OK)
  {
    Error_Handler();
  }
}

/**
  * @brief GPIO Initialization Function
  */
static void MX_GPIO_Init(void)
{
  GPIO_InitTypeDef GPIO_InitStruct = {0};

  __HAL_RCC_GPIOA_CLK_ENABLE();
  __HAL_RCC_GPIOB_CLK_ENABLE();

  HAL_GPIO_WritePin(GPIOA, CLOCK_Pin|DATA_Pin, GPIO_PIN_RESET);
  HAL_GPIO_WritePin(BUZZER_GPIO_Port, BUZZER_Pin, GPIO_PIN_SET);
  HAL_GPIO_WritePin(LATCH_GPIO_Port, LATCH_Pin, GPIO_PIN_RESET);

  GPIO_InitStruct.Pin = BUTTON1_Pin|BUTTON2_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_INPUT;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = BUTTON3_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_IT_FALLING;
  GPIO_InitStruct.Pull = GPIO_PULLUP;
  HAL_GPIO_Init(BUTTON3_GPIO_Port, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = CLOCK_Pin|DATA_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOA, &GPIO_InitStruct);

  GPIO_InitStruct.Pin = BUZZER_Pin|LATCH_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(GPIOB, &GPIO_InitStruct);
}

/* USER CODE BEGIN 4 */
void SysTick_Handler(void)
{
  HAL_IncTick();
  fastTicks++;

  uint8_t d[4] = { hh/10, hh%10, mm/10, mm%10 };
  uint8_t code = seg7_CA[ d[digitToShow] ];

  if (digitToShow == 1) code &= ~0x80;

  uint8_t digitSelect = (1 << digitToShow);

  shiftOut_8bit_noLatch(code);
  shiftOut_8bit_noLatch(digitSelect);
  latchData();

  digitToShow = (digitToShow + 1) % 4;
}
/* USER CODE END 4 */

void Error_Handler(void)
{
  __disable_irq();
  while (1)
  {
  }
}

#ifdef USE_FULL_ASSERT
void assert_failed(uint8_t *file, uint32_t line)
{
  while (1)
  {
  }
}
#endif
```