用stm32写一段代码。任务内容为利用按键KEYO改变指示灯LEDO的闪烁频率，闪烁频率设置为3档:初始状态 时,LEDO按照1 Hz的频率闪烁;第一.次 按键后，LEDO按照5 Hz的频率闪烁; 第二次按键后，LEDO按照20 Hz的频率闪烁，并重复上述过程

以下是使用STM32 HAL库编写的代码，实现了您的需求：

#include "main.h"

uint8_t led_state = 0; // 0: 1Hz, 1: 5Hz, 2: 20Hz

void SystemClock_Config(void);
static void MX_GPIO_Init(void);

int main(void)
{
  HAL_Init();
  SystemClock_Config();
  MX_GPIO_Init();

  while (1)
  {
    HAL_GPIO_TogglePin(LED0_GPIO_Port, LED0_Pin);
    switch (led_state)
    {
      case 0: HAL_Delay(500); break;
      case 1: HAL_Delay(100); break;
      case 2: HAL_Delay(25); break;
    }
  }
}

void HAL_GPIO_EXTI_Callback(uint16_t GPIO_Pin)
{
  if (GPIO_Pin == KEY0_Pin)
  {
    led_state++;
    if (led_state > 2)
    {
      led_state = 0;
    }
  }
}

void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {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_NONE;
  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_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();
  }
}

static void MX_GPIO_Init(void)
{
  GPIO_InitTypeDef GPIO_InitStruct = {0};

  /* GPIO Ports Clock Enable */
  __HAL_RCC_GPIOC_CLK_ENABLE();
  __HAL_RCC_GPIOA_CLK_ENABLE();

  /*Configure GPIO pin Output Level */
  HAL_GPIO_WritePin(LED0_GPIO_Port, LED0_Pin, GPIO_PIN_RESET);

  /*Configure GPIO pin : KEY0_Pin */
  GPIO_InitStruct.Pin = KEY0_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_IT_FALLING;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  HAL_GPIO_Init(KEY0_GPIO_Port, &GPIO_InitStruct);

  /*Configure GPIO pin : LED0_Pin */
  GPIO_InitStruct.Pin = LED0_Pin;
  GPIO_InitStruct.Mode = GPIO_MODE_OUTPUT_PP;
  GPIO_InitStruct.Pull = GPIO_NOPULL;
  GPIO_InitStruct.Speed = GPIO_SPEED_FREQ_LOW;
  HAL_GPIO_Init(LED0_GPIO_Port, &GPIO_InitStruct);

  HAL_NVIC_SetPriority(EXTI0_IRQn, 0, 0);
  HAL_NVIC_EnableIRQ(EXTI0_IRQn);
}

这段代码使用了STM32的外部中断和GPIO模块，实现了按键的响应和指示灯的闪烁。按键的中断回调函数中，通过改变led_state的值实现了不同频率的闪烁。指示灯的闪烁则通过TogglePin和Delay函数实现。