写一段C语言代码：STM32通过PWM实现控制LED亮度，并且通过USART实现上位机通讯

以下是一段C语言代码，用于在STM32上通过PWM实现LED亮度控制，并通过USART实现与上位机的通信：

#include "stm32f4xx.h"
#include "stm32f4xx_gpio.h"
#include "stm32f4xx_rcc.h"
#include "stm32f4xx_tim.h"
#include "stm32f4xx_usart.h"

// 定义LED连接的GPIO引脚
#define LED_PIN GPIO_Pin_13
#define LED_GPIO_PORT GPIOD
#define LED_GPIO_CLK RCC_AHB1Periph_GPIOD

// 定义USART连接的GPIO引脚和串口
#define USART_PIN_TX GPIO_Pin_9
#define USART_PIN_RX GPIO_Pin_10
#define USART_GPIO_PORT GPIOA
#define USART_GPIO_CLK RCC_AHB1Periph_GPIOA
#define USART_PORT USART1
#define USART_CLK RCC_APB2Periph_USART1

// 初始化PWM
void PWM_Configuration(void) {
  TIM_TimeBaseInitTypeDef TIM_TimeBaseStructure;
  TIM_OCInitTypeDef TIM_OCInitStructure;
  GPIO_InitTypeDef GPIO_InitStructure;

  // 启用TIM2和GPIOA的时钟
  RCC_APB1PeriphClockCmd(RCC_APB1Periph_TIM2, ENABLE);
  RCC_AHB1PeriphClockCmd(RCC_AHB1Periph_GPIOA, ENABLE);

  // 配置GPIO引脚为复用功能
  GPIO_InitStructure.GPIO_Pin = GPIO_Pin_0;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_100MHz;
  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
  GPIO_Init(GPIOA, &GPIO_InitStructure);

  // 将GPIO引脚与TIM2通道1复用连接起来
  GPIO_PinAFConfig(GPIOA, GPIO_PinSource0, GPIO_AF_TIM2);

  // 配置TIM2
  TIM_TimeBaseStructure.TIM_Prescaler = 0;
  TIM_TimeBaseStructure.TIM_CounterMode = TIM_CounterMode_Up;
  TIM_TimeBaseStructure.TIM_Period = 999;  // PWM周期为1000个时钟周期
  TIM_TimeBaseStructure.TIM_ClockDivision = TIM_CKD_DIV1;
  TIM_TimeBaseStructure.TIM_RepetitionCounter = 0;
  TIM_TimeBaseInit(TIM2, &TIM_TimeBaseStructure);

  // 配置TIM2通道1为PWM模式
  TIM_OCInitStructure.TIM_OCMode = TIM_OCMode_PWM1;
  TIM_OCInitStructure.TIM_OutputState = TIM_OutputState_Enable;
  TIM_OCInitStructure.TIM_Pulse = 500;  // 初始占空比为50%
  TIM_OCInitStructure.TIM_OCPolarity = TIM_OCPolarity_High;
  TIM_OC1Init(TIM2, &TIM_OCInitStructure);

  // 启用TIM2的PWM输出
  TIM_Cmd(TIM2, ENABLE);
}

// 初始化USART
void USART_Configuration(void) {
  USART_InitTypeDef USART_InitStructure;
  GPIO_InitTypeDef GPIO_InitStructure;

  // 启用USART和GPIO的时钟
  RCC_APB2PeriphClockCmd(USART_CLK, ENABLE);
  RCC_AHB1PeriphClockCmd(USART_GPIO_CLK, ENABLE);

  // 配置GPIO引脚为复用功能
  GPIO_InitStructure.GPIO_Pin = USART_PIN_TX | USART_PIN_RX;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_AF;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_UP;
  GPIO_Init(USART_GPIO_PORT, &GPIO_InitStructure);

  // 将GPIO引脚与USART复用连接起来
  GPIO_PinAFConfig(USART_GPIO_PORT, GPIO_PinSource9, GPIO_AF_USART1);
  GPIO_PinAFConfig(USART_GPIO_PORT, GPIO_PinSource10, GPIO_AF_USART1);

  // 配置USART
  USART_InitStructure.USART_BaudRate = 9600;
  USART_InitStructure.USART_WordLength = USART_WordLength_8b;
  USART_InitStructure.USART_StopBits = USART_StopBits_1;
  USART_InitStructure.USART_Parity = USART_Parity_No;
  USART_InitStructure.USART_HardwareFlowControl = USART_HardwareFlowControl_None;
  USART_InitStructure.USART_Mode = USART_Mode_Rx | USART_Mode_Tx;
  USART_Init(USART_PORT, &USART_InitStructure);

  // 启用USART
  USART_Cmd(USART_PORT, ENABLE);
}

// 发送字符到上位机
void USART_SendChar(char ch) {
  while (USART_GetFlagStatus(USART_PORT, USART_FLAG_TXE) == RESET)
    ;
  USART_SendData(USART_PORT, (uint16_t)ch);
}

int main(void) {
  // 初始化LED引脚和PWM
  GPIO_InitTypeDef GPIO_InitStructure;
  RCC_AHB1PeriphClockCmd(LED_GPIO_CLK, ENABLE);
  GPIO_InitStructure.GPIO_Pin = LED_PIN;
  GPIO_InitStructure.GPIO_Mode = GPIO_Mode_OUT;
  GPIO_InitStructure.GPIO_OType = GPIO_OType_PP;
  GPIO_InitStructure.GPIO_Speed = GPIO_Speed_50MHz;
  GPIO_InitStructure.GPIO_PuPd = GPIO_PuPd_NOPULL;
  GPIO_Init(LED_GPIO_PORT, &GPIO_InitStructure);
  PWM_Configuration();

  // 初始化USART
  USART_Configuration();

  while (1) {
    // 接收上位机发送的命令
    if (USART_GetFlagStatus(USART_PORT, USART_FLAG_RXNE) == SET) {
      char command = USART_ReceiveData(USART_PORT);

      // 根据命令调整LED亮度
      if (command == '0') {
        TIM_SetCompare1(TIM2, 0);  // 关闭LED
      } else if (command == '1') {
        TIM_SetCompare1(TIM2, 500);  // 设置LED亮度为50%
      } else if (command == '2') {
        TIM_SetCompare1(TIM2, 750);  // 设置LED亮度为75%
      } else if (command == '3') {
        TIM_SetCompare1(TIM2, 999);  // 设置LED亮度为100%
      }

      // 将接收到的命令发送回上位机
      USART_SendChar(command);
    }
  }
}

这段代码通过PWM控制LED的亮度，根据接收到的命令调整PWM的占空比，从而控制LED的亮度。同时，通过USART与上位机进行通信，接收上位机发送的命令并将结果发送回上位机。请注意，该代码是基于STM32F4系列的开发板进行开发的，使用了相应的库函数和寄存器定义。如果你使用的是其他型号的STM32开发板或者其他开发平台，可能需要进行相应的适配和修改。