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USB CDC 回环示例

本工程将 STM32F401 配置成 USB CDC（虚拟串口）回环设备。执行 MX_USB_DEVICE_Init() 后，开发板会以 VCP 形式枚举至 PC，用于验证主机与 MCU 之间的收发链路。

  • 上电后固件只发送一次提示:"CDC echo demo ready. Type to see loopback."
  • 串口终端输入的每帧数据都会由 CDC 驱动缓存，并通过 CDC_Transmit_FS() 原样回传。
  • 适合快速确认 USB 协议栈、时钟以及端点配置是否正常。

使用步骤

  1. 编译并烧录固件到 STM32F401。
  2. 通过 USB 连接电脑，等待虚拟串口枚举完成。
  3. 打开串口终端（115200 8N1 即可），确认看到启动提示。
  4. 输入任意字符或命令，实时查看 MCU 回显。

代码结构

  • Core/Src/main.c：维护 cdcRxAppBuffer 并轮询 CDC_IsRxReady()，调用 CDC_ReadRxData() 取出数据后使用 CDC_Transmit_FS() 回环。
  • USB_DEVICE/App/usbd_cdc_if.c/.h：新增接收缓冲、长度与“数据就绪”标志；CDC_Receive_FS() 拷贝 USB OUT 数据并重新挂起下一次接收，对外暴露 CDC_IsRxReady() 与 CDC_ReadRxData()。

详细改动（USB_DEVICE/App/usbd_cdc_if.*）

usbd_cdc_if.h（约 112-113 行）

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uint8_t CDC_IsRxReady(void);
uint32_t CDC_ReadRxData(uint8_t *Buf, uint32_t BufSize);

usbd_cdc_if.c

  1. 引入 <string.h>（行 25）。
  2. 新增私有接收缓冲与标志（行 98-100）。
  3. 在 CDC_Receive_FS 中缓存数据并置位（行 264-273）。
  4. 实现 CDC_IsRxReady 与 CDC_ReadRxData（行 332-345）。

通过以上改动，USB CDC 驱动可以可靠保存主机发送的数据，主循环仅需通过两个接口即可实现回环或扩展自定义协议处理。

usbd_cdc_if.c 关键改动代码

  • 引入字符串库 USB_DEVICE/App/usbd_cdc_if.c:25 

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    #include <string.h>

  • 新增接收缓冲与状态变量 USB_DEVICE/App/usbd_cdc_if.c:98-100 

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    static uint8_t cdcRxBuffer[APP_RX_DATA_SIZE];
static volatile uint32_t cdcRxLength = 0U;
static volatile uint8_t cdcRxReady = 0U;

  • CDC_Receive_FS 中缓存数据并重新挂起接收 USB_DEVICE/App/usbd_cdc_if.c:265-273 

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    if ((Len != NULL) && (*Len > 0U))
{
  uint32_t copyLen = (*Len <= APP_RX_DATA_SIZE) ? *Len : APP_RX_DATA_SIZE;
  memcpy(cdcRxBuffer, Buf, copyLen);
  cdcRxLength = copyLen;
  cdcRxReady = 1U;
}

USBD_CDC_SetRxBuffer(&hUsbDeviceFS, &Buf[0]);
USBD_CDC_ReceivePacket(&hUsbDeviceFS);

  • 对外暴露的查询/读取函数 USB_DEVICE/App/usbd_cdc_if.c:333-345 

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    uint8_t CDC_IsRxReady(void)
{
  return cdcRxReady;
}

uint32_t CDC_ReadRxData(uint8_t *Buf, uint32_t BufSize)
{
  if ((cdcRxReady == 0U) || (cdcRxLength == 0U) || (Buf == NULL) || (BufSize == 0U))
  {
    return 0U;
  }

  uint32_t bytesToCopy = (cdcRxLength <= BufSize) ? cdcRxLength : BufSize;
  memcpy(Buf, cdcRxBuffer, bytesToCopy);
  cdcRxLength = 0U;
  cdcRxReady = 0U;
  return bytesToCopy;
}

驱动以及测试代码

Core/Src/main.c 
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/* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file           : main.c
  * @brief          : Main program body
  ******************************************************************************
  * @attention
  *
  * Copyright (c) 2025 STMicroelectronics.
  * All rights reserved.
  *
  * This software is licensed under terms that can be found in the LICENSE file
  * in the root directory of this software component.
  * If no LICENSE file comes with this software, it is provided AS-IS.
  *
  ******************************************************************************
  */
/* USER CODE END Header */
/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "usb_device.h"
#include "gpio.h"

/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
#include <string.h>
#include "usbd_cdc_if.h"
/* USER CODE END Includes */

/* Private typedef -----------------------------------------------------------*/
/* USER CODE BEGIN PTD */

/* USER CODE END PTD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */

/* USER CODE END PD */

/* Private macro -------------------------------------------------------------*/
/* USER CODE BEGIN PM */

/* USER CODE END PM */

/* Private variables ---------------------------------------------------------*/

/* USER CODE BEGIN PV */
static uint8_t cdcRxAppBuffer[APP_RX_DATA_SIZE];
static uint32_t cdcRxAppLen = 0U;
static uint8_t cdcFeatureMsgSent = 0U;
/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
void SystemClock_Config(void);
/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */

/* USER CODE END 0 */

/**
  * @brief  The application entry point.
  * @retval int
  */
int main(void)
{

  /* USER CODE BEGIN 1 */

  /* USER CODE END 1 */

  /* MCU Configuration--------------------------------------------------------*/

  /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
  HAL_Init();

  /* USER CODE BEGIN Init */

  /* USER CODE END Init */

  /* Configure the system clock */
  SystemClock_Config();

  /* USER CODE BEGIN SysInit */

  /* USER CODE END SysInit */

  /* Initialize all configured peripherals */
  MX_GPIO_Init();
  MX_USB_DEVICE_Init();
  /* USER CODE BEGIN 2 */

  /* USER CODE END 2 */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
  while (1)
  {
    /* Print feature hint once after USB is ready */
    if (cdcFeatureMsgSent == 0U)
    {
      const char *msg = "CDC echo demo ready. Type to see loopback.\r\n";
      if (CDC_Transmit_FS((uint8_t *)msg, (uint16_t)strlen(msg)) == USBD_OK)
      {
        cdcFeatureMsgSent = 1U;
      }
    }

    /* Process received CDC data when stack notifies readiness */
    if (CDC_IsRxReady())
    {
      cdcRxAppLen = CDC_ReadRxData(cdcRxAppBuffer, sizeof(cdcRxAppBuffer));
      if (cdcRxAppLen > 0U)
      {
        /* Echo received payload back to host; replace with custom handling */
        CDC_Transmit_FS(cdcRxAppBuffer, (uint16_t)cdcRxAppLen);
      }
    }

    HAL_Delay(1);
    /* USER CODE END WHILE */

    /* USER CODE BEGIN 3 */
  }
  /* USER CODE END 3 */
}

/**
  * @brief System Clock Configuration
  * @retval None
  */
void SystemClock_Config(void)
{
  RCC_OscInitTypeDef RCC_OscInitStruct = {0};
  RCC_ClkInitTypeDef RCC_ClkInitStruct = {0};

  /** Configure the main internal regulator output voltage
  */
  __HAL_RCC_PWR_CLK_ENABLE();
  __HAL_PWR_VOLTAGESCALING_CONFIG(PWR_REGULATOR_VOLTAGE_SCALE2);

  /** Initializes the RCC Oscillators according to the specified parameters
  * in the RCC_OscInitTypeDef structure.
  */
  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 = 4;
  RCC_OscInitStruct.PLL.PLLN = 168;
  RCC_OscInitStruct.PLL.PLLP = RCC_PLLP_DIV4;
  RCC_OscInitStruct.PLL.PLLQ = 7;
  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_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_2) != HAL_OK)
  {
    Error_Handler();
  }
}

/* USER CODE BEGIN 4 */

/* USER CODE END 4 */

/**
  * @brief  This function is executed in case of error occurrence.
  * @retval None
  */
void Error_Handler(void)
{
  /* USER CODE BEGIN Error_Handler_Debug */
  /* User can add his own implementation to report the HAL error return state */
  __disable_irq();
  while (1)
  {
  }
  /* USER CODE END Error_Handler_Debug */
}

#ifdef  USE_FULL_ASSERT
/**
  * @brief  Reports the name of the source file and the source line number
  *         where the assert_param error has occurred.
  * @param  file: pointer to the source file name
  * @param  line: assert_param error line source number
  * @retval None
  */
void assert_failed(uint8_t *file, uint32_t line)
{
  /* USER CODE BEGIN 6 */
  /* User can add his own implementation to report the file name and line number,
     ex: printf("Wrong parameters value: file %s on line %d\r\n", file, line) */
  /* USER CODE END 6 */
}
#endif /* USE_FULL_ASSERT */
Core/Src/stm32f4xx_it.c 
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/* USER CODE BEGIN Header */
/**
  ******************************************************************************
  * @file    stm32f4xx_it.c
  * @brief   Interrupt Service Routines.
  ******************************************************************************
  * @attention
  *
  * Copyright (c) 2025 STMicroelectronics.
  * All rights reserved.
  *
  * This software is licensed under terms that can be found in the LICENSE file
  * in the root directory of this software component.
  * If no LICENSE file comes with this software, it is provided AS-IS.
  *
  ******************************************************************************
  */
/* USER CODE END Header */

/* Includes ------------------------------------------------------------------*/
#include "main.h"
#include "stm32f4xx_it.h"
/* Private includes ----------------------------------------------------------*/
/* USER CODE BEGIN Includes */
/* USER CODE END Includes */

/* Private typedef -----------------------------------------------------------*/
/* USER CODE BEGIN TD */

/* USER CODE END TD */

/* Private define ------------------------------------------------------------*/
/* USER CODE BEGIN PD */

/* USER CODE END PD */

/* Private macro -------------------------------------------------------------*/
/* USER CODE BEGIN PM */

/* USER CODE END PM */

/* Private variables ---------------------------------------------------------*/
/* USER CODE BEGIN PV */

/* USER CODE END PV */

/* Private function prototypes -----------------------------------------------*/
/* USER CODE BEGIN PFP */

/* USER CODE END PFP */

/* Private user code ---------------------------------------------------------*/
/* USER CODE BEGIN 0 */

/* USER CODE END 0 */

/* External variables --------------------------------------------------------*/
extern PCD_HandleTypeDef hpcd_USB_OTG_FS;
/* USER CODE BEGIN EV */

/* USER CODE END EV */

/******************************************************************************/
/*           Cortex-M4 Processor Interruption and Exception Handlers          */
/******************************************************************************/
/**
  * @brief This function handles Non maskable interrupt.
  */
void NMI_Handler(void)
{
  /* USER CODE BEGIN NonMaskableInt_IRQn 0 */

  /* USER CODE END NonMaskableInt_IRQn 0 */
  /* USER CODE BEGIN NonMaskableInt_IRQn 1 */
   while (1)
  {
  }
  /* USER CODE END NonMaskableInt_IRQn 1 */
}

/**
  * @brief This function handles Hard fault interrupt.
  */
void HardFault_Handler(void)
{
  /* USER CODE BEGIN HardFault_IRQn 0 */

  /* USER CODE END HardFault_IRQn 0 */
  while (1)
  {
    /* USER CODE BEGIN W1_HardFault_IRQn 0 */
    /* USER CODE END W1_HardFault_IRQn 0 */
  }
}

/**
  * @brief This function handles Memory management fault.
  */
void MemManage_Handler(void)
{
  /* USER CODE BEGIN MemoryManagement_IRQn 0 */

  /* USER CODE END MemoryManagement_IRQn 0 */
  while (1)
  {
    /* USER CODE BEGIN W1_MemoryManagement_IRQn 0 */
    /* USER CODE END W1_MemoryManagement_IRQn 0 */
  }
}

/**
  * @brief This function handles Pre-fetch fault, memory access fault.
  */
void BusFault_Handler(void)
{
  /* USER CODE BEGIN BusFault_IRQn 0 */

  /* USER CODE END BusFault_IRQn 0 */
  while (1)
  {
    /* USER CODE BEGIN W1_BusFault_IRQn 0 */
    /* USER CODE END W1_BusFault_IRQn 0 */
  }
}

/**
  * @brief This function handles Undefined instruction or illegal state.
  */
void UsageFault_Handler(void)
{
  /* USER CODE BEGIN UsageFault_IRQn 0 */

  /* USER CODE END UsageFault_IRQn 0 */
  while (1)
  {
    /* USER CODE BEGIN W1_UsageFault_IRQn 0 */
    /* USER CODE END W1_UsageFault_IRQn 0 */
  }
}

/**
  * @brief This function handles System service call via SWI instruction.
  */
void SVC_Handler(void)
{
  /* USER CODE BEGIN SVCall_IRQn 0 */

  /* USER CODE END SVCall_IRQn 0 */
  /* USER CODE BEGIN SVCall_IRQn 1 */

  /* USER CODE END SVCall_IRQn 1 */
}

/**
  * @brief This function handles Debug monitor.
  */
void DebugMon_Handler(void)
{
  /* USER CODE BEGIN DebugMonitor_IRQn 0 */

  /* USER CODE END DebugMonitor_IRQn 0 */
  /* USER CODE BEGIN DebugMonitor_IRQn 1 */

  /* USER CODE END DebugMonitor_IRQn 1 */
}

/**
  * @brief This function handles Pendable request for system service.
  */
void PendSV_Handler(void)
{
  /* USER CODE BEGIN PendSV_IRQn 0 */

  /* USER CODE END PendSV_IRQn 0 */
  /* USER CODE BEGIN PendSV_IRQn 1 */

  /* USER CODE END PendSV_IRQn 1 */
}

/**
  * @brief This function handles System tick timer.
  */
void SysTick_Handler(void)
{
  /* USER CODE BEGIN SysTick_IRQn 0 */

  /* USER CODE END SysTick_IRQn 0 */
  HAL_IncTick();
  /* USER CODE BEGIN SysTick_IRQn 1 */

  /* USER CODE END SysTick_IRQn 1 */
}

/******************************************************************************/
/* STM32F4xx Peripheral Interrupt Handlers                                    */
/* Add here the Interrupt Handlers for the used peripherals.                  */
/* For the available peripheral interrupt handler names,                      */
/* please refer to the startup file (startup_stm32f4xx.s).                    */
/******************************************************************************/

/**
  * @brief This function handles USB On The Go FS global interrupt.
  */
void OTG_FS_IRQHandler(void)
{
  /* USER CODE BEGIN OTG_FS_IRQn 0 */

  /* USER CODE END OTG_FS_IRQn 0 */
  HAL_PCD_IRQHandler(&hpcd_USB_OTG_FS);
  /* USER CODE BEGIN OTG_FS_IRQn 1 */

  /* USER CODE END OTG_FS_IRQn 1 */
}

/* USER CODE BEGIN 1 */

/* USER CODE END 1 */