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Note for using CDC (com m unication device class) or VCP (Virtual Com Port) to exchange data between STM32 and PC via USB .The sam ple code and related tools can be downloaded via:https:///rwm ao/cdc_onstm 32f411rc/downloadshttps:///rwm ao/cdc_onstm 32f411rc/src it is a repository. There is wiki too.如何读取数据,参考: http://www.stm /question/160关于接收数据的讨论在https://m /public/STe2ecom m unities/m cu/Lists/cortex_m x_stm 32/Flat.aspx?RootFolder=/public/STe2ecom m unities/m cu/Lists/cortex_m x_stm 32/Read%20data%20from %20PC%20to%20S ¤tviews=40这是另外一个例子,可以参考用. /hiker-blogs/p/stm 32_usb.htm l关于波特率是这么说的: STM32向PC 发送的是USB 协议的数据包，跟串口没有一毛钱的关系。

PC 端的USB 接口收到USB 协议的数据包后，由驱动程序来解包并放入操作系统的串口缓冲区里，这样，串口助手类的工具就能够从缓冲区里读到数据，串口助手就认为是有 uart 数据到来了。

我刚才说的，是指PC 的uart 受到串口助手的波特率限制时如何摆脱这个限制，跟usb 没关系。

If you are not patient to read through, you can directly go to the last part:Bugs or possible error you may face .Developm ent tools:Keil 5.16a, FDP2.6 pack.Cubem x v1.8pack.Chip: STM32F411RC 64pins.------------------------------------------------------------------------------------Implementation of hardware:On STM32 side, USB m ust be correctly configured.The d- and d+ datalines are sim ply connected to the STM32F411RC m icro-controller (PA11 and PA12).One of the m ost im portant thing is the 1.5k pull-up resistor to identify the devices itself as high speed USB.Without the resistor, PC m ay not be able to detect the device at all.Show Detail2016-02-27 15:59STM32 F411RC 芯片使用CDC/VCP 功能 --更新 -- 软件技巧On PC side, no need for any hardware im plem entation as long as USB port is available.VCP driver is needed for PC to correctly access the data./web/en/catalog/tools/PF257938You also need a software to m onitor the data sent through USB (virtual com port).The software can be found in the repository.--------------------------------------------------------------------------------Software implementation.1. Use STM32F4cubemx to generate the project.Not m uch to configure. Only select USB and I2C. Note I2C is not necessary for VCP. I used it for som ething else.2. Implement the USB TX function.(1). Im plem ent the data sending function, CDC_Transmit_FS. in file:usbd_cdc_if.c/*** @brief CDC_Transmit_FS* Data send over USB IN endpoint are sent over CDC interface* through this function.* @note* @param Buf: Buffer of data to be send* @param Len: Number of data to be send (in bytes)* @retval Result of the operation: USBD_OK if all operations are OK else USBD_FAIL or USBD_BUSY*/uint8_t CDC_Transmit_FS(uint8_t* Buf, uint16_t Len){// uint8_t result = USBD_OK;/* USER CODE BEGIN 7 *///original code is too simple/* USBD_CDC_SetTxBuffer(hUsbDevice_0, Buf, Len);result = USBD_CDC_TransmitPacket(hUsbDevice_0);*/// redefine the function//I revised the code to send long strings exceeding APP_TX_DATA_SIZE. rwmao uint8_t result = USBD_OK;if ( hUsbDevice_0 == NULL ) return USBD_FAIL;USBD_CDC_HandleTypeDef *pCDC =(USBD_CDC_HandleTypeDef *)hUsbDevice_0->pClassData;while(pCDC->TxState) { } //Wait for previous transfer to complete/* USER CODE BEGIN 8 */if (Len > APP_TX_DATA_SIZE){int offset=0;while(offset<Len){int todo = MIN(APP_TX_DATA_SIZE,Len - offset);result = CDC_Transmit_FS(Buf + offset, todo);if ( ( result != USBD_OK ) && ( result != USBD_BUSY ) ) { /* Error: Break out now */return result;}offset+=todo;}return USBD_OK;}pCDC = (USBD_CDC_HandleTypeDef *)hUsbDevice_0->pClassData;/* TODO: Consider a timeout in the following wait loop. */while(pCDC->TxState) { } //Wait for previous transfer to complete int i;for ( i = 0; i < Len; i++ ) {UserTxBufferFS[i] = Buf[i];}USBD_CDC_SetTxBuffer(hUsbDevice_0, &UserTxBufferFS[0], Len);result = USBD_CDC_TransmitPacket(hUsbDevice_0);/* USER CODE END 7 */return result;}You can change the buffer size too.Now the function to send data is ready.For convenience, we define a function to call the subroutine in usb_device.c(.h).Now in m ain.c, you can call the subroutine to send data.A snapshot of the captured data is:--------------------------------------------------------------------------------------从PC端接收数据最主要的是在usbd_cdc_if.c文件中把 callback 函数CDC_Receive_FS 补充完整.每次有接收到数据后系统会自动调用这个函数CDC_Receive_FS并且定义一个变量指示当前是否有数据接收到并且还没有处理过. 这点也很重要.然后定义另外一个函数VCP_retrieveInputData可以用来检查是否接收到数据, 如果返回0表示没有数据, 如果有数据,那么通过传递过去的指针返回数据.这里我们定义了一个队列操作, 可以存储多条数据,不会被覆盖.每次操作VCP_retrieveInputData处理一条数据, 直到所有数据处理完毕,复位变量.#define MaxCommandsInBuffer 10 //max 10 commands can be received and saved without overwriting. Each command has a max size of APP_RX_DATA_SIZEstatic struct{int pos_receive, pos_process; //pos_receive is the current position in buffer to save incoming data. pos_process is the index of data in buffer which has been processed.//if pos_receive=pos_process, it means all data were processed, waiting for new data comingunsigned char IsCommandDataReceived; //anynumber >0 means data were received. 0 means no data is availableuint8_t UserRxBufferFS[MaxCommandsInBuffer][APP_RX_DATA_SIZE];//it could save<MaxCommandsInBuffer> number of commandsuint8_t CommandsLens[MaxCommandsInBuffer]; //save the len of each command} s_RxBuffers;int8_t CDC_Receive_FS (uint8_t* Buf, uint32_t *Len){/* USER CODE BEGIN 6 */s_RxBuffers.IsCommandDataReceived=1; //indicates data were received 指示有数据接收到缓存中s_mandsLens[s_RxBuffers.pos_receive]=*Len; //only set the length, data wasdirectly saved to buffers_RxBuffers.pos_receive++;//move to next position to receive dataif(s_RxBuffers.pos_receive>=MaxCommandsInBuffer) //reach the last buffer, need to rewind to 0 {s_RxBuffers.pos_receive=0;}//prepare to receive the next dataUSBD_CDC_SetRxBuffer(hUsbDevice_0, s_erRxBufferFS[s_RxBuffers.pos_receive]);//Set the buffer to receive incoming dataUSBD_CDC_ReceivePacket(hUsbDevice_0);// Tell that you are ready to receive the next packet 这点也很重要return USBD_OK;/* USER CODE END 6 */}Define in the same file as that of CDC_Receive_FS/*** @brief VCP_retrieveInputData, defined by user* Call this function frequently to check if data is received.**** @param Buf: Buffer of data to be received* @param Len: Number of data received (in bytes)* @retval 0 means no data was received.*/int8_t VCP_retrieveInputData(uint8_t* Buf, uint32_t *Len){if(s_RxBuffers.IsCommandDataReceived==0)return0; //no data receivedint index=s_RxBuffers.pos_process;*Len=s_mandsLens[index]; //return the lengthmemcpy(Buf,s_erRxBufferFS[index],*Len);Buf[*Len]='\0'; //testing only. make sure there is ending char in the returned command string //check if all data were processed.s_RxBuffers.pos_process++;if(s_RxBuffers.pos_process>=MaxCommandsInBuffer) //reach the last buffer, need to rewind to 0 {s_RxBuffers.pos_process=0;}if(s_RxBuffers.pos_process==s_RxBuffers.pos_receive)s_RxBuffers.IsCommandDataReceived=0; //check if all data were processedreturn1;}In main() frequently call above defined VCP_retrieveInputDatawhile(1){// DO something else hereif(VCP_retrieveInputData(hexbuffer,&pos32)!=0){// you could do data processing here.//by demo, i just send it back to PCsprintf(textbuf,"i=%d,data=%s\n",i_temp0,hexbuffer);//export it to charbuffer first.SendTextMsgToUSB(textbuf); //send the text to PC via cdchexbuffer[0]='\0';}TM_DelayMillis(10);//it is better to delay a little bit time, 0.1ms up to a few ms i_temp0++;}3. Bugs or possible error you may face.(1). PC doesn't response at all when you plug in the usb cable.This is the m ost headache you m ay face. There are lots of possibility.One of them is the m issing pull-up resistor. Look at the first page. You m ay need to connect a 1.5kohm pull up resistor.The idea is the device needs to be correctly identified.bm adesim /um s_3.htm(2). PC response, but it prompts "USB device not recognized"The problem is that m y own board uses self-powered usb. Doesn't use the VBUS.Therefore it should be disabled.(3).USB is recognized by PC, but reported as unknown or "This device cannot start"This problem was discussed in the forum as link:This one is easy to correct. Go to startup fileand change stacksize and heap size from 400-> 4000, 200->2000.Last note, internal clock is OK for the USB data transfer.2015-10-23。