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Info about receiving buffer in usb cdc vcp application with stm32cube

Question asked by thy on Apr 10, 2015
Latest reply on Apr 22, 2015 by thy
Hi everybody,
I am a newbie trying to develop a simple application for an stm32f429 discovery board with freertos, STM32Cube_FW_F4_V1.5.0 and vcp over usb capabilities. I used stm32cubemx to generate the project that works fine. I can run a simple echo program via terminal with the micro connected to a pc with windows7.

However there are some things that are very unclear to me and I want to ask for an explanation. I unsuccesfully googled a lot, gave a look at the examples in STM32Cube_FW_F4_V1.5.0 and read the User manual UM1734 (STM32Cube USB device library)

Particularly, what I don't get is related to usbd_cdc_if.c. In the version generated by stm32cubemx there are
01./* Define size for the receive and transmit buffer over CDC */
02./* It's up to user to redefine and/or remove those define */
03.#define APP_RX_DATA_SIZE  4
07./* Create buffer for reception and transmission           */
08./* It's up to user to redefine and/or remove those define */
09./* Received Data over USB are stored in this buffer       */
10.uint8_t UserRxBufferHS[APP_RX_DATA_SIZE];
12./* Send Data over USB CDC are stored in this buffer       */
13.uint8_t UserTxBufferHS[APP_TX_DATA_SIZE];
18.  * @brief  CDC_Init_HS
19.  *         Initializes the CDC media low layer over the USB HS IP
20.  * @param  None
21.  * @retval Result of the operation: USBD_OK if all operations are OK else USBD_FAIL
22.  */
23.static int8_t CDC_Init_HS(void)
25.  hUsbDevice_1 = &hUsbDeviceHS;
26.  /* USER CODE BEGIN 8 */
27.  /* Set Application Buffers */
28.  USBD_CDC_SetTxBuffer(hUsbDevice_1, UserTxBufferHS, 0);
29.  USBD_CDC_SetRxBuffer(hUsbDevice_1, UserRxBufferHS);
30.  return (USBD_OK);
31.  /* USER CODE END 8 */
36.static int8_t CDC_Receive_HS (uint8_t* Buf, uint32_t *Len)
38.  /* USER CODE BEGIN 11 */
39.  return (USBD_OK);
40.  /* USER CODE END 11 */

The (I think defenitely wrong) idea I got in my mind is that in UserRxBufferHS is stored by the lower layer what I type over the terminal (that is what arrives at the endpoint out). The lower layer also calls CDC_Receive_HS. Basicly Buf, which is defined by the interface of CDC_Receive_HS, is copied to UserRxBufferHS. I noticed that *Len is always 1 whatever I type and that in UserRxBufferHS there are always the last character I typed and two more odd characters (like interrupt characters, e.g. 0@). That is if I type "t" then I get "t0@" in UserRxBufferHS.

According to this I wonder why one would set APP_RX_DATA_SIZE to wathever value greater than 3 since in UserRxBufferHS there always are 3 elements.

Is this right? Can anybody give me a better insight?

Thank you and regards.