USB CDC Increase Reading Speed

[Kakyouka]

Hello,
I'm currently working on a USB project and i encounter some issues.

I would like to send data to the STM32 and the STM32 must re-send it as fast as possible. I already test the sending speed and it work great for me but the reading speed is low.
The function CDC_Receive_FS work as a callback function and is call at the right speed (as fast as the sending speed) but between the sending and the writing, there's a delay... I've looked into the library but i don't understant where it comes from or how to fix it.
The STM32 seems to be able to receive et write data with a latency of 10ms but combining these two operation make the board have a latency of (avg)124ms.

I suspect the USBD_CDC_ReceivePacket function to be the cause of this issue.

Do you have any idea ?

```C

static int8_t CDC_Receive_FS(uint8_t* Buf, uint32_t *Len)

{

/* USER CODE BEGIN 6 */

incrtest++;

TCountMsgReceive = StartingValueTCount;

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

CDC_Transmit_FS(UserRxBufferFS, *Len);

USBD_CDC_ReceivePacket(&hUsbDeviceFS);

if(((USBD_CDC_HandleTypeDef*) hUsbDeviceFS.pClassData)->RxState==0)

{

//USB_CAN_SwitchReceive();

}

return (USBD_OK);

/* USER CODE END 6 */

}

```

[Kakyouka]

Little precision:

I'm using a STM32F411E-Disco

[Pavel A.]

Even 10 ms latency seems a bit too much. Depends on the size of the data, though.

The code snippet that you've shared does not provide enough clue to answer.

[Kakyouka]

I don't know what part of code i must share because my question i about the HAL library provided by ST.

10ms is too much but i don't need less than that. The issue i encounter is that the switch between reading and writing is slow. If i just read, i can acheive 10ms without problem and it's the same if i just write.

I don't understand why the retransmit seems to be blocked while the board is receiving data. 

do you have any explanation why?

[AScha.3]

Just - how often you call the "USB service" , MX_USB_HOST_Process(); , in main  ?

I call it about every 300us and USB running fine.

[Kakyouka]

The board is in Device mode and that's how my main function looks like.

I handle the data in the function CDC_Receive_FS() in the usbd_cdc_if.c file.

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();
  MX_TIM4_Init();
  /* USER CODE BEGIN 2 */

  HAL_TIM_Base_Start_IT(&htim4);

  /* USER CODE END 2 */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
  while (1)
  {
    /* USER CODE END WHILE */

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

static int8_t CDC_Receive_FS(uint8_t* Buf, uint32_t *Len)
{
  /* USER CODE BEGIN 6 */
	incrtest++;

  TCountMsgReceive = StartingValueTCount;

  USBD_CDC_SetRxBuffer(&hUsbDeviceFS, &Buf[0]);
  CDC_Transmit_FS(UserRxBufferFS, *Len);

  USBD_CDC_ReceivePacket(&hUsbDeviceFS);


  if(((USBD_CDC_HandleTypeDef*) hUsbDeviceFS.pClassData)->RxState==0)
  {
	  //USB_CAN_SwitchReceive();

  }


  return (USBD_OK);
  /* USER CODE END 6 */
}

In this function, i call CDC_Transmit_FS to re-send the data receive from the host PC. But no matter what i try, the transmit doesn't work until i stop sending data from the host to the board. If datas are send every 130ms there's no problem but below 130ms it cannot follow.

[AScha.3]

Oo, sorry, i did not see : device only. 

(I never did much with device ...more host .)

To receive i have :

static int8_t CDC_Receive_FS(uint8_t* Buf, uint32_t *Len)
{
  /* USER CODE BEGIN 6 */
  USBD_CDC_SetRxBuffer(&hUsbDeviceFS, &Buf[0]);
  USBD_CDC_ReceivePacket(&hUsbDeviceFS);
  return (USBD_OK);
  /* USER CODE END 6 */
}

and to transmit:

uint8_t CDC_Transmit_FS(uint8_t* Buf, uint16_t Len)
{
  uint8_t result = USBD_OK;
  /* USER CODE BEGIN 7 */
  USBD_CDC_HandleTypeDef *hcdc = (USBD_CDC_HandleTypeDef*)hUsbDeviceFS.pClassData;
  if (hcdc->TxState != 0){
    return USBD_BUSY;
  }
  USBD_CDC_SetTxBuffer(&hUsbDeviceFS, Buf, Len);
  result = USBD_CDC_TransmitPacket(&hUsbDeviceFS);
  /* USER CODE END 7 */
  return result;
}

and program in main.c : " if xxx received , send (CDC_Transmit...) " . = > your program here. <

(To call transmit at receive interrupt level...maybe not best idea. And then call receive ... strange . (Dont do this.))

[Kakyouka]

Ok thanks, i'll try but it seems weird because i've done what ST told to do in their tutorial.

I'll try and give you my results.

[Kakyouka]

it doesn't work because CDC_Receive_FS is called by an interrupt so the main() function is blocked during the execution of CDC_Receive_FS.

I also tried using a Timer with a higher priority but i don't success to make it work. Depending on the frequency and the period i gave to CubeMX the latency is reduced or not.

Sometimes i acheive a transfert with 124ms and sometimes with 30ms.... The timer is running correctly but don't transmit the data directly....

[AScha.3]

Ok, i dont remember details ... i did about 11 years ago, got (close to limit) almost 1 MB/s (sending).

Send and receive same time ... max. 50% of this.

So look on git , at examples, how to do.

https://github.com/search?q=stm32%20cdc%20&type=repositories

+

read in HAL usb functions , to find , how to check "something received" -> read it.

(Maybe : USBD_CtlReceiveStatus() to start it, then check with  USBD_GetRxCount(..), if > 0 then copy it. )

Not call just (blocking) "receive now" ... look at examples, ok ?