Electronic – Throttling data from USB serial connection on STM32 microcontroller (flow control)

flow controlserialstm32usb

I've just integrated the code from ST's STM32 USB Virtual Com Port example (stm32_usb-fs-device_lib.zip from here) with my project in order to get the STM32 to appear as a Virtual Serial Port.

However the code example throttles the data it receives by having the callback function EP3_OUT_Callback block (in an interrupt) until all data is dealt with. The exact comment in the code is "USB data will be immediately processed, this allow next USB traffic being NAKed till the end of the USART Xfer"

In my project, I need to process the data that has been received in the main loop (outside of an interrupt), so I need a way of throttling the data that I get from USB without blocking in EP3_OUT_Callback.

What I would like is a function:

Do_Not_Call_EP3_OUT_Callback_Right_Now(bool yes_or_no);

Best Answer

I appear to have got this working:

void EP3_OUT_Callback(void) {
 ...
  //SetEPRxValid(ENDP3); //<-- Commented out
  SetEPRxStatus(ENDP3, Do_I_Have_Room_For_More_Data() ? EP_RX_VALID : EP_RX_NAK); //<-- Added this
}

void SOF_Callback(void) {
  if(bDeviceState == CONFIGURED) {
    SetEPRxStatus(ENDP3, Do_I_Have_Room_For_More_Data() ? EP_RX_VALID : EP_RX_NAK); //<-- Added this
    ...
    // Original TX code here
  }
}

What's happening is after getting data, and in the Start of Frame callback (every 1ms), I set whether the endpoint is accepting data.

Note: The docs never seem to say just how much data USB_SIL_Read will read - potentially leading to a buffer overrun. However I believe the maximum is what is in VIRTUAL_COM_PORT_DATA_SIZE - and this seems to work great for me.

This would appear to work - however I have no idea if it is the 'right' way of doing it. Please let me know if it's wrong and I'll update it!

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Electronic – How to Efficiently Decode Non-Standard Serial Signal

Another answer: Stop using interrupts.

People jump to use interrupts too easily. Personally, I rarely use them because they actually waste a lot of time, as you are discovering.

It's often possible to write a main loop which polls everything so rapidly that's it's latency is within spec, and very little time is wasted.

loop
{
    if (serial_bit_ready)
    {
        // shift serial bit into a byte
    }

    if (serial_byte_ready)
    {
        // decode serial data
    }

    if (enough_serial_bytes_available)
    {
        // more decoding
    }        

    if (usb_queue_not_empty)
    {
        // handle USB data
    }        
}

There might be some things in the loop which happen far more often than others. Perhaps the incoming bits for example, in which case, add more of those tests, so that more of the processor is dedicated to that task.

loop
{
    if (serial_bit_ready)
    {
        // shift serial bit into a byte
    }

    if (serial_byte_ready)
    {
        // decode serial data
    }

    if (serial_bit_ready)
    {
        // shift serial bit into a byte
    }

    if (enough_serial_bytes_available)
    {
        // more decoding
    }        

    if (serial_bit_ready)
    {
        // shift serial bit into a byte
    }

    if (usb_queue_not_empty)
    {
        // handle USB data
    }        
}

There might be some events for which the latency of this approach is too high. For example, you might need a very accurately timed event. In which case, have that event on interrupt, and have everything else in the loop.

STM32F4 VCP transmit/receive data

In my VCP project:

file: usbd_cdc_vcp.c

uint16_t VCP_DataTx (uint8_t* Buf, uint32_t Len); <-You call it to send data from STM to PC

and

uint16_t VCP_DataRx (uint8_t* Buf, uint32_t Len) {
  // Called when data from PC to STM is available.  

  // YOUR CODE IS HERE

  return USBD_OK
}

Best Answer

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STM32F4 VCP transmit/receive data

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