VCom port, emptying buffer.

[PG.1]

Hello!

Not sure it's the right place to post.

I have a custom board based on STM32F767.

I'm using USB with VCOM port to communicate by UART. Basically it works (I mean, I can receive bytes

from a terminal (I'm working on Ubuntu with STM32CubeIDE v. 1.15).

So what I do is that as soon as I receive a message, I reply with a "OK!\n" to the terminal.

1. In usbd_cdc_if.c, in the receive function is implemented like this:

static uint8_t CDC_Receice_FS(uint8 * Buf, uint32 * Len) {

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

USBD_CDC_ReceivePacket(&hUsbDeviceFS);

// Added to the code:

CDC_RxCpltCallback(Buf, *Len);

return USBD_OK;

}

2. Start the program.

Set a breakpoint in the above CDC_Recive_FS function.

If I open 2 terminals (Linux) like this:

first terminal: cat < /dev/ttyACM1

second terminal : echo cmd > /dev/ttyACM1

then as long as I don't launch the echo command, the breakpoint is not hit and the cat

command doesn't print anything.

Then if I enter anything in the first terminal, for example echo GO! > /dev/ttyACM1, then the

interrupt gets it, and if I contunue I get "OK\n" printed in the second terminal which looks normal.

But it immediately stops again at the breakpoint (although I don't echo in the first terminal).

At first stop at the breakpoint, the Buf contains "Go!" and *Len contains 3, which is the length

of "Go!". But the subsequent stops contained "\r\n" (length = 2), then "^\n!" with a length of 1,

which isn't even right.

Can anybody explain me how to get what I sent and only what I sent? Do I have to clear

some buffer or issue some kind of acknowledge to the VCom port so that it knows it has to

wait for the next data?

Thanks.

[FBL]

Hi @PG.1 

If still having the same issue, could you attach your project?

[PG.1]

Hello!

Thanks for your reply and sorry for the delay. I had to work on other things and months passed...

Yes, I have the same problem.

Once the connection is setup, on my first buffer send (cat something > dev/ttyAcm1), the interrupt

is triggered, and then it loops forever. So I suspect I have to reset a flag somewhere so that the input

becomes idle again, but I have no idea where it could be.

Here is the demo. I have made a completely new project with a board you certainly have in your lab,

the nucleo 767.

So basically you just build it and launch the debugger to run it.

Open a terminal, run cat < /dev/ttyACM0 (or whatever port it connects to)

Open another terminal, run echo "Hello" > /dev/ttyACM0

You will hit the breakpoint at CDC_receive_FS in usbd_cdc_if.c which is normal.

Then press continue. The receiving terminal (with the cat command) will echo a string, and the command

CDC_receive_FS will be called again although the echo command is finished.

Question: how to call the CDC_Receive_FS only once when something was sent by cat, and then return

to the idle mode and wait for the next command.

Thanks for any hint!

PG