USB UART output buffer remains full after serial disconnect

[douglas.malnati]

Windows 11, NCS v2.3.0, VSCode

Hi, I am using the uart_poll_out API for communicating via USB serial.

For each character I want to output, I do this:

uint32_t dtr;
uart_line_ctrl_get(DEV_UART_USB, UART_LINE_CTRL_DTR, &dtr);
if (dtr)
{
    uart_poll_out(DEV_UART_USB, c);
}

The other side of the serial connection can disconnect at any time.

I don't want my code to hang trying to write, that's why I checked the DTS beforehand, and basically throw away any data that won't get sent immediately.

The problem I have is:

- I am writing continuously

- The other side disconnects

- I attempt to detect that, and do, eventually, but not before the uart_poll_out puts a few characters into what I assume is an output buffer which fills up a short time later

Then, when the other side re-connects, the bytes that are sitting in the output buffer get sent.  This corrupts the datastream to the receiver as those bytes were for a different connection.

Is there a way to deal with this?

I would basically like to detect that I can't send data, and clear any existing output buffer at that time.

Or any other way to deal with this situation?

I'm not a serial expert and I only had the line_ctrl code because I saw it in an example.  Please let me know.

Thanks.

[Kenneth]

Hello,

I don't see any better way than to always check the uart_line_ctrl_get(dev, UART_LINE_CTRL_DTR, &dtr) before trying to send data. 

I assume you are calling usb_enable() here, can you try to add a callback function such as:

static void status_cb(enum usb_dc_status_code status, const uint8_t *param)
{
	LOG_INF("Status %d", status);
	// Clear buffers here if status is USB_DC_DISCONNECTED.
}

...
ret = usb_enable(status_cb);
...

I do believe you should be able to clear all buffers on disconnect event.

The complete list of status codes are:

/**
 * @brief USB Driver Status Codes
 *
 * Status codes reported by the registered device status callback.
 */
enum usb_dc_status_code {
	/** USB error reported by the controller */
	USB_DC_ERROR,
	/** USB reset */
	USB_DC_RESET,
	/** USB connection established, hardware enumeration is completed */
	USB_DC_CONNECTED,
	/** USB configuration done */
	USB_DC_CONFIGURED,
	/** USB connection lost */
	USB_DC_DISCONNECTED,
	/** USB connection suspended by the HOST */
	USB_DC_SUSPEND,
	/** USB connection resumed by the HOST */
	USB_DC_RESUME,
	/** USB interface selected */
	USB_DC_INTERFACE,
	/** Set Feature ENDPOINT_HALT received */
	USB_DC_SET_HALT,
	/** Clear Feature ENDPOINT_HALT received */
	USB_DC_CLEAR_HALT,
	/** Start of Frame received */
	USB_DC_SOF,
	/** Initial USB connection status */
	USB_DC_UNKNOWN
};

Maybe you can use those to also investigate your other issue you have:
usb_cdc_acm: cdc_acm_poll_out: Ring buffer full, drain buffer 

Best regards,
Kenneth

[douglas.malnati]

Thank you.

Kenneth said:

I assume you are calling usb_enable() here, can you try to add a callback function such as:

Yes I receive a callback in that way.

Kenneth said:

I do believe you should be able to clear all buffers on disconnect event.

How do I do that?  Is there documentation or an example?  I don't understand how to do this from your reply.

Kenneth said:

I don't see any better way than to always check the uart_line_ctrl_get(dev, UART_LINE_CTRL_DTR, &dtr) before trying to send data. 

Are you saying there is no way to stop this from happening?

Is the problem description clear enough?

I am saying I believe several bytes get loaded into some area in memory before I am notified of disconnect (usb callback) and also during that time I am still getting the dtr (probably because the bytes are in ram to be sent out later but aren't blocking yet.

Thanks.

Doug

[Kenneth]

Maybe you can share some soure code I can look at?

Kenneth

[douglas.malnati]

Hi, what code do you want to see to help you answer this?

Let's be specific so we can get to the answer quickly.

[Kenneth]

Ideally I want a project I can run on an nRF52840-DK to recreate this.

Kenneth