printf with UART - blocking way?

[Globuloz]

Hello,

I used the uart example to print text over serial communication from the SDK v11.

The problem is when I write lot of text consecutively, I don't receive all data on the terminal. (It clearly appears when I change the UART_TX_BUF_SIZE to a small value) I found that the file \sdk\components\libraries\uart\retarget.c implements the IO primitives. In the function _write, it appears that the FIFO memory is quiclky full.

int _write(int file, const char * p_char, int len) {
    int i;
    UNUSED_PARAMETER(file);

    for (i = 0; i < len; i++)
    {
        if (app_uart_put(*p_char++) == NRF_ERROR_NO_MEM)
		blinkErr(); // or return i; to allow printf to report error to application code
    }
    return len;
}

And it blinks. I also found that app_uart_put is a non-blocking call. It explains that I can fill the FIFO before it is empty.

According to this question, there was a bug (devzone.nordicsemi.com/.../) but it should be fixed in the SDK v11.

In any case, I am looking for a way to send data over UART synchronously (yes, 'A' is for asynchronous...) Do you have any ideas ? Make a loop, uart option ?

Thank you in advance !

[ovrebekk]

The app_uart library returns an event when the TX buffer is empty, and you can wait for this event to make the library appear blocking:

Pseudo code:

// Declare variable
static volatile bool uart_tx_in_progress = false;

// Write to the uart
uart_tx_in_progress = true;
app_uart_put(...);
while(uart_tx_in_progress);

// Excerpt from UART event handler
if(p_event->evt_type == APP_UART_TX_EMPTY)
{
uart_tx_in_progress = false;
}

Regards Torbjørn

[studzinski]

We are still waiting for a fix 

[ovrebekk]

Hi Andre

I just tested this myself, and as long as I make sure to run the while loop in the main (thread) context it works fine. 

If I run the while loop in an interrupt context it will not work, and the code will get stuck in the while loop as described by Globuloz. 

Can you confirm whether or not the code runs in main context?

Best regards
Torbjørn

[Mathias]

Hi Torbjørn,

I'm wondering why the code gets stuck when used in the interrupt context? Why does this code only work in the main context and not in interrupt context?

Thanks.

Kind regards,

Mathias

[ovrebekk]

Hi Mathias

When you run the code from interrupt context you will block the UART interrupts, which prevents the UART driver from working properly to empty the UART buffers. 

An alternative workaround to this is to make sure the UART interrupts run at a higher interrupt priority (lower pri number) than the interrupt context you are currently in. 

Best regards
Torbjørn

[Mathias]

Ok, got it! Thanks so much for replying Torbjørn!

[ovrebekk]

Happy to help Mathias, best of luck with your project