How does nRF5's NRF_UART0->EVENTS_TXDRDY works?

[Jarmo]

In the simple_uart.c there is a code:

void simple_uart_put(uint8_t cr)
{
    NRF_UART0->TXD = (uint8_t)cr;    
    while (NRF_UART0->EVENTS_TXDRDY != 1)
    {
        // Wait for the current TXD data to be sent.
    }
    NRF_UART0->EVENTS_TXDRDY = 0;
}

To make it more efficient, I tried to put the wait-loop in the beginning, so that the function would block only before the next character needs to be written into the TXD register. However, the function stopped working and lead me wonder if the while-loop could miss TXDRDY event in some case. Is the hardware clearing TXDRDY register automatically after a timeout, or why the software is not able to catch it in the version of the function below? In the very beginning of the program (initialization), I do write NRF_UART0->EVENTS_TXDRDY = 1, so that the while loop should pass when writing the first character. However, it seems to block.

void simple_uart_put(uint8_t cr)
{
    while (NRF_UART0->EVENTS_TXDRDY != 1)
    {
        // Wait for the previous TXD data to be sent.
    }
    NRF_UART0->EVENTS_TXDRDY = 0;
    NRF_UART0->TXD = (uint8_t)cr;
}

It looks a bit like hardware would clear TXDRDY automatically back to zero (after a while?), so why the line NRF_UART0->EVENTS_TXDRDY = 0 is still needed? And if it does clear automatically, would it be possible that in some cases (i.e. some interrupt occurring bad time) even the original code could miss the TXDRDY event and the function would block forever?

[FormerMember]

According to the documentation, a byte will be transmitted when it is written the TXD register, and the event EVENTS_TXDRDY will occur when the transmission has finished.

When I test here, it seems like it is not possible to set an event to "1" by writing to it, it is only possible to clear it, i.e. NRF_UART0->EVENTS_TXDRDY = 1 does not work. That may be why your version of simple_uart_put(..) is hanging.

[Jarmo]

Yep, that sounds like an understandable answer. So the very first character should be handled in different way, i.e. not to wait based on TXDRDY register.