How to empty RX buffer on nrf52840 UARTE0

RE: How to empty RX buffer on nrf52840 UARTE0

Oystein — Wed, 27 Aug 2025 18:34:18 GMT

Thansk a lot, the information about scanf, I was not aware of all the smart features. I will implemt as needed.

Vidar wrote:

Because of this I woud only expect \n to remain in your FIFO

yes, when I read the FIFO buffer (using the RXD:PTR pointer there is only one characters \n, but there is also a static bufffer

static app_fifo_t                  m_rx_fifo;                               /**< RX FIFO buffer for storing data received on the UART until the application fetches them using app_uart_get(). */

I guess this static buffer contains the last two characters. I did not investiage it anymore. I just run getchar 2 times to solve it.

//Run getchar() 2 times to empty the UARTE0 RX buffer. 
    getchar();
    getchar();

Øystein

RE: How to empty RX buffer on nrf52840 UARTE0

Vidar Berg — Fri, 22 Aug 2025 08:50:57 GMT

Thanks for the additional information. It sounds like our CLI library might be more suitable for what you are trying do. Anyway, scanf will call __getchar() -> app_uart_get() in a loop until it has read the first termination character from the UART driver’s FIFO buffer before exiting. Because of this I woud only expect \n to remain in your FIFO. It may also be worth adding that not all terminals use the same line endings by default.

[quote user="Oystein"]This times I know its two bytes, but maybe next time in some other program it may be 10 bytes. I would like to run a routine that check how many unread bytes there is and clear all of them?[/quote]

The app uart library which is used by the stdio functions does not expose any functions to peek into the FIFO buffer unfortunately.

[quote user="Oystein"]smarter low level UARTE0 register I can set to clear RX buffer[/quote]

It seems like you have scanf ignore a leading newline by inserting a whitespace before the format specifier. So scanf(" %s", string1); instead of scanf("%s", string1);

https://cplusplus.com/reference/cstdio/scanf/

[quote user="Oystein"]program it may be 10 bytes.[/quote]

I would consider setting a max width for scanf to not allow the string buffer to potentially overflow.

RE: How to empty RX buffer on nrf52840 UARTE0

Oystein — Thu, 21 Aug 2025 21:03:55 GMT

        memset(string1, 0, sizeof(string1));
        memset(string2, 0, sizeof(string2));  
        scanf("%s", string1);
        i=0;

        while( (i < 256) && (string1[i]!='(') && (string1[i]!=0) ) {
          string2[i] = string1[i];
          i=i+1;
        }

I use nRF5_SDK_17.1.0_ddde560, use the UART pheripheral example/PCA10056 as a start base for my project and develped it from there.

I use scanf function to read in te command string which always is appended with \n\r by my UART serial terminal program (REALTERM)

I am not completly sure about this but as I understand it, SCANF function reads in all characters until a whitepspace, or \r, or \n is sent. So the last two characters sent \n\r is never read out from the RX buffer. So thats why always the first two characters I read in when I run

else if ( strcmp(string2, "send_binary_lpc_code") == 0 )

are always \r\n, and I would like to clear the RX buffer before using getchar(). I have solved it for this use case bu just running getchar two times first, but is there some smarter low level UARTE0 register I can set to clear RX buffer. This times I know its two bytes, but maybe next time in some other program it may be 10 bytes. I would like to run a routine that check how many unread bytes there is and clear all of them?

RE: How to empty RX buffer on nrf52840 UARTE0

Vidar Berg — Thu, 21 Aug 2025 11:12:03 GMT

Hi,

Would it be an option to just append \n\r to every command?

E.g.,

else if ( strcmp(string2, "send_binary_lpc_code\r\n") == 0 ) {

I’m not aware of any examples using getchar from stdio in our SDKs, and the actual implementation will also depend on which SDK and toolchain you are using.