app_uart_fifo, dma and buffers

[Fabien Comte]

Hello, I have some communication overflows on UART on nRF52, SDK 11 alpha, DMA active, big buffers (1024 bytes). I don't understand, nRF52 is not blocked in an interrupt and source is a poor 115200 bauds GPS.

When I look into SDK, DMA use 1 byte buffer. Am I right ? If it's correct what is the interrest of DMA ? Is there any DMA timeout like in some others CPU (call event hen enough bytes received or timeout elapsed) ?

[Hung Bui]

Hi Fabien,

I assume you are using the app_uart_fifo.c from SDK v11 alpha. If you are using the uart example included in the SDK, you should modify to increase UART_RX_BUF_SIZE to the size you want. It was default in the example configured to be 1, maybe that's the reason you got the overflow.

The EasyDMA support multiple bytes buffer. When the easy DMA reached the number of max byte in buffer MAXCNT, you will receive ENDRX event.

Note that the app_uart_fifo.c doesn't really take advantage of the EasyDMA feature (we are updating it) so if you want to really use EasyDMA, I would suggest to use the UARTE peripheral directly.

But for normal use, as suggest above you can simply increase UART_RX_BUF_SIZE and you should be fine.

[Fabien Comte]

UART_RX_BUF_SIZE is not used for DMA but for FIFO, if you go deeper it use thèses 2 buffers.

static uint8_t tx_buffer[1];
static uint8_t rx_buffer[1];

[Fabien Comte]

RXTO event can be used to generate a timeout. How can i fire NRF_UARTE_EVENT_RXTO ?

[Fabien Comte]

I have this feeling that app_uart_fifo.c is sometimes dangerous.

    uint32_t app_uart_put(uint8_t byte)
    {
        uint32_t err_code;
    
        tx_tmp = byte;
        err_code = nrf_drv_uart_tx(&tx_tmp, 1);

// if TX END interrupt fire here, your byte will be never sent. For my point of view (can be false or course) you have to disable TX interrupt around this part.
    
        if (err_code == NRF_ERROR_BUSY)
        {
            err_code = app_fifo_put(&m_tx_fifo, byte);
        }
    
        return err_code;
    }

[Hung Bui]

Hi Fabien,

• Yes, as mentioned earlier, the app_uart doesn't really utilize the advantage of EasyDMA on the UARTE. But the nrf_drv_uart does use EasyDMA. So you may want to use nrf_drv_uart directly.

• You can fire NRF_UARTE_EVENT_RXTO by trigger a NRF_UARTE_TASK_STOPRX task (nrf_drv_uart_rx_abort)

• We have updated this function in the current SDK v11 production that released last week. Could you check if the potential issue is also solved. ?