Improve Hardware UART Speed

[davidz]

I am using NRFSDK 1702. I have one NRF52840 as central and multiple peripherals. The central receives data from the peripherals and then transmit the received data via hardware UART interface to the computer. The UART baud rate is set at 115,200. The UART initialization function at the NRF52840 (central) is as follows:

static void uart_init(void)
{
    ret_code_t err_code;

    app_uart_comm_params_t const comm_params =
    {
        .rx_pin_no    = RX_PIN_NUMBER,
        .tx_pin_no    = TX_PIN_NUMBER,
        .rts_pin_no   = RTS_PIN_NUMBER,
        .cts_pin_no   = CTS_PIN_NUMBER,
        .flow_control = APP_UART_FLOW_CONTROL_DISABLED,
        .use_parity   = false,
        .baud_rate    = UART_BAUDRATE_BAUDRATE_Baud115200
    };

    APP_UART_FIFO_INIT(&comm_params,
                       UART_RX_BUF_SIZE,
                       UART_TX_BUF_SIZE,
                       uart_event_handle,
                       APP_IRQ_PRIORITY_LOWEST,
                       err_code);

    APP_ERROR_CHECK(err_code);
}

Previously, the data length from the NRF52840 to the computer is less than 20 bytes each time and the UART is working well. Recently, I found one issue with the hardware UART: when the data length is at 240 a packet, then the UART transfer speed can not catch up and will cause the NRF52840 reset. If I lower the data rate from the peripheral to the central, then the NRF52840 central is working well. 

The UART data transfer function is very simple (taken from the SDK examples):

static void uart_send(uint8_t * p_data, uint16_t data_len)
{
   ret_code_t ret_val;
      for (uint16_t i = 0; i < data_len; i++)
        {
            do{
              ret_val=app_uart_put(p_data[i]);
              if ((ret_val != NRF_SUCCESS) && (ret_val != NRF_ERROR_BUSY)) 
                APP_ERROR_CHECK(ret_val);
            }while (ret_val == NRF_ERROR_BUSY);
         }
}

The above function will error out when there are multiple data coming in. Is there a way to improve the UART speed? For example, how to transfer multiple bytes a time instead of just one byte a time via the UART function. 

[Vidar Berg]

Hello,

The app uart library can only send one byte at a time and thus not as efficient when you need to send larger chunks of data. I think it maybe the easiest way to improve the transfer speed is to work directly with the UART driver (UART) and set up larger transmit buffers to utilize the DMA support in UARTE.  

Best regards,

Vidar

[davidz]

Thank you very much Vidar. I have two more questions:

1. How big can the UART_TX_BUF_SIZE and UART_RX_BUF_SIZE be? Is it safe to give a number of 16384 or bigger?

2. Since there is no need for me to assign a big number for the UART_RX_BUF_SIZE, can I give different buffer size to UART RX and TX? From APP_UART_FIFO_INIT definition, it looks like it is safe to do so. 

[Vidar Berg]

No problem! 1. The buffer is limited by the size of the EasyDMA pointer, which on the nRF52840, is 16 bits long. This means that a single DMA buffer can be up to 65 kB. 2. Yes, the buffer for TX and RX is configured independently.

[davidz]

Thank you and this is very helpful. It is all clear now :)