UART Hardware Flow control does not seem to work - Connecting two nRF52840 dev boards together

[Olivier]

Hi all,
I am trying to get my head around UART Flow control on Nordic nRF52. I cannot get it working...
I have two nRF52840 dev boards (PCA10056) that I connected between each others:

Board 1 (Sender)     Board 2 (Receiver)
P0.3 ------------------- P0.4
P0.4 ------------------- P0.3
P0.28 ------------------ P0.29
P0.29 ------------------ P0.28

I have not got UART logging enabled. I use a logic analyzer to see what is being sent.
I noticed the Receiver does not block while waiting to receive.

Here is the source code:

The same source code is build with `-DSENDER=1` for the sender and no declaration for the receiver.
When the receiver is switched off, I can see my packet being sent from the sender.

#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include "app_uart.h"
#include "app_error.h"
#include "nrf_delay.h"
#include "nrf.h"
#include "bsp.h"
#if defined (UART_PRESENT)
#include "nrf_uart.h"
#endif
#if defined (UARTE_PRESENT)
#include "nrf_uarte.h"
#endif
#include "nrf_drv_uart.h"

#include "nrf_gpio.h"

#include "nrf_log.h"
#include "nrf_log_ctrl.h"
#include "nrf_log_default_backends.h"

uint8_t command[] = { 7, 198, 4, 8, 255, 159, 1, 253, 136 };

#define UART_RX_PIN   3
#define UART_TX_PIN   4
#define UART_RTS_PIN  28
#define UART_CTS_PIN  29

#define SUPPORT_HARDWARE_FLOW_CONTROL

uint8_t response[32];

/**
 * @brief Function for main application entry.
 */
int main(void)
{
    uint32_t err_code;
    ret_code_t ret;

    APP_ERROR_CHECK(NRF_LOG_INIT(NULL));
    NRF_LOG_DEFAULT_BACKENDS_INIT();

    /* Configure board. */
    bsp_board_leds_init();

#if SENDER
    NRF_LOG_ERROR("Sender");
#else
    NRF_LOG_ERROR("Receiver");
#endif

    static nrf_drv_uart_t app_uart_inst = NRF_DRV_UART_INSTANCE(0);

    nrf_drv_uart_config_t uart_config = NRF_DRV_UART_DEFAULT_CONFIG;
    uart_config.baudrate = 0x00275000UL; // 9600
#ifdef SUPPORT_HARDWARE_FLOW_CONTROL
    uart_config.hwfc = 1;
#else
    uart_config.hwfc = 0;
#endif
    uart_config.pselrxd = UART_RX_PIN;
    uart_config.pseltxd = UART_TX_PIN;
#ifdef SUPPORT_HARDWARE_FLOW_CONTROL
    uart_config.pselrts = UART_RTS_PIN;
    uart_config.pselcts = UART_CTS_PIN;
#endif
    uart_config.parity = 0; // No Parity
    uart_config.use_easy_dma = true;

    err_code = nrf_drv_uart_init(&app_uart_inst, &uart_config, NULL);
    if (err_code != NRF_SUCCESS) {
        NRF_LOG_ERROR("Error1 ErrCode:0x%x\n", err_code);
        while(1);
    }

    nrf_drv_uart_rx_enable(&app_uart_inst);

#if SENDER
    ret =  nrf_drv_uart_tx(&app_uart_inst, command, sizeof(command));
#else
    ret = nrf_drv_uart_rx(&app_uart_inst, response, sizeof(command));
#endif

    if (NRF_ERROR_BUSY == ret)
    {
        bsp_board_led_invert(1);
        NRF_LOG_ERROR("Error2");
    }
    else if (ret != NRF_SUCCESS)
    {
        bsp_board_led_invert(2);
        NRF_LOG_ERROR("Error3 ret:0x%x (error_reg:0x%02x)", ret, nrf_drv_uart_errorsrc_get(&app_uart_inst));
    }
    else
    {
        bsp_board_led_invert(0);
        bsp_board_led_invert(3);

#ifndef SENDER
        NRF_LOG_ERROR("Received: %02x%02x%02x%02x", response[0], response[1], response[2], response[3]);
#endif
    }

    while (true)
    {
    // Do nothing.
    }
}


/** @} */

[Matt]

Why do you need the "#if SENDER ... #else... #endif" when you have already done the physical cross wiring between the two units?

[Olivier]

Thanks Matt, you are right. By reversing the wire and the pinmux it did not help...

I have made some progress (and I also added some logging using the other UART). I edited the code with the latest version

Now, the receiver complains:

Error3 ret:0x3 (error_reg:0x4)

From the doc (http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52840.ps/uarte.html?cp=2_0_0_33_9_4#register.ERRORSRC), it means it is a FRAMING error (A valid stop bit is not detected on the serial data input after all bits in a character have been received.).

I am guessing I forgot something when sending my command. But I cannot find what is missing from the sender...

[Olivier]

Actually the logic analyzer see the framing error. But I cannot find the error in my code.

[Edvin]

Have you tested any of the uart examples, either SDK\examples\peripheral\uart or SDK\examples\peripheral\serial?

Do you see the same behavior there?

 

Best regards,
Edvin

[Olivier]

I have just tried with a slightly modified version of SDK\examples\peripheral\uart to be used between two nRF52840 DK boards with HW flow control.

The boards are connected as follow:

Board 1 (Sender)     Board 2 (Receiver)
P0.3 (TX)  ------------------- P0.3 (RX)
P0.4  (RTS) ------------------ P0.4 (CTS)
P0.28 (CTS) ------------------ P0.28 (RTS)

I changed the code as follow (I am using nRF5_SDK_14.2.0):

--- a/nRF5_SDK_14.2.0_17b948a/examples/peripheral/uart/main.c
+++ b/../../../main.c
@@ -62,6 +62,17 @@
 #include "nrf_uarte.h"
 #endif
 
+#ifdef SENDER
+#define UART_RX_PIN_NUMBER  8  /* Received characters from the terminal */
+#define UART_TX_PIN_NUMBER  3  /* Send it to P0.3 */
+#define UART_CTS_PIN_NUMBER 4  /* CTS: P0.4 */
+#define UART_RTS_PIN_NUMBER 28 /* RTS: P0.28 */
+#else
+#define UART_RX_PIN_NUMBER  3  /* Receive characters from Sender */
+#define UART_TX_PIN_NUMBER  6  /* Send characters to the terminal */
+#define UART_CTS_PIN_NUMBER 28 /* RTS: P0.28 */
+#define UART_RTS_PIN_NUMBER 4  /* RTS: P0.4 */
+#endif
 
 //#define ENABLE_LOOPBACK_TEST  /**< if defined, then this example will be a loopback test, which means that TX should be connected to RX to get data loopback. */
 
@@ -126,9 +137,10 @@ static void uart_loopback_test()
 }
 #else
 /* When UART is used for communication with the host do not use flow control.*/
-#define UART_HWFC APP_UART_FLOW_CONTROL_DISABLED
+//#define UART_HWFC APP_UART_FLOW_CONTROL_ENABLED
 #endif
 
 /**
  * @brief Function for main application entry.
@@ -141,10 +153,10 @@ int main(void)
 
     const app_uart_comm_params_t comm_params =
       {
-          RX_PIN_NUMBER,
-          TX_PIN_NUMBER,
-          RTS_PIN_NUMBER,
-          CTS_PIN_NUMBER,
+          UART_RX_PIN_NUMBER,
+         UART_TX_PIN_NUMBER,
+         UART_RTS_PIN_NUMBER,
+         UART_CTS_PIN_NUMBER,
           UART_HWFC,
           false,
           NRF_UART_BAUDRATE_115200
@@ -159,6 +171,12 @@ int main(void)

I cannot see the RTS/CTS signal change. I was expecting to see changes with RTS and CTS signals.