Why can't the UARTE of nRF52840 perform loopback testing?

Question

I am developing UARTE using nRF52840.I use a wire to connect the TX and RX pins together, and send something out the TX. 
 However, RX did not receive any data, and the interrupt handler's RX_DONE did not trigger, nor did the RXDR event. Only the TX_DONE triggered in the interrupt, indicating that data had been sent out. 
 However, everything works fine when not performing loopback testing; nRF52840 can receive messages from other peripherals, and the interrupt handler's RX_DONE can trigger, with the RXDR event also occurring. Why is this happening, and how should I solve this problem? Thanks.

hmolesworth · Accepted Answer

Yes simultaneous transmit and receive works in UARTE loopback; try this code. Remember Rx has to be started before issuing the Tx, using DMA for both. This is tested code. The Rx complete can just be a delay or use Rx Events, but short delay is good for testing. 
 static void SetupUART(const uint32_t BaudRate);
static void StartUART_Transmit(const uint8_t * const pMsg, const uint16_t MsgLength);
static void DisableUART(void);
static uint16_t SetupUART_Receive(uint8_t * const pMsg, const uint16_t MaxMsgLength);
static uint16_t WaitUART_Receive(void);

static NRF_UARTE_Type *pUART = NRF_UARTE0;

#define PIN_UART_TX PIN_FEATHER_TXD
#define PIN_UART_RX PIN_FEATHER_RXD
#define MAX_TX_PACKET_SIZE 128 //2048 nRF52832 8-bit, nRF52840 16-bit

static uint8_t TxMsg[MAX_TX_PACKET_SIZE] = "Baudrate generator for the UART is the top 20 bits of a 32-bit field; add in rounding 0.5 ls bit";
#define TX_PACKET_SIZE sizeof(TxMsg)
#define RX_PACKET_SIZE (TX_PACKET_SIZE - 1)
static uint8_t RxMsg[RX_PACKET_SIZE] = "";

void UART_Loopback(void)
{
 // Enable crystal osc for external decices, don't care for loopback
 // Setup UART from scratch for each test
 SetupUART(NRF_UART_BAUDRATE_230400);
 SetupUART_Receive(RxMsg, sizeof(RxMsg));
 StartUART_Transmit(TxMsg, TX_PACKET_SIZE);
 WaitUART_Receive();
 DisableUART();
 if ((memcmp(TxMsg, RxMsg, RX_PACKET_SIZE) == 0) && (pUART->EVENTS_ERROR == 0) && (pUART->ERRORSRC == 0x0000))
 {
 PassCount++;
 }
 while (1) ;
}

static void SetupUART(const uint32_t BaudRate)
{
 // Disable the UARTE
 pUART->ENABLE = 0;
 // Configure UARTE with no flow control, no parity bit and selected baud rate
 pUART->CONFIG = 0;
 pUART->BAUDRATE = BaudRate;
 // Select TX and RX pin default disconnected mode to avoid sending break indication on disabling uart ie NRF_GPIO_PIN_INPUT_DISCONNECT on Rx
 nrf_gpio_cfg(PIN_UART_RX, NRF_GPIO_PIN_DIR_INPUT, NRF_GPIO_PIN_INPUT_CONNECT, NRF_GPIO_PIN_PULLUP, NRF_GPIO_PIN_S0S1, NRF_GPIO_PIN_NOSENSE);
 nrf_gpio_cfg(PIN_UART_TX, NRF_GPIO_PIN_DIR_OUTPUT, NRF_GPIO_PIN_INPUT_CONNECT, NRF_GPIO_PIN_PULLUP, NRF_GPIO_PIN_H0H1, NRF_GPIO_PIN_NOSENSE);
 // Select TX and RX pins; not for Loopback testing or 1-wire or RS485 a single pin can be used for both Rx and Tx
 pUART->PSEL.TXD = PIN_UART_TX;
 pUART->PSEL.RXD = PIN_UART_TX; // For single pin loopback use PIN_UART_TX here, normal uart use PIN_UART_RX
 // Disable all interrupts and clear events
 pUART->INTENCLR = 0xFFFFFFFFUL;
 pUART->EVENTS_TXSTARTED = 0;
 pUART->EVENTS_TXSTOPPED = 0;
 pUART->EVENTS_RXSTARTED = 0;
 pUART->EVENTS_ERROR = 0;
 pUART->ERRORSRC = 0x000F; // Write '1's to clear any errors left set
 pUART->EVENTS_ENDTX = 0;
 pUART->EVENTS_ENDRX = 0;
 // Enable the UART, note 0x04 for UART no DMA and 0x08 for UARTE with DMA
 pUART->ENABLE = 0x08;
}
STATIC_ASSERT (UARTE_ENABLE_ENABLE_Enabled == 0x08, "UARTE_ENABLE_ENABLE_Enabled == 0x08 Failed");

static void StartUART_Transmit(const uint8_t * const pMsg, const uint16_t MsgLength)
{
 // Configure transmit buffer and start the transfer
 pUART->TXD.MAXCNT = MsgLength;
 pUART->TXD.PTR = (uint32_t)pMsg;
 pUART->TASKS_STARTTX = 1;
 __DSB();
 // Wait until the transfer start event is indicated
 while (pUART->EVENTS_TXSTARTED == 0) ;
 // Wait until the transfer is complete
 while (pUART->EVENTS_ENDTX == 0) ; //add timeout here also add LEDs blinking during tests

 // Stop the UART TX
 pUART->TASKS_STOPTX = 1;
 __DSB();
 // Wait until we receive the stopped event
 while (pUART->EVENTS_TXSTOPPED == 0) ;
}

static void DisableUART(void)
{
 // De-Select TX and RX pins
 pUART->PSEL.TXD = 0x80000000;
 pUART->PSEL.RXD = 0x80000000;
 // Disable the UARTE
 pUART->ENABLE = 0;
}

static uint16_t SetupUART_Receive(uint8_t * const pMsg, const uint16_t MaxMsgLength)
{
 volatile uint32_t Timeout;
 // Clear receive buffer
 memcpy(pMsg, "-----------------", MaxMsgLength);
 // Configure receive buffer and start reception
 pUART->RXD.MAXCNT = MaxMsgLength;
 pUART->RXD.PTR = (uint32_t)pMsg;
 pUART->EVENTS_ENDRX = 0;
 pUART->TASKS_STARTRX = 1;
 __DSB();
 // Wait until the transfer start event is indicated
 while (pUART->EVENTS_RXSTARTED == 0) ;
 return 0;
}

static uint16_t WaitUART_Receive(void)
{
 // Wait until the transfer is complete
 //while (pUART->EVENTS_ENDRX == 0) ; //add timeout here also add LEDs blinking during tests
 // just delay ..
 nrf_delay_ms(1);
 // Stop the UART RX
 pUART->TASKS_STOPRX = 1;
 __DSB();
 // Wait until we receive the stopped event
 //while (pUART->EVENTS_ENDRX == 0) ;
 // Return packet when postLastRxByteTime indicates end-of-packet timeout
 return 0;
}

Why can't the UARTE of nRF52840 perform loopback testing?

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