nrf52832 uart tx works only with NRF_UARTE0 read after EndTx

[FormerMember]

Greetings, There is a bit of strange operation in UARTE peripheral in nrf52832 with the DK. In the simple code below, if both of the if statements with comments 'Check 1' and 'Check 2' are removed the 'hello' prints are not sent and I get some junk value. If either of the two reads to the UARTE registers is made in these if statement, then the 'hello' prints are sent out correctly. What's happening? I don't see any errata related to UARTE peripheral.

Thanks.

Toolchain: GCC 4.9.3q2 on Linux

#include <stdbool.h>
#include <stdint.h>
#include "nrf_delay.h"
#include "nrf52-gpio.h"
#include "nrf52-clock.h"
#include "board.h"
#include "nrf52-uart.h"

int main(void)
{
	nrf_gpio_cfg_output(LED1);
	nrf_gpio_cfg_output(LED2);
	nrf_gpio_cfg_output(LED3);
	nrf_gpio_cfg_output(LED4);

	/* Configure TX and RX pins from board.h */
	nrf_gpio_cfg_output(TX_PIN_NUMBER);
	nrf_gpio_cfg_input(RX_PIN_NUMBER, GPIO_PIN_CNF_PULL_Disabled);
	NRF_UARTE0->PSEL.TXD = TX_PIN_NUMBER;
	NRF_UARTE0->PSEL.RTS = RX_PIN_NUMBER;

	nrf_gpio_cfg_output(RTS_PIN_NUMBER);
	nrf_gpio_cfg_input(CTS_PIN_NUMBER, GPIO_PIN_CNF_PULL_Disabled);
	NRF_UARTE0->PSEL.RTS = RTS_PIN_NUMBER;
	NRF_UARTE0->PSEL.CTS = CTS_PIN_NUMBER;
	NRF_UARTE0->CONFIG = (UARTE_CONFIG_HWFC_Enabled << UARTE_CONFIG_HWFC_Pos);

	/* Configure other UART parameters, BAUD rate is defined in nrf52-uart.h	*/
	NRF_UARTE0->BAUDRATE = (UARTE_BAUDRATE << UARTE_BAUDRATE_BAUDRATE_Pos);
	NRF_UARTE0->ENABLE = (UARTE_ENABLE_ENABLE_Enabled << UARTE_ENABLE_ENABLE_Pos);

    // Toggle LEDs.
    while (true)
    {
    	uint32_t i;
        for (i = 0; i < 2; i++)
        {
        	if(i){
        		NRF_P0->OUTSET = (1<<LED1);
        	} else {
        		NRF_P0->OUTCLR = (1<<LED1);
        	}
        	nrf_delay_ms(500);

            uint8_t str[] = "hello\n";
            uint32_t len = 6;

            NRF_UARTE0->EVENTS_ENDTX = 0;
            NRF_UARTE0->TASKS_STOPTX = 0;

        	NRF_UARTE0->TXD.PTR = (uint32_t)((uint8_t *) str);
        	NRF_UARTE0->TXD.MAXCNT = (uint32_t) len;

        	NRF_UARTE0->TASKS_STARTTX = 1;

        	while((0 == NRF_UARTE0->EVENTS_ENDTX)&&(0 == NRF_UARTE0->EVENTS_TXSTOPPED)){}

            /*****Check 1*****/
        	if(NRF_UARTE0->EVENTS_ENDTX){
        		NRF_P0->OUTSET = (1<<LED3);
        	}

            /*****Check 2*****/    
        	if(6 == NRF_UARTE0->TXD.AMOUNT){
        		NRF_P0->OUTSET = (1<<LED2);
        	}

            NRF_UARTE0->EVENTS_ENDTX = 0;
            NRF_UARTE0->TASKS_STOPTX = 0;
        }
    }
}

[Susheel Nuguru]

Finally, I am answering assuming that you are working on windows. This is GCC bug, if you are using 4_9-2015q2-20150609 There is some issue with it when you enable optimizations.

NRF_UARTE0->TASKS_STOPTX = 0;

You cannot do this, or more specifically it will have no effect.

files to analyse main.asm.dump main.no_asm.dump

[RK]

Aryan, I'm massively relieved if it's not a GCC bug so no need to apologise for that.

PS I don't know if you do this or not, you said that optimized gcc is hard to follow (and it is). I always compile optimised but still add full debug info, the code is still scrambled but at least it has source in it.

I've been looking at that code for a while and don't see what's wrong. Wondered if it could be due to the write buffer on the Cortex M4 delaying the TASKS_STARTTX=1 write and the while loop exiting instantly but that doesn't seem very likely. Have you tried setting EVENTS_TXSTOPPED to 0 explicitly before starting the transmission?