timestamping w/o active timer (APP_TIMER_KEEPS_RTC_ACTIVE = 1)

[martin]

hello,

i am using the nrf52840 preview devkit and am currently trying to find a solution for timestamping / for a time-base. the idea was to use the RTC for that since it provides a pretty accurate time reference and i really don't want to create ISRs or my own tick-implementation.

i found the APP_TIMER module, looked at the WDT example to get up the lf-clock and also found the configuration macro to keep the RTC alive (APP_TIMER_KEEPS_RTC_ACTIVE) but it somehow doesn't seem to work, i.e., if i don't have some dummy counter running then i can't get any values with app_timer_cnt_get (always zero see other questions).

is this a known issue? the following is a reduced example which i basically copied together from the csense driver to get the app_timer_cnt_get function up and running but only with a dummy timer:

#define APP_TIMER_PRESCALER     0
#define APP_TIMER_OP_QUEUE_SIZE 2

/* Time between RTC interrupts. */
#define APP_TIMER_TICKS_TIMEOUT 2000

static ret_code_t clock_config(void)
{
    ret_code_t err_code;

    err_code = nrf_drv_clock_init();
    if (err_code != NRF_SUCCESS)
    {
        return err_code;
    }

    nrf_drv_clock_lfclk_request(NULL);

    return NRF_SUCCESS;
}

static void timeout_handler(void * p_context)
{
    NRF_LOG_INFO("Busy.\r\n");
}

void start_app_timer(void)
{
    ret_code_t err_code;
        
    /* APP_TIMER definition for csense example. */
    APP_TIMER_DEF(timer_0);

    err_code = app_timer_create(&timer_0, APP_TIMER_MODE_REPEATED, timeout_handler);
    APP_ERROR_CHECK(err_code);

    err_code = app_timer_start(timer_0, APP_TIMER_TICKS_TIMEOUT, NULL);
    APP_ERROR_CHECK(err_code);
}


uint32_t rtc_t0;

int main(void)
{
    uint32_t err_code = NRF_SUCCESS;
    bsp_board_leds_init();

    err_code = clock_config();
    APP_ERROR_CHECK(err_code);
    APP_TIMER_INIT(APP_TIMER_PRESCALER, APP_TIMER_OP_QUEUE_SIZE, NULL);
    
    start_app_timer(); 

    while (1)
    {
        rtc_t0 = app_timer_cnt_get ();
        __WFI();
    }
}

the timestamp rc_t0 will stay zero if i do not execute start_app_timer regardless of the SDK configuration (macro APP_TIMER_KEEPS_RTC_ACTIVE).

the workaround would be to use the dummy app-timer but i'd really like to avoid this since i cannot initialize a timer without callback.

also something that i still need to investigate: does this dummy timer then also have an influence on the resolution i get from app_timer_cnt_get ?

[Jørgen Holmefjord]

rtc1_start() will not be called in app_timer before a timer is created. You can bypass this by adding a call to rtc1_start() at the end of app_timer_init().

[martin]

thanks for the quick reply! that seems to do something but does not produce the expected outcome. somehow i can only see that a new value (e.g., rtc_t0) is only generated once i break ... so it behaves quite differently. or is there something else i'm missing?

[Jørgen Holmefjord]

How do you check the variable? I tested your code and it seems to work fine. Attachecd project for SDK 13.1.0 (only Keil project and hex-files updated): app_timer.zip. Can you test this on your board? It will output value to UART.

[martin]

hello - i tried the attached project. you still have the start_app_timer () in there - thus a dummy-timer will be initialized and everything works. once you remove this line your output should also stall. tried it with UART as well as J-Link (using ozone).

to summarize: when i remove the start_app_timer() line plus add the rtc1_start() in app_timer_init i get ~2 prints of the RTC, then everything stops working. only if i break and resume the rtc_t0 gets updated.

[Hung Bui]

Hi martin,

What about using the RTC directly and skip the apptimer ?

Hung