RTC1 problem of accuracy

[TheRaven]

Hello,

I'm trying to use the APP_TIMER API to have tempo, but I've noticed a problem of accuracy when using the RTC1. I use 0 as prescaler, so the RTC1 runs at 32768Hz. But when I want a tempo of 32768 ticks (in order to have 1 second of tempo), I've got only 936ms (measured with an oscilloscope).

Here is my code :

#define APP_TIMER_PRESCALER              0                                        
#define APP_TIMER_MAX_TIMERS             9                                        
#define APP_TIMER_OP_QUEUE_SIZE          4                                                                        

static app_timer_id_t                    m_uneSeconde_timer_id;   



void initialisationLed(void)
{
	nrf_gpio_cfg_output(21);
	nrf_gpio_pin_clear(21);
}


static void forToggle_handler(void * p_context)
{
    nrf_gpio_pin_toggle(21);
	UNUSED_PARAMETER(p_context);
}


static void timers_init(void)
{
    uint32_t err_code;

    // Initialize timer module.
    APP_TIMER_INIT(APP_TIMER_PRESCALER, APP_TIMER_MAX_TIMERS, APP_TIMER_OP_QUEUE_SIZE, false);

    // Create timers.
    err_code = app_timer_create(&m_uneSeconde_timer_id,
                                APP_TIMER_MODE_REPEATED,
                                forToggle_handler);
    APP_ERROR_CHECK(err_code);
		NRF_CLOCK->TASKS_LFCLKSTART = 1;
	
}


static void application_timers_start(void)
{
    uint32_t err_code;

    // Start application timers.
    err_code = app_timer_start(m_uneSeconde_timer_id, 32767, NULL);
    APP_ERROR_CHECK(err_code);
}




int main(void)
{
    // Initialize.
    timers_init();
		initialisationLed();

    // Start execution.
    application_timers_start();

    // Enter main loop.
    for (;;)
    {
				__WFI();
    }
}

Is there an erro in my code, or do I have to put about 35000 ticks to have a 1.000s tempo ?

[Susheel Nuguru]

Interesting .... Your code looks correct, RTC1 uses LFCLK, which LFCLK have you selected? I see that the code starts the LFCLK but cant see which one. I am not sure if APP_TIMER selects any clock by default (and i do not have the code handy to verify) If you (or app_timer) have selected internal RC LFCLK, then the error seems to be inline with this clock jitter. Try to select the XTAL LFCLK explicitly and see if there is any improvement

[TheRaven]

I haven't selected a LFCLK. After selecting the XTAL LFCLK, 32768 represents exactly 1.000s on my osxilloscope, so it seems that APP_TIMER uses the internal clock by default !

Thank you very much for helping me to solve my problem !!