Timer3 Interrupt with compare with pin toggle - time is varying

[ricardo.oliveira]

Hi there.

I'm using nRF5 SDK 17.1.0 with nrf52840.

I've configured my timer3 to interrupt by compare with 10us ticks. When it does, I toggle a pin. But when I measure the high and low periods of the pin with osciloscope, high period is shorter than low (high=8.4 us, low=11.6 us) , and I don't know what may be happening.

Does anyone know why it's happening?

#define OUTPUT_3_PIN                NRF_GPIO_PIN_MAP(0,13)

void Gpio_Config(void){
    
    nrf_gpio_cfg_output(OUTPUT_3_PIN);
}

void TimerSpeed_Config(void){
    uint32_t time_ticks;
    uint32_t err_code = NRF_SUCCESS;

    nrf_drv_timer_config_t timer_cfg = NRF_DRV_TIMER_DEFAULT_CONFIG;

    err_code = nrf_drv_timer_init(&TIMER3, &timer_cfg, Timer3_EventHandler);
    APP_ERROR_CHECK(err_code);

    time_ticks = nrf_drv_timer_us_to_ticks(&TIMER3, 10);
    nrf_drv_timer_extended_compare(&TIMER3, NRF_TIMER_CC_CHANNEL3, time_ticks, NRF_TIMER_SHORT_COMPARE3_CLEAR_MASK, true);

    nrf_drv_timer_enable(&TIMER3);
}

void Timer3_EventHandler(nrf_timer_event_t event_type, void* p_context){
    switch (event_type){
        case NRF_TIMER_EVENT_COMPARE3: 
        {
            nrf_gpio_pin_toggle(OUTPUT_3_PIN);
        }
    }
}

[Sigurd]

Hi!

IRQ latency will vary depending on if there are other interrupts happening.

If you want highest possible precision, use GPIOTE+TIMER+PPI to do the toggling.

[ricardo.oliveira]

Hi, thanks for replying.
What is not clear for me is if the timer resets after compare interruption by itself or if it keeps counting until overflow it's limits (for example, if it's set to 16 bits, will it count till 65535 even after compare interrupt?)

[Sigurd]

ricardo.oliveira said:

What is not clear for me is if the timer resets after compare interruption by itself or if it keeps counting until overflow it's limits

since you are using NRF_TIMER_SHORT_COMPARE3_CLEAR_MASK , it's configured in HW to clear/reset itself when the compare3 event happens.

[ricardo.oliveira]

thanks!