RTC with PPI, interrupts stops after two events

[Ros]

Hi,

I'm trying to use RTC to generate interrupts at every second. The rtc event handler is below:

void rtc_event_handler(nrfx_rtc_int_type_t int_type)
{
    static int32_t cnt = 0;
    LOG_DBG("RTC Interrupt: %d, %d", int_type, cnt++);
    //nrfx_rtc_counter_clear(&rtc);
    //nrfx_rtc_cc_set(&rtc, 0, 32768, true);
}

In the RTC examples, nrfx_rtc_counter_clear is used to reset the RTC count; therefore, the interrupt event continues. Since my intention is to use this to trigger an ADC at 1 Hz, I used a PPI channel to clear the counter. However, there are only two interrupts after power-up, and then no more interrupts occur unless I uncomment either nrfx_rtc_counter_clear or nrfx_rtc_cc_set. The RTC initialization is shown below

const nrfx_rtc_t rtc = NRFX_RTC_INSTANCE(2);
#define NRFX_RTC_MS_TO_TICKS(us,freq) (((us) * (freq)) / 1000U)
void init_rtc()
{
    nrfx_err_t err;
    nrfx_rtc_config_t rtc_cfg = NRFX_RTC_DEFAULT_CONFIG;
    IRQ_CONNECT(DT_IRQN(DT_NODELABEL(rtc2)),
                DT_IRQ(DT_NODELABEL(rtc2), priority), 
                nrfx_isr, NRFX_RTC_IRQ_HANDLER(RTC_INSTANCE), 0);

    err = nrfx_rtc_init(&rtc, &rtc_cfg, rtc_event_handler);
    if (err != NRFX_SUCCESS) {
        LOG_ERR("Failed to initialize RTC-0, %d", err);
        return;
    }

    uint32_t rtc_ticks = NRFX_RTC_MS_TO_TICKS(SAMPLE_INTERVAL_MS, NRF_RTC_INPUT_FREQ);

    err = nrfx_rtc_cc_set(&rtc, 0, rtc_ticks, true);
    if (err != NRFX_SUCCESS) {
        LOG_ERR("Failed to set RTC comparator");
    }

    static uint8_t m_rtc_counter_clear;
    nrfx_ppi_channel_alloc(&m_rtc_counter_clear);

    /* channel, eep, tep task endpoint address*/
    nrfx_ppi_channel_assign(m_rtc_counter_clear, 
        nrfx_rtc_event_address_get(&rtc, NRF_RTC_EVENT_COMPARE_0),
            nrfx_rtc_task_address_get(&rtc, NRF_RTC_TASK_CLEAR));


    nrfx_ppi_channel_enable(m_rtc_counter_clear);

    nrfx_rtc_enable(&rtc);
}

And the output is

```

*** Booting nRF Connect SDK v2.9.0-7787b2649840 ***
*** Using Zephyr OS v3.7.99-1f8f3dc29142 ***
I: Starting Application
D: RTC Interrupt: 0, 0
D: RTC Interrupt: 0, 1

```

[Sigurd]

Hi!

Ros said:

Since my intention is to use this to trigger an ADC at 1 Hz

What chip are you using? I see you have a nRF52820 tag, but the nRF52820 does not have ADC.

https://docs.nordicsemi.com/bundle/struct_nrf52/page/struct/nrf52.html

[Ros]

I'm using nRF52840.

[Sigurd]

Hi,

I would recommend using a kernel timer for this instead.

https://docs.zephyrproject.org/latest/kernel/services/timing/timers.html#using-a-timer-expiry-function

void my_work_handler(struct k_work *work)
{
    /* do the processing that needs to be done periodically */
    ...
}

K_WORK_DEFINE(my_work, my_work_handler);

void my_timer_handler(struct k_timer *dummy)
{
    k_work_submit(&my_work);
}

K_TIMER_DEFINE(my_timer, my_timer_handler, NULL);

...

/* start a periodic timer that expires once every second */
k_timer_start(&my_timer, K_SECONDS(1), K_SECONDS(1));

[Ros]

Sigurd said:

I would recommend using a kernel timer for this instead.

This is a low-power application, so I wish not wake-up cpu before ADC sampling completes.

[Sigurd]

Hi!

The zephyr kernel timer uses RTC. Set it up to get trigger callback every 1 second, and start the sampling in the callback.