nRF9161 to lowest power mode

[Mart]

I am trying to make the nrf9161 go to very low power mode for 10 seconds and then come back online. 

Actually I would not mind if it were to start all over again when coming back around but currently on my nrf9161dk I cannot achieve < 3.2mA. 

I could not find any right source with an example but if someone has a good example source, please let me know. Otherwise, why doesn't the code that I provide here bring me anywhere below 3.2mA? 

#include <zephyr/kernel.h>
#include <zephyr/sys/printk.h>
#include <hal/nrf_power.h>
#include <hal/nrf_clock.h>
#include <hal/nrf_rtc.h>
#include <modem/lte_lc.h>

#define WAKEUP_SECONDS 10U  // Sleep duration in seconds

// Start LFCLK (needed for RTC)
static void start_lfclk(void)
{
    if (!NRF_CLOCK->LFCLKRUN) {
        NRF_CLOCK->LFCLKSRC = 2U; // LFXO (external crystal)
        NRF_CLOCK->TASKS_LFCLKSTART = 1;
        while (!NRF_CLOCK->EVENTS_LFCLKSTARTED) {}
    }
}

// Configure RTC0 as wake timer
static void setup_rtc_wakeup(uint32_t seconds)
{
    // RTC0 runs at 32.768 kHz / (prescaler + 1)
    nrf_rtc_prescaler_set(NRF_RTC0, 0); 
    nrf_rtc_cc_set(NRF_RTC0, 0, seconds * 32768UL);
    nrf_rtc_int_enable(NRF_RTC0, NRF_RTC_INT_COMPARE0_MASK);
    nrf_rtc_task_trigger(NRF_RTC0, NRF_RTC_TASK_START);
}

// Enter System OFF (deep sleep)
static void enter_system_off(void)
{
    // Must power down LTE modem first
    lte_lc_power_off();

    // Ensure all memory operations completed
    __DSB();
    __WFI(); // Wait for interrupt; chip powers down
}

// Check if we woke from System OFF
static bool woke_from_system_off(void)
{
    return (NRF_POWER->RESETREAS & (1U << 2)) != 0; // Bit 2 = OFF
}

// Clear wake reason
static void clear_wake_reason(void)
{
    NRF_POWER->RESETREAS = NRF_POWER->RESETREAS;
}

int main(void)
{
    while (1) {
        if (woke_from_system_off()) {
            printk("Woke from System OFF!\n");
            clear_wake_reason();
        } else {
            printk("Power-on or other reset\n");
        }

        // Start LFCLK for RTC
        start_lfclk();

        // Set RTC wakeup
        setup_rtc_wakeup(WAKEUP_SECONDS);
        printk("Entering System OFF for %d seconds...\n", WAKEUP_SECONDS);

        // Enter deep sleep
        enter_system_off();

        // Execution never reaches here in System OFF
        while (1) {
            k_sleep(K_MSEC(1000));
        }
    }
    return 0;
}

[Achim Kraus]

In general:

if you want to know the very lowest current of your board, just switch all off and don't care about the wake-up.

If you then want to achieve a more "real world" behavior, check, if PSM is available in your network and enable that.

In zephyr it's not common to try to do such things as sleeping and wake up on a low level. The modem will already sleep with PSM, the application MCU may also put in sleep with k_sleep. If you want to wake up on an external input, you may setup a GPIO as input interrupt.

Once your solution works with that, it may be possible to reduce the power consumption of the modem using RAl, but that also depends on the network and used protocol. If you want to use TLS/TCP, RAI may turn out to be not too realistic, but that's left to your own experience.