Large current spikes (1Hz) with softdevice enabled

[NLemire]

Hello, 

I am using an nRF52840 on a custom board running the BLE stack from SDK16.0.0. I have an application that periodically advertises for 10s and is idle in between these advertising events. During this idle period I measure very large current spikes (up to ~15mA) which occur at 1Hz. The WDT is enabled with a reload value of 2000, and set to CONFIG_BEHAVIOUR 8 (Pause in SLEEP, Run in HALT). Captures of the current waveform are provided below. 

Curiously, I modified the CONFIG_BEHAVIOUR to 0 (Pause in SLEEP and HALT), and the current spikes were still present but with reduced peak values (up to ~5mA).

I also went through and stepped through each part of initialization, and found that these spikes do not occur until I enabled the softdevice before going idle. Completely disabling the watchdog does not yield any further reductions. This leads me to believe the softdevice is responsible for this behavior, but I have not had luck at figuring how to fix this. Any suggestions would be appreciated, thank you!

[NLemire]

I dug into the idle state management and found what is causing the CPU to wake up every 1s. Looking at nrf_pwr_mgnt.c, a timer is created with a period of 1s (shown below). I was able to modify this timer and saw the current spikes shift respectively as well. PWR_MGMT_TIMER_REQUIRED is defined due to NRF_PWR_MGMT_CONFIG_AUTO_SHUTDOWN_RETRY being defined in sdk_config.h. Setting this value to 0, disables this timer and gets rid of the current spikes altogether, which is great. However, I do not understand what implication this might have on the application because I don't have more detail on what a "blocked shutdown procedure" is. Can you please explain what this event is so I can understand how it affects the application?

snippet from nrf_pwr_mgnt.c

#ifdef PWR_MGMT_TIMER_REQUIRED
    #include "app_timer.h"
    #define PWR_MGMT_TIMER_CREATE()     pwr_mgmt_timer_create()

    APP_TIMER_DEF(m_pwr_mgmt_timer);    /**< Timer used by this module. */

    /**@brief Handle events from m_pwr_mgmt_timer.
     */
    static void nrf_pwr_mgmt_timeout_handler(void * p_context)
    {
        PWR_MGMT_CPU_USAGE_MONITOR_UPDATE();
        PWR_MGMT_AUTO_SHUTDOWN_RETRY();
        PWR_MGMT_STANDBY_TIMEOUT_CHECK();
    }

    __STATIC_INLINE ret_code_t pwr_mgmt_timer_create(void)
    {
        ret_code_t ret_code = app_timer_create(&m_pwr_mgmt_timer,
                                               APP_TIMER_MODE_REPEATED,
                                               nrf_pwr_mgmt_timeout_handler);
        if (ret_code != NRF_SUCCESS)
        {
            return ret_code;
        }

        return app_timer_start(m_pwr_mgmt_timer, APP_TIMER_TICKS(1000), NULL);
    }

#if NRF_PWR_MGMT_CONFIG_AUTO_SHUTDOWN_RETRY
    #undef  PWR_MGMT_TIMER_REQUIRED
    #define PWR_MGMT_TIMER_REQUIRED

snippet from sdk_config.h

// <q> NRF_PWR_MGMT_CONFIG_AUTO_SHUTDOWN_RETRY  - Blocked shutdown procedure will be retried every second.
 

#ifndef NRF_PWR_MGMT_CONFIG_AUTO_SHUTDOWN_RETRY
#define NRF_PWR_MGMT_CONFIG_AUTO_SHUTDOWN_RETRY 1
#endif