Fixes the issue that gen3 devices cannot be woken up by RTC.

[XuGuohui]

Problem

System.sleep(SystemSleepConfiguration().mode(SystemSleepMode::STOP).duration(3s)); doesn't work on Gen3 platforms.

Solution

This issue is caused by not clearing RTC event in its interrupt handler. After device being woken up in hal_sleep_enter(), it will repeat entering the RTC interrupt handler.

Steps to Test

• Build and flash the example below
• Build and flash the test application in user/tests/wiring/sleep20

Example App

#include "Particle.h"

SYSTEM_MODE(MANUAL);

void setup() {
    delay(1000);
    System.sleep(SystemSleepConfiguration().mode(SystemSleepMode::STOP).duration(3s));
}

void loop() {
}

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Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[avtolstoy]

Do we know which event in particular causes this? While it is fine to just clear everything in the handler, we should have been able to do that in hal_timer_init() after waking up from sleep and enable only what is required.

[XuGuohui]

We make use of the NRF_RTC_EVENT_COMPARE_0 event. Clearing events in hal_timer_init() doesn't work after waking up from sleep, as the interrupt handler is executed immediately following __enable_irq();, which is before hal_timer_init() being executed.

[avtolstoy]

That should not be happening though, right? We unbump the priority level of both the GPIOTE and RTC interrupts right before calling __enable_irq(). If they do get triggered that's a bug in the sleep code. This has also been working previously just fine for quite a while. This is not new code.

[avtolstoy]

Just to clarify: I meant that calling __enable_irq() at that point in sleep code does not unblock interrupts with priorities >= 2 because we are still under effect of BASEPRI. We do re-enable interrupts with priority >= 5 while waiting for SoftDevice to do certain things, but that should also not apply to RTC as it's configured normally at 6.