Using an ATSAMD21 with the Arduino core, I'm running into an issue where the RTCZero alarm callback won't continue running the loop() function. If I schedule another alarm inside the callback, the second alarm will execute the loop() function after the callback is done. Every second time it works, every other time it doesn't.

Minimal example in which I expect the LED to change state every 2 seconds, which does not happen:

#include <RTCZero.h>
#include <time.h>

RTCZero rtc;

void setup()
  digitalWrite(LED_BUILTIN, LOW);


void loop()
  digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
  rtc.standbyMode();    // Sleep until next alarm match

void schedule_wake_up() {
  time_t wakeupTimestamp = rtc.getEpoch();
  wakeupTimestamp += 2;
  struct tm *wakeupTimeStruct = gmtime(&wakeupTimestamp);

  rtc.setAlarmSeconds( wakeupTimeStruct->tm_sec );
  rtc.setAlarmMinutes( wakeupTimeStruct->tm_min );
  rtc.setAlarmHours( wakeupTimeStruct->tm_hour );

void alarmMatch()
  // Do nothing, just wake up
  • Arduino IDE 1.8.9
  • RTCZero ver 1.6.0
  • SAMD core ver 1.8.4 and 1.6.20 tried

1 Answer 1


I ended up deleting my copy of RTCZero and installing a new one via the Arduino IDE library manager. This solved the issue.

The difference between my local copy and the upstream one is in the standbyMode() function. I had the following PR merged into my local copy of this function: https://github.com/arduino-libraries/RTCZero/pull/46/files

After I installed the clean copy I went and replaced the standbyMode() function's content with the following: https://github.com/arduino-libraries/ArduinoLowPower/blob/892594adbccf45b198fc067ea994048c76523936/src/samd/ArduinoLowPower.cpp#L18

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