🛠 RTC timestamps and a minute alarm (no battery!)

Goal

Use the RA4M1 real-time clock to keep time, print a timestamp on every minute change, and fire an event (LED + serial line) as an alarm. Caveat up front: the board has no backup battery, so the clock restarts from the sketch value at every boot.

Code

#include "RTC.h"
RTCtime now;
uint8_t lastMinute = 255;

void setup {
  Serial.begin(115200);
  RTC.begin;
  RTCtime t(2026, 10, 14, 8, 0, 0);  // Y M D H M S, set once per boot
  RTC.set(t);
}

void loop {
  RTC.get(now);
  if (now.minute != lastMinute) {
    lastMinute = now.minute;
    Serial.printf("%04d-%02d-%02d %02d:%02d:%02d\n",
      now.year, now.month, now.dayofmonth,
      now.hour, now.minute, now.second);
    digitalWrite(13, !digitalRead(13));  // blink each minute
  }
  delay(100);
}

Expected result

One timestamp line per minute, LED toggling with it. Poll faster than once per minute or you will miss boundaries.

Common mistakes

• Expecting the time to survive a power cut: without a battery it resets each boot ; re-sync over Serial after upload.
• Calling RTC.set in loop: you freeze the clock.
• No daylight-saving logic in the library - handle it yourself.