Add example sketch.

examples/alarm_ex11/alarm_ex11.ino

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+// Arduino DS3232RTC Library
+// https://github.com/JChristensen/DS3232RTC
+// Copyright (C) 2022 by Jack Christensen and licensed under
+// GNU GPL v3.0, https://www.gnu.org/licenses/gpl.html
+//
+// Example sketch to create an interrupt with Alarm 1
+// at regular intervals.
+// Assumes the RTC is already set and running.
+//
+// Notes:
+// Using the INT/SQW pin for alarms is mutually exclusive with using
+// it to output a square wave. However, alarms can still be set when
+// a square wave is output, but then the alarm() function will need
+// to be used to determine if an alarm has triggered. Even though
+// the DS3231 power-on default for the INT/SQW pin is as an alarm
+// output, it's good practice to call DS3232RTC::squareWave(SQWAVE_NONE)
+// before setting alarms.
+//
+// I recommend calling DS3232RTC::alarm() before DS3232RTC::alarmInterrupt()
+// to ensure the RTC's alarm flag is cleared.
+//
+// Tested with Arduino 1.8.19, Arduino Uno, DS3231.
+// Connect RTC SDA to Arduino pin A4.
+// Connect RTC SCL to Arduino pin A5.
+// Connect RTC INT/SQW to Arduino pin 3.
+//
+// Jack Christensen 01Apr2022
+
+#include <DS3232RTC.h>              // https://github.com/JChristensen/DS3232RTC
+#include <Streaming.h>              // https://github.com/janelia-arduino/Streaming
+
+constexpr uint8_t sqwPin{3};        // RTC provides an interrupt signal on this pin
+                                    // (Can use Pin 2 (INT0) or Pin 3 (INT1) with Arduino Uno)
+constexpr time_t alarmInterval{60}; // alarm interval in seconds
+DS3232RTC myRTC;
+
+void setup()
+{
+    Serial.begin(115200);
+    Serial << F( "\n" __FILE__ "\n" __DATE__ " " __TIME__ "\n\n" );
+
+    // initialize the alarms to known values, clear the alarm flags, clear the alarm interrupt flags
+    myRTC.begin();
+    myRTC.setAlarm(DS3232RTC::ALM1_MATCH_DATE, 0, 0, 0, 1);
+    myRTC.setAlarm(DS3232RTC::ALM2_MATCH_DATE, 0, 0, 0, 1);
+    myRTC.alarm(DS3232RTC::ALARM_1);
+    myRTC.alarm(DS3232RTC::ALARM_2);
+    myRTC.alarmInterrupt(DS3232RTC::ALARM_1, false);
+    myRTC.alarmInterrupt(DS3232RTC::ALARM_2, false);
+    myRTC.squareWave(DS3232RTC::SQWAVE_NONE);
+
+    // configure an interrupt on the falling edge from the SQW pin
+    pinMode(sqwPin, INPUT_PULLUP);
+    attachInterrupt(digitalPinToInterrupt(sqwPin), alarmIsr, FALLING);
+
+    Serial << "Alarm interval is " << alarmInterval << " seconds\n";
+    time_t t = myRTC.get();
+    time_t a = t + alarmInterval - t % alarmInterval;
+    if (a <= t) a += alarmInterval;
+    // set the alarm
+    myRTC.setAlarm(DS3232RTC::ALM1_MATCH_HOURS, second(a), minute(a), hour(a), 0);
+    myRTC.alarm(DS3232RTC::ALARM_1);    // clear the alarm flag
+    myRTC.alarmInterrupt(DS3232RTC::ALARM_1, true);
+    printDateTime(t);
+    Serial << "  Current RTC time\n";
+    printDateTime(a);
+    Serial << "  Alarm1 set time\n";
+}
+
+volatile bool alarmFlag{false};
+
+void alarmIsr()
+{
+    alarmFlag = true;
+}
+
+void loop()
+{
+    if (alarmFlag) {
+        alarmFlag = false;
+        time_t t = myRTC.get();
+        time_t a = t + alarmInterval;
+        // set the alarm
+        myRTC.setAlarm(DS3232RTC::ALM1_MATCH_HOURS, second(a), minute(a), hour(a), 0);
+        myRTC.alarm(DS3232RTC::ALARM_1);    // clear the alarm flag
+        //myRTC.alarmInterrupt(DS3232RTC::ALARM_1, true);
+        printDateTime(t);
+        Serial << "  Alarm1 interrupt occurred\n";
+        printDateTime(a);
+        Serial << "  Alarm1 set time\n";
+    }
+}
+
+void printDateTime(time_t t)
+{
+    Serial << ((day(t)<10) ? "0" : "") << _DEC(day(t));
+    Serial << monthShortStr(month(t)) << _DEC(year(t)) << ' ';
+    Serial << ((hour(t)<10) ? "0" : "") << _DEC(hour(t)) << ':';
+    Serial << ((minute(t)<10) ? "0" : "") << _DEC(minute(t)) << ':';
+    Serial << ((second(t)<10) ? "0" : "") << _DEC(second(t));
+}