earn more about interrupts with the ESP8266 here<\/a>).<\/p>\n\n\n\/*********\n Rui Santos & Sara Santos - Random Nerd Tutorials\n Complete instructions at https:\/\/RandomNerdTutorials.com\/esp8266-nodemcu-ds3231-real-time-clock-arduino\/\n*********\/\n\/\/ Example based on the library: implementation of an alarm using DS3231\n#include <RTClib.h>\n\nRTC_DS3231 rtc;\n\nchar daysOfTheWeek[7][12] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};\n\n\/\/ the pin that is connected to SQW\n#define CLOCK_INTERRUPT_PIN 14\n\n\/\/ LED for visual indication\nconst int ledPin = 2;\n\n\/\/ set the alarms\n\/\/ (year, month, day, hour, minutes, seconds)\nDateTime alarm1Time = DateTime(2025, 01, 03, 18, 42, 0);\nDateTime alarm2Time = DateTime(2025, 01, 03, 18, 48, 0);\n\nvoid printCurrentTime(){\n \/\/ Get the current time from the RTC\n DateTime now = rtc.now();\n \n \/\/ Getting each time field in individual variables\n \/\/ And adding a leading zero when needed;\n String yearStr = String(now.year(), DEC);\n String monthStr = (now.month() < 10 ? "0" : "") + String(now.month(), DEC);\n String dayStr = (now.day() < 10 ? "0" : "") + String(now.day(), DEC);\n String hourStr = (now.hour() < 10 ? "0" : "") + String(now.hour(), DEC); \n String minuteStr = (now.minute() < 10 ? "0" : "") + String(now.minute(), DEC);\n String secondStr = (now.second() < 10 ? "0" : "") + String(now.second(), DEC);\n String dayOfWeek = daysOfTheWeek[now.dayOfTheWeek()];\n\n \/\/ Complete time string\n String formattedTime = dayOfWeek + ", " + yearStr + "-" + monthStr + "-" + dayStr + " " + hourStr + ":" + minuteStr + ":" + secondStr;\n\n \/\/ Print the complete formatted time\n Serial.println(formattedTime);\n}\n\nICACHE_RAM_ATTR void onAlarm() {\n Serial.println("Alarm occured!");\n \/\/ toggle the current LED state\n int state = digitalRead(ledPin);\n digitalWrite(ledPin, !state);\n}\n\nvoid setup() {\n Serial.begin(115200);\n pinMode (ledPin, OUTPUT);\n digitalWrite(ledPin, LOW);\n\n \/\/ initializing the rtc\n if(!rtc.begin()) {\n Serial.println("Couldn't find RTC!");\n Serial.flush();\n while (1) delay(10);\n }\n\n if(rtc.lostPower()) {\n \/\/ this will adjust to the date and time at compilation\n rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));\n }\n \/\/ Uncomment if you need to adjust the time\n \/\/ rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));\n\n \/\/we don't need the 32K Pin, so disable it\n rtc.disable32K();\n\n \/\/ Trigger an interrupt when the alarm happens\n pinMode(CLOCK_INTERRUPT_PIN, INPUT_PULLUP);\n attachInterrupt(digitalPinToInterrupt(CLOCK_INTERRUPT_PIN), onAlarm, FALLING);\n\n \/\/ set alarm 1, 2 flag to false (so alarm 1, 2 didn't happen so far)\n \/\/ if not done, this easily leads to problems, as both register aren't reset on reboot\/recompile\n rtc.clearAlarm(1);\n rtc.clearAlarm(2);\n\n \/\/ stop oscillating signals at SQW Pin, otherwise setAlarm1 will fail\n rtc.writeSqwPinMode(DS3231_OFF);\n\n \/\/ turn off alarm 2 (in case it isn't off already)\n \/\/ again, this isn't done at reboot, so a previously set alarm could easily go overlooked\n rtc.disableAlarm(2);\n\n \/\/ Schedule Alarm1 to fire when the minutes match\n if(!rtc.setAlarm1(alarm1Time, DS3231_A1_Second)) { \/\/ this mode triggers the alarm when the minutes match\n Serial.println("Error, alarm wasn't set!");\n }else {\n Serial.println("Alarm 1 will happen at specified time");\n }\n}\n\nvoid loop() {\n \/\/ print current date and time\n printCurrentTime();\n\n \/\/ Get Details about the alarm1\n DateTime alarm1 = rtc.getAlarm1();\n Ds3231Alarm1Mode alarm1mode = rtc.getAlarm1Mode();\n char alarm1Date[12] = "DD hh:mm:ss";\n alarm1.toString(alarm1Date);\n Serial.print("[Alarm1: ");\n Serial.print(alarm1Date);\n Serial.print(", Mode: ");\n switch (alarm1mode) {\n case DS3231_A1_PerSecond: Serial.print("PerSecond"); break;\n case DS3231_A1_Second: Serial.print("Second"); break;\n case DS3231_A1_Minute: Serial.print("Minute"); break;\n case DS3231_A1_Hour: Serial.print("Hour"); break;\n case DS3231_A1_Date: Serial.print("Date"); break;\n case DS3231_A1_Day: Serial.print("Day"); break;\n }\n \/\/ the value at SQW-Pin (because of pullup 1 means no alarm)\n Serial.print("] SQW: ");\n Serial.print(digitalRead(CLOCK_INTERRUPT_PIN));\n \/\/ whether a alarm fired\n Serial.print(" Fired: ");\n Serial.print(rtc.alarmFired(1));\n\n \/\/ Only one alarm can be set at a time, reset alarm 1 and activate alarm 2\n \/\/ resetting SQW and alarm 1 flag\n \/\/ the next alarm could now be configurated\n if (rtc.alarmFired(1)) {\n rtc.clearAlarm(1);\n Serial.println(" - Alarm cleared");\n \n \/\/ Set Alarm 2\n if(!rtc.setAlarm2(alarm2Time, DS3231_A2_Minute)) { \/\/ this mode triggers the alarm when the minutes match\n Serial.println("Error, alarm wasn't set!");\n }else {\n Serial.println("Alarm 2 will happen at specified time");\n }\n\n \/\/ Get Details about the alarm2\n DateTime alarm1 = rtc.getAlarm2();\n Ds3231Alarm2Mode alarm2mode = rtc.getAlarm2Mode();\n char alarm2Date[12] = "DD hh:mm:ss";\n alarm1.toString(alarm2Date);\n Serial.print("[Alarm2: ");\n Serial.print(alarm2Date);\n Serial.print(", Mode: ");\n switch (alarm2mode) {\n case DS3231_A2_PerMinute: Serial.print("Every Minute"); break;\n case DS3231_A2_Minute: Serial.print("Minute"); break;\n case DS3231_A2_Hour: Serial.print("Hour"); break;\n case DS3231_A2_Date: Serial.print("Date"); break;\n case DS3231_A2_Day: Serial.print("Day"); break;\n }\n \/\/ the value at SQW-Pin (because of pullup 1 means no alarm)\n Serial.print("] SQW: ");\n Serial.print(digitalRead(CLOCK_INTERRUPT_PIN));\n \/\/ whether a alarm fired\n Serial.print(" Fired: ");\n Serial.print(rtc.alarmFired(1));\n }\n\n Serial.println();\n delay(2000);\n}\n<\/code><\/pre>\n\tView raw code<\/a><\/p>\n\n\n\nImportant notes about the alarms:<\/p>\n\n\n\n
\n- the RTC allows you to save up to two alarms;<\/li>\n\n\n\n
- you can only have one alarm active at a time;<\/li>\n\n\n\n
- after an alarm is triggered, you must clear its flag to avoid triggering and crashing the ESP8266;<\/li>\n\n\n\n
- you must deactivate an alarm before activating the other.<\/li>\n<\/ul>\n\n\n\n
How Does the Code Work?<\/h3>\n\n\n\n
We’ve already covered how to initialize the RTC, and set and get the time from the DS3231 module. We’ll just explain the relevant parts related to the alarms.<\/p>\n\n\n\n
Define the GPIO that is connected to the SQW pin. In our case, we’re using GPIO 14, you can use any other suitable digital GPIO<\/a>.<\/p>\n\n\n\n\/\/ the pin that is connected to SQW\n#define CLOCK_INTERRUPT_PIN 14<\/code><\/pre>\n\n\n\nFirst, we create two DateTime<\/span> objects to set the time for alarm 1 and alarm 2.<\/p>\n\n\n\nDateTime alarm1Time = DateTime(2025, 01, 03, 18, 42, 0);\nDateTime alarm2Time = DateTime(2025, 01, 03, 18, 48, 0);<\/code><\/pre>\n\n\n\nThen, we need to disable the 32k pin because we won’t use it.<\/p>\n\n\n\n
rtc.disable32K();<\/code><\/pre>\n\n\n\nSet the SQW pin as an interrupt so that we can trigger a function when an alarm is fired. The SQW is active low, which means its state is normally HIGH and changes to LOW when an alarm is fired. When the alarm is triggered, we’ll call the onAlarm<\/span> function.<\/p>\n\n\n\n\/\/ Trigger an interrupt when the alarm happens\npinMode(CLOCK_INTERRUPT_PIN, INPUT_PULLUP);\nattachInterrupt(digitalPinToInterrupt(CLOCK_INTERRUPT_PIN), onAlarm, FALLING);<\/code><\/pre>\n\n\n\nThe onAlarm()<\/span> function, defined previously in the code, prints a message in the Serial Monitor and inverts the current state of GPIO 2 (connected to the ESP8266 onboard LED).<\/p>\n\n\n\nICACHE_RAM_ATTR void onAlarm() {\n Serial.println(\"Alarm occured!\");\n \/\/ toggle the current LED state\n int state = digitalRead(ledPin);\n digitalWrite(ledPin, !state);\n}<\/code><\/pre>\n\n\n\nNote:<\/strong> the function to be called when the interrupt is triggered needs to have the ICACHE_RAM_ATTR<\/span> before the function definition to run the interrupt code in RAM.<\/p>\n\n\n\nClear both alarm 1 and alarm 2 before setting any alarm.<\/p>\n\n\n\n
rtc.clearAlarm(1);\nrtc.clearAlarm(2);<\/code><\/pre>\n\n\n\n
![\"ESP8266](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/ESP8266-DS3231-RTC-Arduino-IDE.jpg?resize=1200%2C675&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"RTC](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2024\/12\/RTC3231-DS1307-RTC-Modules.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"RTC](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2024\/12\/RTC-DS32321-Module.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"RTC](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2024\/12\/RTC-DS32321-Module-back.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure>\n<\/div>\n\n\n\n![\"DS3231](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2024\/12\/RTC-DS3231-Battery-Holder.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure>\n<\/div>\n<\/div>\n\n\n\n![\"RTC](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2024\/12\/RTC-DS32321-remove-resistor-for-nor-rechargeable-battery.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"DS3231](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2024\/12\/ds3231-i2c-address.png?resize=666%2C253&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2017\/10\/header-200.png?w=1200&quality=100&strip=all&ssl=1\")
<\/a><\/p>\n\n\n\n![\"ESP32](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/esp8266-ds3231_wiring.png?resize=993%2C533&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"Arduino](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2024\/11\/install-RTCLib.png?resize=750%2C391&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"ESP8266](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/ESP8266-DS3231-RTC-Module-Get-Time.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"ESP8266](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/ESP8266-Set-Get-Time-Temperature-DS3231.png?resize=738%2C352&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"ESP8266](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/DS3231-ESP8266-Setting-Alarms.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n