RTC_DS3231 rtc;<\/code><\/pre>\n\n\n\nThen, you create a char array with the days of the week.<\/p>\n\n\n\n
char daysOfTheWeek[7][12] = {\"Sunday\", \"Monday\", \"Tuesday\", \"Wednesday\", \"Thursday\", \"Friday\", \"Saturday\"};<\/code><\/pre>\n\n\n\nYou can access each day by its index. For example:<\/p>\n\n\n\n
\ndaysOfTheWeek[0]<\/span> will return “Sunday”.<\/li>\n\n\n\ndaysOfTheWeek[1]<\/span> will return “Monday”.<\/li>\n<\/ul>\n\n\n\nIn the setup()<\/span>, initialize the Serial Monitor.<\/p>\n\n\n\nSerial.begin(9600);<\/code><\/pre>\n\n\n\nInitialize the RTC module as follows:<\/p>\n\n\n\n
if (! rtc.begin()) {\n Serial.println(\"Couldn't find RTC\");\n Serial.flush();\n while (1) delay(10);\n}<\/code><\/pre>\n\n\n\nChecking the RTC Status<\/h4>\n\n\n\n Then, check if the RTC has lost power with the lostPower()<\/span> function. If it is not running, because it is a new device or because the battery backup failed, we’ll print a message in the Serial Monitor and set the time.<\/p>\n\n\n\nif (rtc.lostPower()) {\n Serial.println(\"RTC lost power, let's set the time!\");<\/code><\/pre>\n\n\n\nSetting the Time<\/h4>\n\n\n\n To set the time on the RTC, we can use the adjust()<\/span> method on our rtc<\/span> object. The following line sets the RTC\u2019s date and time to the current date and time when this sketch was last compiled.<\/p>\n\n\n\nrtc.adjust(DateTime(F(__DATE__), F(__TIME__)));<\/code><\/pre>\n\n\n\n__DATE__<\/span> and __TIME__<\/span> are macros that provide the current date and time at compilation.<\/p>\n\n\n\nAlternatively, you can set the date and time manually. Pass the time fields in this order: year, month, day, hour, minute, second. This line is commented on the code.<\/p>\n\n\n\n
\/\/ January 21, 2014 at 3am you would call:\nrtc.adjust(DateTime(2014, 1, 21, 3, 0, 0));<\/code><\/pre>\n\n\n\nIf you need to re-set the time on a previously configured device, you can call one of the previous two lines to set the time without checking if it has lost power or not (this is commented on the code).<\/p>\n\n\n\n
\/\/ When time needs to be re-set on a previously configured device, the\n\/\/ following line sets the RTC to the date & time this sketch was compiled\n\/\/rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));\n\/\/ This line sets the RTC with an explicit date & time, for example to set\n\/\/ January 21, 2014 at 3am you would call:\n\/\/rtc.adjust(DateTime(2014, 1, 21, 3, 0, 0));<\/code><\/pre>\n\n\n\nGetting Date and Time<\/h4>\n\n\n\n In the loop()<\/span>, we get the date and time every three seconds, and we print it in the Serial Monitor.<\/p>\n\n\n\nWe call rtc.now()<\/span> to get the current date and time from the RTC module.<\/p>\n\n\n\nDateTime now = rtc.now();<\/code><\/pre>\n\n\n\nIt returns a DateTime<\/span> object containing the values for the current year, month, day, hour, minute, and second.<\/p>\n\n\n\nTo access each field of the date and time, we can use the following methods:<\/p>\n\n\n\nnow.year()<\/span><\/td> Gets the current year (e.g., 2024)<\/td><\/tr> now.month()<\/span><\/td> Gets the current month (1\u201312)<\/td><\/tr> now.day()<\/span><\/td> Gets the current day of the month (1\u201331)<\/td><\/tr> now.dayOfTheWeek()<\/span><\/td> Gets the day of the week (0-6), where 0 is Sunday, and 6 is Saturday<\/td><\/tr> now.hour()<\/span><\/td> Gets the current hour (0\u201323)<\/td><\/tr> now.minute()<\/span><\/td> Gets the current minute (0\u201359)<\/td><\/tr> now.second()<\/span><\/td> Gets the current second (0\u201359)<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nTo convert the result into a string, we can use the String()<\/span> method. We also pass DEC<\/span> as a second argument to the string()<\/span> method to get a decimal number.<\/p>\n\n\n\nString yearStr = String(now.year(), DEC);<\/code><\/pre>\n\n\n\nIn the case of the month, day, hour, minute, and second, we add a leading zero when the number is smaller than 10. So, instead of having, for example: 3:5:6 (which is weird for a time format), you’ll get 03:05:06<\/span>.<\/p>\n\n\n\nString monthStr = (now.month() < 10 ? \"0\" : \"\") + String(now.month(), DEC);\nString dayStr = (now.day() < 10 ? \"0\" : \"\") + String(now.day(), DEC);\nString hourStr = (now.hour() < 10 ? \"0\" : \"\") + String(now.hour(), DEC); \nString minuteStr = (now.minute() < 10 ? \"0\" : \"\") + String(now.minute(), DEC);\nString secondStr = (now.second() < 10 ? \"0\" : \"\") + String(now.second(), DEC);<\/code><\/pre>\n\n\n\nTo get the name of the day of the week, we use the daysOfTheWeek<\/span> array we created at the beginning of the code.<\/p>\n\n\n\nString dayOfWeek = daysOfTheWeek[now.dayOfTheWeek()];<\/code><\/pre>\n\n\n\nIn the end, we concatenate all the time fields in a variable and display it in the Serial Monitor.<\/p>\n\n\n\n
\/\/ Complete time string\nString formattedTime = dayOfWeek + \", \" + yearStr + \"-\" + monthStr + \"-\" + dayStr + \" \" + hourStr + \":\" + minuteStr + \":\" + secondStr;\n\n\/\/ Print the complete formatted time\nSerial.println(formattedTime);<\/code><\/pre>\n\n\n\nThe DS3231 gives temperature-compensated results. We can get the temperature it measures using the getTemperature()<\/span> method on the rtc<\/span> object. This returns the temperature in Celsius degrees.<\/p>\n\n\n\n\/\/ Getting temperature\nSerial.print(rtc.getTemperature());\nSerial.println(\"\u00baC\");<\/code><\/pre>\n\n\n\nTesting the Example<\/h3>\n\n\n\n With the RTC connected to the Arduino, upload the code to your board.<\/p>\n\n\n\n
Open the Serial Monitor at a baud rate of 9600. The Arduino will set the RTC time and display the current time and temperature every three seconds.<\/p>\n\n\n
\n
DS3231 with the Arduino: Setting Alarms<\/h2>\n\n\n\n
The DS3231 RTC Module allows you to set up to two alarms: alarm 1 and alarm 2. When using the RTCLib<\/span> Library, these are the accepted modes for the alarms:<\/p>\n\n\n\nAlarm<\/strong><\/td>Mode<\/strong><\/td>Meaning<\/strong> (Trigger the alarm…)<\/td><\/tr>Alarm 1<\/td> DS3231_A1_PerSecond<\/span><\/td> every second<\/td><\/tr> Alarm 1<\/td> DS3231_A1_Second<\/span><\/td> when the seconds match<\/td><\/tr> Alarm 1<\/td> DS3231_A1_Minute<\/span><\/td> when the minutes match<\/td><\/tr> Alarm 1<\/td> DS3231_A1_Hour<\/span><\/td> when the hour matches<\/td><\/tr> Alarm 1<\/td> DS3231_A1_Date<\/span><\/td> when the date matches<\/td><\/tr> Alarm 1<\/td> DS3231_A1_Day<\/span><\/td> when the day matches<\/td><\/tr> Alarm 2<\/td> DS3231_A2_PerMinute<\/span><\/td> every minute<\/td><\/tr> Alarm 2<\/td> DS3231_A2_Minute<\/span><\/td> when the minutes match<\/td><\/tr> Alarm 2<\/td> DS3231_A2_Hour<\/span><\/td> when the hour matches<\/td><\/tr> Alarm 2<\/td> DS3231_A2_Date<\/span><\/td> when the date matches<\/td><\/tr> Alarm 2<\/td> DS3231_A2_Day<\/span><\/td> when the day matches<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\nThe following example shows how to set up two alarms and how to use them to trigger an interrupt on the Arduino. When an interrupt is triggered, we’ll toggle the state of an LED connected to Pin 3 (so, to test this example, add an LED to your circuit by connecting it to Pin 3).<\/p>\n\n\n
\/*********\n Rui Santos & Sara Santos - Random Nerd Tutorials\n Complete instructions at https:\/\/RandomNerdTutorials.com\/arduino-ds3231-real-time-clock\/\n*********\/\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 2\n\n\/\/ LED for visual indication\nconst int ledPin = 3;\n\n\/\/ set the alarms\n\/\/ (year, month, day, hour, minutes, seconds)\nDateTime alarm1Time = DateTime(2025, 1, 7, 19, 05, 0);\nDateTime alarm2Time = DateTime(2025, 1, 7, 19, 06, 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\nvoid onAlarm() {\n Serial.println("Alarm occurred!");\n \/\/ toggle the current LED state\n int state = digitalRead(ledPin);\n digitalWrite(ledPin, !state);\n}\n\nvoid setup() {\n Serial.begin(9600);\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\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_Minute)) { \/\/ 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
\nthe 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 re-triggering and crashing the board;<\/li>\n\n\n\n you must deactivate an alarm before activating the other.<\/li>\n<\/ul>\n\n\n\nHow Does the Code Work?<\/h3>\n\n\n\n We’ve already covered how to initialize the RTC, 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 pin that is connected to the SQW pin. In our case, we’re using Pin 2, you can use any other pin that can be used as an interrupt.<\/p>\n\n\n\n
#define CLOCK_INTERRUPT_PIN 2<\/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, 1, 7, 15, 25, 0);\nDateTime alarm2Time = DateTime(2025, 1, 7, 15, 26, 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 Pin 3 (connected to an LED).<\/p>\n\n\n\nvoid onAlarm() {\n Serial.println(\"Alarm occured!\");\n int state = digitalRead(ledPin);\n digitalWrite(ledPin, !state);\n}<\/code><\/pre>\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\nWe’ll use the SQW pin to trigger the alarms. We won’t use its functionality to generate square waves. So, we need to stop that.<\/p>\n\n\n\n
rtc.writeSqwPinMode(DS3231_OFF);<\/code><\/pre>\n\n\n\nWe can only have one alarm going at a time. So, disable alarm 2 before setting up alarm 1.<\/p>\n\n\n\n
rtc.disableAlarm(2);<\/code><\/pre>\n\n\n\nSet up the alarm 1 using the setAlarm1()<\/span> function on the rtc<\/span> object. Pass as arguments the time for alarm 1 and the alarm mode. We’ve already seen the alarm mode options for alarm 1 and alarm 2 before. In this case, we’re setting it to DS3231_A1_Minute<\/span>, which means the alarm will fire when the minutes match.<\/p>\n\n\n\n\/\/ Schedule Alarm1 to fire when the minutes match\nif(!rtc.setAlarm1(alarm1Time, DS3231_A1_Minute)) { \/\/ 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}<\/code><\/pre>\n\n\n\nIn the loop()<\/span>, we print the details of the current alarm.<\/p>\n\n\n\n
![\"Arduino](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/Arduino-DS3231-RTC-Guide.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![](\"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![\"Arduino](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/arduino-ds3231_circuit-diagram.jpg?resize=750%2C385&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![\"Arduino](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/Arduino-with-DS3231.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"Arduino](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/ds3231-arduino-set-get-time.png?resize=752%2C374&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n![\"Arduino](\"https:\/\/i0.wp.com\/randomnerdtutorials.com\/wp-content\/uploads\/2025\/01\/Arduino-DS3231-Setting-Alarms.jpg?resize=750%2C422&quality=100&strip=all&ssl=1\")
<\/figure><\/div>\n\n\n