Real-Time Clock

The RTC (Real-Time Clock) is integrated into the UNO R4 WiFi’s microcontroller (RA4M1). The RTC is an autonomous clock module capable of operating even when the main power supply is disconnected, thanks to a backup power source such as a battery. This makes the RTC incredibly versatile for various applications like scheduling timed tasks in home automation systems or time-stamping individual data points in data logging applications.


The UNO R4 WiFi has a VRTC pin that maintains the onboard RTC’s operation even when the board loses power. To utilize this feature, apply a voltage between 1.6 and 3.6 V to the VRTC pin.

Scheduled Repetitive Tasks

In certain use-cases, you may need to execute specific tasks at regular intervals. To establish periodic interrupts, you’ll first need to initialize a periodic callback function. Below is an Arduino code example that uses a periodic interrupt to blink an LED every 2 seconds.

Upload the Code

Open the 03-rtc.ino file located at elite-explorer-kit-main\r4_new_feature\03-rtc, or paste the following code into your Arduino IDE.

Code Explanation

  1. Initializing Components and Libraries

    #include "RTC.h"
    volatile bool irqFlag = false;
    bool ledState = false;
    const int led = LED_BUILTIN;
    void setup() {
      pinMode(led, OUTPUT);
    • #include "RTC.h": Includes the RTC library.

    • volatile bool irqFlag = false;: Declares a volatile boolean flag. volatile ensures that the variable can be safely accessed or modified by an interrupt. For more details on volatile, refer to Arduino volatile documentation.

    • bool ledState = false;: Declares a boolean to keep track of the LED state.

    • const int led = LED_BUILTIN;: Declares a constant for the built-in LED pin.

    • pinMode(led, OUTPUT);: Sets the LED pin as an output.

    • Serial.begin(9600);: Initializes serial communication at 9600 baud rate.

    • RTC.begin();: Initializes the RTC.

  2. Setting Up the Real-Time Clock

    RTCTime mytime(30, Month::JUNE, 2023, 13, 37, 00, DayOfWeek::WEDNESDAY, SaveLight::SAVING_TIME_ACTIVE);
    • RTCTime mytime(...);: Creates an RTCTime object and initializes it with a specific date and time.

    • RTC.setTime(mytime);: Sets the RTC with the initialized time.

    Setting and Checking Periodic Callback

    if (!RTC.setPeriodicCallback(periodicCallback, Period::ONCE_EVERY_2_SEC)) {
      Serial.println("ERROR: periodic callback not set");
    void loop() {
      if (irqFlag) {
        Serial.println("Timed CallBack");
        ledState = !ledState;
        digitalWrite(led, ledState);
        irqFlag = false;
    void periodicCallback() {
      irqFlag = true;
    • RTC.setPeriodicCallback(...);: Sets a periodic callback to trigger every 2 seconds.The period can be specified using the following enumerations:

      • ONCE_EVERY_2_SEC

      • ONCE_EVERY_1_SEC







      • N128_TIMES_EVERY_SEC

      • N256_TIMES_EVERY_SEC

    • void loop() {...}: Checks if the callback has been triggered. If so, toggles the LED state.

    • void periodicCallback() {...}: The callback function sets irqFlag = true when triggered.