Arduino UNO R4 - DIYables Bluetooth App RTC
Overview
The Bluetooth RTC example provides real-time clock synchronization through the DIYables Bluetooth STEM app. Designed for Arduino UNO R4 WiFi using BLE (Bluetooth Low Energy) to sync the board's built-in hardware RTC with your smartphone's clock and display the time. The Arduino UNO R4 WiFi has a built-in RTC module, making it ideal for timekeeping projects without needing external RTC hardware. Perfect for clocks, data logging with timestamps, scheduled automation, and time-based projects.
Note: The Arduino UNO R4 WiFi only supports BLE (Bluetooth Low Energy). It does not support Classic Bluetooth. The DIYables Bluetooth App supports both BLE and Classic Bluetooth on Android, and BLE on iOS. Since this board uses BLE, the app works on both Android and iOS.
Features
- Built-in Hardware RTC: Uses Arduino UNO R4 WiFi's onboard RTC — no external module needed
- Phone Time Sync: Sync time from smartphone via Unix timestamp or local time components
- Real-Time Display: Show current time on the app, updated every second
- Time Request: App can request current time from the board
- Persistent Timekeeping: RTC keeps time while board is powered
- Works on Android & iOS: BLE is supported on both platforms
- No Pairing Required: BLE auto-connects without manual pairing
Hardware Preparation
Or you can buy the following kits:
| 1 | × | DIYables STEM V4 IoT Starter Kit (Arduino included) | |
| 1 | × | DIYables Sensor Kit (18 sensors/displays) |
Disclosure: Some of the links provided in this section are Amazon affiliate links. We may receive a commission for any purchases made through these links at no additional cost to you.
Additionally, some of these links are for products from our own brand, DIYables .
Additionally, some of these links are for products from our own brand, DIYables .
Note: No external RTC module is needed! The Arduino UNO R4 WiFi has a built-in hardware RTC accessible via the RTC.h library.
Arduino UNO R4 WiFi Code
Detailed Instructions
Follow these instructions step by step:
- If this is your first time using the Arduino UNO R4 WiFi, refer to the Arduino UNO R4 WiFi getting started guide.
- Connect the Arduino UNO R4 WiFi board to your computer using a USB cable.
- Launch the Arduino IDE on your computer.
- Select Arduino UNO R4 WiFi board and the appropriate COM port.
- Navigate to the Libraries icon on the left bar of the Arduino IDE.
- Search "DIYables Bluetooth", then find the DIYables Bluetooth library by DIYables
- Click Install button to install the library.
- You will be asked for installing some other library dependencies
- Click Install All button to install all library dependencies.
BLE Code
- On Arduino IDE, Go to File Examples DIYables Bluetooth ArduinoBLE_RTC example, or copy the above code and paste it to the editor of Arduino IDE
/*
* DIYables Bluetooth Library - Bluetooth RTC Example
* Works with DIYables Bluetooth STEM app on Android and iOS
*
* This example demonstrates the Bluetooth RTC (Real-Time Clock) feature:
* - Real-time clock display for both Arduino and mobile app
* - One-click time synchronization from mobile app to Arduino
* - Hardware RTC integration for persistent timekeeping
* - Visual time difference monitoring
*
* Compatible Boards:
* - Arduino UNO R4 WiFi (with built-in RTC)
* Note: This example requires a board with hardware RTC.
* Other BLE boards can be used with an external RTC module (e.g., DS3231).
*
* Setup:
* 1. Upload the sketch to your Arduino
* 2. Open Serial Monitor to see connection status
* 3. Use DIYables Bluetooth App to connect and sync time
*
* Tutorial: https://diyables.io/bluetooth-app
* Author: DIYables
*/
#include <DIYables_BluetoothServer.h>
#include <DIYables_BluetoothRTC.h>
#include <platforms/DIYables_ArduinoBLE.h>
#include "RTC.h"
// BLE Configuration
const char* DEVICE_NAME = "Arduino_RTC";
const char* SERVICE_UUID = "19B10000-E8F2-537E-4F6C-D104768A1214";
const char* TX_UUID = "19B10001-E8F2-537E-4F6C-D104768A1214";
const char* RX_UUID = "19B10002-E8F2-537E-4F6C-D104768A1214";
// Create Bluetooth instances
DIYables_ArduinoBLE bluetooth(DEVICE_NAME, SERVICE_UUID, TX_UUID, RX_UUID);
DIYables_BluetoothServer bluetoothServer(bluetooth);
// Create RTC app instance
DIYables_BluetoothRTC bluetoothRTC;
void setup() {
Serial.begin(9600);
delay(1000);
Serial.println("DIYables Bluetooth - RTC Example");
// Initialize RTC
RTC.begin();
// Check if RTC is running and set initial time if needed
RTCTime savedTime;
RTC.getTime(savedTime);
if (!RTC.isRunning() || savedTime.getYear() == 2000) {
Serial.println("RTC is NOT running, setting initial time...");
// Set a default time - you can modify this to match current time
RTCTime startTime(28, Month::AUGUST, 2025, 12, 0, 0, DayOfWeek::THURSDAY, SaveLight::SAVING_TIME_ACTIVE);
RTC.setTime(startTime);
Serial.println("RTC initialized with default time");
} else {
Serial.println("RTC is already running");
}
// Print initial RTC time
RTCTime initialTime;
RTC.getTime(initialTime);
Serial.print("Initial RTC Time: ");
Serial.print(initialTime.getYear());
Serial.print("/");
Serial.print(Month2int(initialTime.getMonth()));
Serial.print("/");
Serial.print(initialTime.getDayOfMonth());
Serial.print(" - ");
if (initialTime.getHour() < 10) Serial.print("0");
Serial.print(initialTime.getHour());
Serial.print(":");
if (initialTime.getMinutes() < 10) Serial.print("0");
Serial.print(initialTime.getMinutes());
Serial.print(":");
if (initialTime.getSeconds() < 10) Serial.print("0");
Serial.print(initialTime.getSeconds());
Serial.println();
// Initialize Bluetooth server with platform-specific implementation
bluetoothServer.begin();
// Add RTC app to server
bluetoothServer.addApp(&bluetoothRTC);
// Set up connection event callbacks
bluetoothServer.setOnConnected([]() {
Serial.println("Bluetooth connected!");
// Send current time to app on connection
sendCurrentTimeToApp();
});
bluetoothServer.setOnDisconnected([]() {
Serial.println("Bluetooth disconnected!");
});
// Set callback for time sync from mobile app (Unix timestamp)
bluetoothRTC.onTimeSync(onTimeSyncReceived);
// Set callback for local time sync from mobile app (date/time components)
bluetoothRTC.onLocalTimeSync(onLocalTimeSyncReceived);
// Set callback for time request from mobile app
bluetoothRTC.onTimeRequest(onTimeRequested);
Serial.println("Waiting for Bluetooth connection...");
Serial.println("Connect via app to sync time");
}
void loop() {
// Handle Bluetooth server communications
bluetoothServer.loop();
// Send current time to mobile app and print to Serial every 1 second
static unsigned long lastUpdate = 0;
if (millis() - lastUpdate >= 1000) {
lastUpdate = millis();
// Get current RTC time
RTCTime currentTime;
RTC.getTime(currentTime);
// Send time to mobile app in human readable format
bluetoothRTC.sendTime(currentTime.getYear(), Month2int(currentTime.getMonth()),
currentTime.getDayOfMonth(), currentTime.getHour(),
currentTime.getMinutes(), currentTime.getSeconds());
// Print time to Serial Monitor
Serial.print("RTC Time: ");
Serial.print(currentTime.getYear());
Serial.print("/");
Serial.print(Month2int(currentTime.getMonth()));
Serial.print("/");
Serial.print(currentTime.getDayOfMonth());
Serial.print(" - ");
if (currentTime.getHour() < 10) Serial.print("0");
Serial.print(currentTime.getHour());
Serial.print(":");
if (currentTime.getMinutes() < 10) Serial.print("0");
Serial.print(currentTime.getMinutes());
Serial.print(":");
if (currentTime.getSeconds() < 10) Serial.print("0");
Serial.print(currentTime.getSeconds());
Serial.println();
}
delay(10);
}
// Callback function called when mobile app sends time sync command
void onTimeSyncReceived(unsigned long unixTimestamp) {
Serial.print("Time sync received (Unix): ");
Serial.println(unixTimestamp);
// Convert Unix timestamp to RTCTime
RTCTime newTime;
newTime.setUnixTime(unixTimestamp);
// Set RTC time
RTC.setTime(newTime);
Serial.println("Arduino RTC synchronized from Unix timestamp!");
}
// Callback function called when mobile app sends local time sync with components
void onLocalTimeSyncReceived(int year, int month, int day, int hour, int minute, int second) {
Serial.print("Local time sync received: ");
Serial.print(year);
Serial.print("/");
Serial.print(month);
Serial.print("/");
Serial.print(day);
Serial.print(" ");
Serial.print(hour);
Serial.print(":");
Serial.print(minute);
Serial.print(":");
Serial.println(second);
// Create RTCTime from components (local time)
// Convert month integer to Month enum
Month monthEnum;
switch(month) {
case 1: monthEnum = Month::JANUARY; break;
case 2: monthEnum = Month::FEBRUARY; break;
case 3: monthEnum = Month::MARCH; break;
case 4: monthEnum = Month::APRIL; break;
case 5: monthEnum = Month::MAY; break;
case 6: monthEnum = Month::JUNE; break;
case 7: monthEnum = Month::JULY; break;
case 8: monthEnum = Month::AUGUST; break;
case 9: monthEnum = Month::SEPTEMBER; break;
case 10: monthEnum = Month::OCTOBER; break;
case 11: monthEnum = Month::NOVEMBER; break;
case 12: monthEnum = Month::DECEMBER; break;
default: monthEnum = Month::JANUARY; break;
}
RTCTime newTime(day, monthEnum, year, hour, minute, second, DayOfWeek::MONDAY, SaveLight::SAVING_TIME_ACTIVE);
// Set RTC time
RTC.setTime(newTime);
Serial.println("Arduino RTC synchronized from local time components!");
}
// Callback function called when mobile app requests current Arduino time
void onTimeRequested() {
Serial.println("Time requested by app");
sendCurrentTimeToApp();
}
// Helper function to send current time to mobile app
void sendCurrentTimeToApp() {
// Get current RTC time and send to app in human readable format
RTCTime currentTime;
RTC.getTime(currentTime);
bluetoothRTC.sendTime(currentTime.getYear(), Month2int(currentTime.getMonth()),
currentTime.getDayOfMonth(), currentTime.getHour(),
currentTime.getMinutes(), currentTime.getSeconds());
}
- Click Upload button on Arduino IDE to upload code to Arduino UNO R4 WiFi
- Open the Serial Monitor
- Check out the result on Serial Monitor. It looks like the below:
8
Serial.println("Hello World!");
Message (Enter to send message to 'Arduino Uno R4 WiFi' on 'COM15')
New Line
9600 baud
DIYables Bluetooth - RTC Example
Waiting for Bluetooth connection...
RTC not running or year is 2000, waiting for time sync...
Mobile App
Note: The DIYables Bluetooth App supports both BLE and Classic Bluetooth on Android, and BLE on iOS. Since the Arduino UNO R4 WiFi uses BLE, the app works on both Android and iOS. No manual pairing is needed for BLE — just scan and connect.
- Open the DIYables Bluetooth App
- When opening the app for the first time, it will ask for permissions. Please grant the following:
- Nearby Devices permission (Android 12+) / Bluetooth permission (iOS) - required to scan and connect to Bluetooth devices
- Location permission (Android 11 and below only) - required by older Android versions to scan for BLE devices
- Make sure Bluetooth is turned on on your phone
- On the home screen, tap the Connect button. The app will scan for BLE devices.
- Find and tap "Arduino_RTC" in the scan results to connect.
- Once connected, the app automatically goes back to the home screen. Select the RTC app from the app menu.
Note: You can tap the settings icon on the home screen to hide/show apps on the home screen. For more details, see the DIYables Bluetooth App User Manual.
- The app will display the current time from the Arduino's RTC
- Use the Sync button to sync the phone's time to the Arduino
- The time updates every second
Now look back at the Serial Monitor on Arduino IDE. You will see:
8
Serial.println("Hello World!");
Message (Enter to send message to 'Arduino Uno R4 WiFi' on 'COM15')
New Line
9600 baud
Bluetooth connected!
Time sync received (unix): 1719849600
RTC set to: 2025/07/01 12:00:00
Current time: 2025/07/01 12:00:01
Current time: 2025/07/01 12:00:02
Creative Customization - Adapt the Code to Your Project
Time Sync Methods
The app can sync time to the Arduino using two methods:
// Method 1: Unix timestamp sync
bluetoothRTC.onTimeSync([](unsigned long unixTime) {
// Convert Unix timestamp and set RTC
Serial.print("Unix time: ");
Serial.println(unixTime);
});
// Method 2: Local time components sync
bluetoothRTC.onLocalTimeSync([](int year, int month, int day, int hour, int minute, int second) {
// Set RTC directly with components
Serial.print("Local time: ");
Serial.print(year);
Serial.print("/");
Serial.print(month);
Serial.print("/");
Serial.println(day);
});
Send Time to App
// Send current time to the app
bluetoothRTC.sendTime(year, month, day, hour, minute, second);
Handle Time Requests
bluetoothRTC.onTimeRequest([]() {
// App is requesting the current time
// Read RTC and send time back
RTCTime currentTime;
RTC.getTime(currentTime);
bluetoothRTC.sendTime(
currentTime.getYear(),
Month2int(currentTime.getMonth()),
currentTime.getDayOfMonth(),
currentTime.getHour(),
currentTime.getMinutes(),
currentTime.getSeconds()
);
});
Using the Built-in RTC
The Arduino UNO R4 WiFi's built-in RTC is accessed via the RTC.h library:
#include "RTC.h"
void setup() {
RTC.begin(); // Initialize the hardware RTC
}
// Set time on the RTC
RTCTime timeToSet;
timeToSet.setYear(2025);
timeToSet.setMonth(Month::JULY);
timeToSet.setDayOfMonth(1);
timeToSet.setHour(12);
timeToSet.setMinute(0);
timeToSet.setSecond(0);
RTC.setTime(timeToSet);
// Read time from the RTC
RTCTime currentTime;
RTC.getTime(currentTime);
int year = currentTime.getYear();
int month = Month2int(currentTime.getMonth());
int day = currentTime.getDayOfMonth();
int hour = currentTime.getHour();
int minute = currentTime.getMinutes();
int second = currentTime.getSeconds();
Programming Examples
Clock with Serial Display
void loop() {
bluetoothServer.loop();
static unsigned long lastTime = 0;
if (millis() - lastTime >= 1000) {
lastTime = millis();
RTCTime currentTime;
if (RTC.getTime(currentTime)) {
char timeStr[20];
sprintf(timeStr, "%04d/%02d/%02d %02d:%02d:%02d",
currentTime.getYear(),
Month2int(currentTime.getMonth()),
currentTime.getDayOfMonth(),
currentTime.getHour(),
currentTime.getMinutes(),
currentTime.getSeconds());
Serial.println(timeStr);
// Also send to the app
bluetoothRTC.sendTime(
currentTime.getYear(),
Month2int(currentTime.getMonth()),
currentTime.getDayOfMonth(),
currentTime.getHour(),
currentTime.getMinutes(),
currentTime.getSeconds()
);
}
}
}
Data Logger with Timestamps
void logSensorData() {
RTCTime currentTime;
RTC.getTime(currentTime);
float temperature = readSensor();
char logEntry[50];
sprintf(logEntry, "%04d/%02d/%02d %02d:%02d:%02d - Temp: %.1f°C",
currentTime.getYear(),
Month2int(currentTime.getMonth()),
currentTime.getDayOfMonth(),
currentTime.getHour(),
currentTime.getMinutes(),
currentTime.getSeconds(),
temperature);
Serial.println(logEntry);
}
Troubleshooting
Common Issues
1. Cannot find the device in the app
- Make sure the Arduino UNO R4 WiFi is powered on and the sketch is uploaded
- Ensure your phone's Bluetooth is enabled
- On Android 11 and below, also enable Location services
2. Time shows 2000/01/01 or incorrect time
- The RTC needs to be synced at least once after power-up
- Use the Sync button in the app to set the time
- The RTC loses time when the board is powered off (no battery backup)
3. Time not syncing from app
- Verify the onTimeSync and onLocalTimeSync callbacks are set up
- Check the Serial Monitor for sync messages
- Ensure the BLE connection is stable
4. RTC drifts over time
- The built-in RTC crystal has limited accuracy
- Re-sync periodically via the app
- For critical timing, consider using NTP over WiFi as well
5. Month conversion issues
- The Arduino UNO R4 WiFi RTC uses a Month enum, not integer
- Use a conversion function (like Month2int() in the example) for integer months
- Months are 1-based (January = 1)
6. Upload fails or board not recognized
- Install the latest Arduino UNO R4 board package via Board Manager
- Try a different USB cable or port
Project Ideas
- Digital clock with BLE time sync
- Data logger with accurate timestamps
- Scheduled task automation (turn on/off at specific times)
- Alarm clock with smartphone control
- Time-stamped event recorder
Next Steps
After mastering the Bluetooth RTC example, try:
- Bluetooth Monitor - For text-based status display with timestamps
- Bluetooth Table - For structured data with time fields
- Bluetooth Chat - For two-way communication
- Multiple Bluetooth Apps - Combining RTC with other apps
Support
For additional help:
- Check the API Reference documentation
- Visit DIYables tutorials
- Arduino community forums