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Programming the PCF8523 RTC with C in the Arduino IDE (ESP32 Edition)

By Wilfried Voss on April 9, 2025

PCF8523 Programming with ESP32Overview

The PCF8523 is a low-power real-time clock (RTC) module that communicates over I2C. This module is ideal for applications requiring accurate timekeeping, even when the main processor like the ESP32 is powered off.

This report explains how to:

  • Wire the PCF8523 to an ESP32

  • Set up the Arduino IDE

  • Write C-style code to initialize, read, and set time

  • Use both a library-based and raw I2C approach

1. Hardware Requirements

  • ESP32 development board (e.g., DevKit v1, WROOM, etc.)

  • PCF8523 RTC module

  • CR1220 or CR2032 battery (for backup power)

  • Jumper wires

  • Breadboard (optional)

2. Wiring (ESP32 and PCF8523)

The ESP32 uses custom I2C pins, so you’ll define these in code. Default I2C pins are usually:

PCF8523 Pin ESP32 Pin (default)
VCC 3.3V (⚠️ Not 5V)
GND GND
SDA GPIO21
SCL GPIO22

⚠️ The ESP32 is a 3.3V device. Ensure your PCF8523 module supports 3.3V, or use a level shifter if needed.

3. Arduino IDE Setup

Step 1: Board Setup

  • Install ESP32 board support via the Board Manager

  • Go to File > Preferences → Add the following to “Additional Board URLs”:

    https://raw.githubusercontent.com/espressif/arduino-esp32/gh-pages/package_esp32_index.json

Step 2: Install Required Libraries

  • Open Sketch > Include Library > Manage Libraries

  • Install Adafruit RTClib (supports PCF8523)

4. Example Code (with ESP32 Pin Definition)

A. Initialize and Set Time (C-style Code Using Library)

#include <Wire.h>
#include "RTClib.h"#define I2C_SDA 21
#define I2C_SCL 22RTC_PCF8523 rtc;void setup() {
Serial.begin(115200);Wire.begin(I2C_SDA, I2C_SCL);if (!rtc.begin()) {
Serial.println(“Couldn’t find RTC”);
while (1);
}if (!rtc.initialized() || rtc.lostPower()) {
Serial.println(“RTC lost power, setting time!”);
rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); // Compile-time set
}rtc.start(); // Ensure the clock is running
}

void loop() {
DateTime now = rtc.now();

Serial.printf(“%04d/%02d/%02d %02d:%02d:%02d\n”,
now.year(), now.month(), now.day(),
now.hour(), now.minute(), now.second());

delay(1000);
}

B. Manually Set a Specific Time

rtc.adjust(DateTime(2025, 4, 9, 15, 0, 0)); // YYYY, MM, DD, HH, MM, SS

5. Using Raw I2C (Without RTC Library)

For full control, use Wire directly with PCF8523’s I2C address (0x68).

Example: Reading Control Register

#include <Wire.h>

#define PCF8523_ADDR 0x68
#define SDA_PIN 21
#define SCL_PIN 22

void setup() {
Serial.begin(115200);
Wire.begin(SDA_PIN, SCL_PIN);

Wire.beginTransmission(PCF8523_ADDR);
Wire.write(0x00); // Control_1 register
Wire.endTransmission();

Wire.requestFrom(PCF8523_ADDR, 1);
if (Wire.available()) {
uint8_t ctrl1 = Wire.read();
Serial.printf(“Control_1 Register: 0x%02X\n”, ctrl1);
}
}

void loop() {
delay(3000);
}

6. ESP32 Power Management & Battery Switchover

The PCF8523 features a battery switchover circuit, ensuring it keeps time even when ESP32 is off. You can control power mode using:

  • Register 0x10 → Battery switch-over mode

  • rtc.lostPower() to detect brownouts

You can also write to these registers manually via I2C for fine-grained power control.

7. Notes & Best Practices

  • ✅ Use a coin cell battery for backup power

  • ✅ Use a multimeter to verify your SDA/SCL voltage levels

  • ⚠️ Do not power the PCF8523 module with 5V when using an ESP32

  • ⚙️ Alarms and timers require manual I2C programming (not supported in Adafruit RTClib)

8. Online References

Published in Blog, ESP32 Programming and Home Page News

Wilfried Voss
Wilfried Voss

Wilfried Voss is a seasoned entrepreneur and technologist with deep international roots. Born and raised in Germany, he brings a rich blend of European precision and global insight to his business ventures. Now based in Vermont, Wilfried lives with his family and a spirited mix of cats, dogs, chickens, and horses. His passion for technology and innovation is reflected in his work, which bridges cultures and continents, offering solutions with a truly international perspective.

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