Overview
The rtc_pcf8523 node interfaces with the PCF8523 low-power real-time clock module
over I2C. The PCF8523 features automatic battery switchover circuitry, a programmable clockout pin, and extremely low
battery current draw (150 nA typical), making it ideal for battery-powered and low-energy IoT applications.
It is commonly found on Adafruit and other popular RTC breakout boards.
Properties
| Property | Type | Default | Options | Description |
|---|---|---|---|---|
| i2cBus | select | "/dev/i2c-1" | /dev/i2c-0, /dev/i2c-1 | I2C bus device path |
| address | string | "0x68" | - | I2C device address |
| use24Hour | boolean | true | - | Use 24-hour time format instead of 12-hour |
| batteryLowInt | boolean | false | - | Enable interrupt on battery low condition |
| clockoutFreq | select | "off" | off, 32768hz, 16384hz, 8192hz, 4096hz, 1024hz, 32hz, 1hz | Programmable clockout frequency on CLKOUT pin |
Output Format
{
"year": 2026,
"month": 2,
"day": 12,
"hour": 14,
"minute": 30,
"second": 45,
"dayOfWeek": 4,
"batteryLow": false,
"timestamp": 1771080645
} Note: The batteryLow flag is read from the
PCF8523 control register and indicates when the backup battery voltage drops below the threshold. Monitor this
field to proactively replace batteries before timekeeping is lost.
Wiring / Connection Diagram
Raspberry Pi PCF8523 Module
+-----------+ +---------------+
| | | |
| 3.3V (1) +------------+ VCC |
| | | |
| SDA (3) +------------+ SDA |
| | | |
| SCL (5) +------------+ SCL |
| | | |
| GND (6) +------------+ GND |
| | | |
| GPIO (7) +----opt-----+ INT/CLKOUT |
| | | |
+-----------+ +---------------+
| CR1220 |
| (backup) |
+------------+
INT/CLKOUT pin is optional:
- Used for clockout frequency output
- Battery low interrupt signal
Example Use Cases
Battery Health Monitor
Monitor the PCF8523 battery status and send alerts when the backup battery is low.
// Function node: check battery status
var rtc = msg.payload;
if (rtc.batteryLow) {
msg.payload = {
alert: "PCF8523 battery low",
lastTime: rtc.year + "-" + rtc.month + "-" + rtc.day
+ " " + rtc.hour + ":" + rtc.minute,
action: "Replace CR1220 battery"
};
return msg;
}
return null;Solar-Powered Data Logger
Timestamp sensor readings in a solar-powered deployment where power conservation is critical.
[
{
"id": "sensor-trigger",
"type": "inject",
"repeat": "300",
"payload": ""
},
{
"id": "rtc-read",
"type": "rtc_pcf8523",
"i2cBus": "/dev/i2c-1",
"address": "0x68",
"use24Hour": true,
"batteryLowInt": true
},
{
"id": "format-log",
"type": "function",
"func": "msg.timestamp = msg.payload.timestamp;\nmsg.batteryOk = !msg.payload.batteryLow;\nreturn msg;"
},
{
"id": "store",
"type": "file",
"filename": "/var/log/sensor-data.csv"
}
]1Hz Clockout for Precise Timing
Use the CLKOUT pin to generate a 1Hz reference pulse for external timing circuits.
// Configure PCF8523 with 1Hz clockout
// The CLKOUT pin will output a precise 1Hz square wave
// that can be connected to a GPIO interrupt for
// accurate one-second tick events.
{
"id": "rtc-config",
"type": "rtc_pcf8523",
"i2cBus": "/dev/i2c-1",
"address": "0x68",
"clockoutFreq": "1hz"
}Common Use Cases
Solar IoT Deployments
Ultra-low battery draw ideal for solar-powered remote sensors
Battery-Backed Systems
Automatic battery switchover preserves time during power loss
Reference Clock Source
Generate precise clock signals from 1Hz to 32.768kHz
Battery Monitoring
Built-in low-battery detection with optional interrupt output