Datasheet

Product data sheet Rev. 6 — 17 September 2013 6 of 78

NXP Semiconductors

PCF8523

Real-Time Clock (RTC) and calendar

8. Functional description

The PCF8523 contains:

• 20 8-bit registers with an auto-incrementing address register

• An on-chip 32.768 kHz oscillator with two integrated load capacitors

• A frequency divider, which provides the source clock for the Real-Time Clock (RTC)

• A programmable clock output

• A 1 Mbit/s I

C-bus interface

• An offset register, which allows fine-tuning of the clock

All 20 registers are designed as addressable 8-bit registers although not all bits are

implemented.

• The first three registers (memory address 00h, 01h, and 02h) are used as control and

status registers

• The addresses 03h through 09h are used as counters for the clock function (seconds

up to years)

• Addresses 0Ah through 0Dh define the alarm condition

• Address 0Eh defines the offset calibration

• Address 0Fh defines the clock-out mode and the addresses 10h and 12h the timer

mode

• Addresses 11h and 13h are used for the timers

The registers Seconds, Minutes, Hours, Days, Weekdays, Months, and Years are all

coded in Binary Coded Decimal (BCD) format. Other registers are either bit-wise or

standard binary. When one of the RTC registers is read, the contents of all counters are

frozen. Therefore, faulty reading of the clock and calendar during a carry condition is

prevented.

The PCF8523 has a battery backup input pin and battery switch-over circuit. The battery

switch-over circuit monitors the main power supply and switches automatically to the

backup battery when a power failure condition is detected. Accurate timekeeping is

maintained even when the main power supply is interrupted.

A battery low detection circuit monitors the status of the battery. When the battery voltage

goes below a certain threshold value, a flag is set to indicate that the battery must be

replaced soon. This ensures the integrity of the data during periods of battery backup.