PCF2129AT datasheet(5/68 Pages) NXP

5 / 68 page

PCF2129A_1

Product data sheet

Rev. 01 — 13 January 2010

5 of 68

NXP Semiconductors

PCF2129A

Integrated RTC, TCXO and quartz crystal

Functional description

The PCF2129A is a Real Time Clock and calendar (RTC) with an on-chip Temperature

Compensated crystal (Xtal) Oscillator (TCXO) and a 32.768 kHz quartz crystal integrated

into the same package.

Address and data are transferred by a selectable 1 MHz Fast-mode Plus (Fm+) I2C-bus or

a 3 line SPI-bus with separate data input and output (see Section 9). The maximum speed

of the SPI-bus is 6.5 Mbit/s.

The PCF2129A contains 28 8-bit registers, that are used for many different functions,

such as clock, alarm, watchdog, timestamp etc. (see Section 8.1).

The PCF2129A has a backup battery input pin and backup battery switch-over circuit

which monitors the main power supply and automatically switches to the backup battery

when a power failure condition is detected (see Section 8.5.1). Accurate timekeeping is

maintained even when the main power supply is interrupted.

A battery low detection circuit monitors the status of the battery (see Section 8.5.3). When

the battery voltage goes below a 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.

8.1 Register overview

The PCF2129A contains 28 8-bit registers (see Table 4) with an auto-incrementing

address register: the built-in address register will increment automatically after each read

or write of a data byte up to the register 1Bh. After register 1Bh the auto-incrementing will

wrap around to address 00h.

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

registers (see Section 8.2).

• The memory addresses 03h through to 09h are used as counters for the clock

function (seconds up to years). The date is automatically adjusted for months with

fewer than 31 days, including corrections for leap years. The clock can operate in

12-hour mode with an AM/PM indication or in 24-hour mode (see Section 8.8).

• Addresses 0Ah through 0Eh define the alarm function. It can be selected that an

interrupt is generated when an alarm event occurs (see Section 8.9).

• The register 0Fh defines the temperature measurement period and the clock out

mode. The temperature measurement can be selected from every 4 minutes (default)

down to every 30 seconds (see Table 9). CLKOUT frequencies of 32.768 kHz

(default) down to 1 Hz for use as a system clock, a microcontroller clock etc. can be

chosen (see Section 8.3.2).

• Address registers 10h and 11h are used for the watchdog timer functions. The

watchdog timer has four selectable source clocks allowing for timer periods from less

than 1 ms to greater than 4 hours. An interrupt will be generated when the watchdog

times out.

• Address registers 12h to 18h are used for the timestamp function. When the trigger

event happens, the actual time is saved in the timestamp registers (see Section 8.11).

• Address register 19h is used for the correction of the crystal aging effect (see

Section 8.4.1).

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