PCF2129A datasheet(5/69 Pages) NXP

5 / 69 page

PCF2129A_2

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Product data sheet

Rev. 02 — 7 May 2010

5 of 69

NXP Semiconductors

PCF2129A

Integrated RTC, TCXO and quartz crystal

Functional description

The PCF2129A is a Real Time Clock (RTC) and calendar 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 400 kHz Fast-mode 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 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.

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