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PCF2127T

Accurate RTC with integrated quartz crystal for industrial applications

NXP Semiconductors. 

NXP Semiconductors

PCF2127

Accurate RTC with integrated quartz crystal for industrial applications

8. Functional description

The PCF2127 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 (see Section 8.3.3).

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 PCF2127 has a backup battery input pin and backup battery switch-over circuit which

monitors the main power supply. The backup battery switch-over circuit automatically

switches to the backup battery when a power failure condition is detected (see

Section 8.6.1). 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 8.6.2). When

the battery voltage drops 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.

8.1 Register overview

The PCF2127 contains 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

(see Figure 5).

PCF2127

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Fig 5. Handling address registers

• 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 8.9).

• The registers at addresses 0Ah through 0Eh define the alarm function. It can be

selected that an interrupt is generated when an alarm event occurs (see

Section 8.10).

All information provided in this document is subject to legal disclaimers.

Rev. 8 — 19 December 2014

© NXP Semiconductors N.V. 2014. All rights reserved.

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