PCF2123TS datasheet(6/55 Pages) NXP

6 / 55 page

PCF2123_2

Product data sheet

Rev. 02 — 4 December 2009

6 of 55

NXP Semiconductors

PCF2123

SPI Real time clock/calendar

Functional description

The PCF2123 contains 16 8-bit registers with an auto-incrementing address counter, 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, and a 6.25 Mbit/s SPI-bus. An offset register allows ﬁne tuning of the clock.

All 16 registers are designed as addressable 8-bit parallel registers although not all bits

are implemented.

• The ﬁrst two registers (memory address 00h and 01h) are used as control registers.

• The memory addresses 02h through 08h are used as counters for the clock function

(seconds up to years). The registers Seconds, Minutes, Hours, Days, Weekdays,

Months, and Years are all coded in Binary Coded Decimal (BCD) format. 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.

• Addresses 09h through 0Ch deﬁne the alarm condition.

• Address 0Dh deﬁnes the offset calibration.

• Address 0Eh deﬁnes the clock out and timer mode.

• Address registers 0Eh and 0Fh are used for the countdown timer function. The

countdown timer has four selectable source clocks allowing for countdown periods in

the range from 244

µs up to four hours. There are also two pre-deﬁned timers which

can be used to generate an interrupt once per second or once per minute. These are

deﬁned in register Control_2 (01h).

9.1 Low power operation

Minimum power operation will be achieved by reducing the number and frequency of

switching signals inside the IC, i.e., low frequency timer clocks and a low frequency

CLKOUT will result in lower operating power. A second prime consideration is the series

resistance Rs of the quartz used.

9.1.1 Power consumption with respect to quartz series resistance

The series resistance acts as a loss element. Low Rs will reduce current consumption

further.

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