PCA2125_1 © NXP B.V. 2008. All rights reserved.
Product data sheet Rev. 01 — 28 July 2008 4 of 36
NXP Semiconductors
PCA2125
SPI Real-time clock/calendar
• The first two registers at addresses 00h and 01h (Control_1 and Control_2) are used
as control registers.
• Registers at addresses 02h to 08h (Seconds, Minutes, Hours, Days, Weekdays,
Months, Years) are used as counters for the clock function. Seconds, minutes, hours,
days, 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/calendar during a carry condition is prevented.
• Registers at addresses 09h to 0Ch (Minute_alarm, Hour_alarm, Day_alarm,
Weekday_alarm) define the alarm condition.
• Register at address 0Dh (CLKOUT_control) defines the clock out mode.
• Registers at addresses 0Eh and 0Fh (Timer_control and Countdown_timer) are used
for the countdown timer function. The countdown timer has four selectable source
clocks allowing for countdown periods in the range from less than 1 ms to more than 4
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).
8.1 Register overview
The time registers are encoded in BCD to simplify application use. Other registers are
either bit-wise or standard binary.
[1] Ten’s place.
Table 4. Register overview
Bits labeled ‘-’ are not implemented and will return a logic 0 when read. Bit positions labeled ‘0’ should always be written with
logic 0.
Address Register name Bit
7 6 5 4 3 2 1 0
00h Control_1 EXT_TEST 0 STOP 0 POR_OVRD 12_24 0 0
01h Control_2 MI SI MSF TI_TP AF TF AIE TIE
02h Seconds RF SECONDS
[1]
SECONDS
03h Minutes - MINUTES
[1]
MINUTES
04h Hours - - AMPM HOURS
[1]
HOURS
- - HOURS
[2]
HOURS
05h Days - - DAYS
[1]
DAYS
06h Weekdays - - - - - WEEKDAYS
07h Months - - - MONTHS
[1]
MONTHS
08h Years YEARS
[1]
YEARS
09h Minute_alarm AEN_M MINUTE_ALARM
[1]
MINUTE_ALARM
0Ah Hour_alarm AEN_H - AMPM HOUR_ALARM
[1]
HOUR_ALARM
- HOUR_ALARM
[2]
HOUR_ALARM
0Bh Day_alarm AEN_D - DAY_ALARM
[1]
DAY_ALARM
0Ch Weekday_alarm AEN_W - - - - WEEKDAY_ALARM
0Dh CLKOUT_control - - - - - COF
0Eh Timer_control TE - - - - - CTD
0Fh Countdown_timer COUNTDOWN_TIMER
