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
8.1 Register overview
The time registers are encoded in BCD to simplify application use. Other registers are
either bit-wise or standard binary.
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
Address Register name
EXT_TEST 0 STOP
POR_OVRD 12_24 0
MI SI MSF
AF TF AIE TIE
- AMPM HOUR_ALARM
0Ch Weekday_alarm AEN_W
TE - -
- - CTD
 Ten’s place.
Product data sheet
Rev. 01 — 28 July 2008
© NXP B.V. 2008. All rights reserved.
4 of 36