PIC18F87K90 Datasheet, PDF (228/566 Pages) Microchip Technology – 64/80-Pin, High-Performance Microcontrollers with LCD Driver and nanoWatt XLP Technology

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17.2.2 CLOCK SOURCE
As mentioned earlier, the RTCC module is intended to
be clocked by an external Real-Time Clock (RTCC)
crystal oscillating at 32.768 kHz, but an internal oscillator
can be used. The RTCC clock selection is decided by
the RTCOSC bit (CONFIG3L<0>).
Calibration of the crystal can be done through this
module to yield an error of 3 seconds or less per month.
(For further details, see Section 17.2.9 “Calibration”.)
FIGURE 17-4:
CLOCK SOURCE MULTIPLEXING
32.768 kHz XTAL
from SOSC
Internal RC
1:16384
Clock Prescaler⁽¹⁾
Half Second
Clock
Half Second⁽¹⁾
One Second Clock
CONFIG3L<0>
Second
Hour:Minute
Day
Day of Week
Month
Year
Note 1: Writing to the lower half of the MINSEC register resets all counters, allowing fraction of a second
synchronization; clock prescaler is held in Reset when RTCEN = 0.
17.2.2.1 Real-Time Clock Enable
The RTCC module can be clocked by an external
32.768 kHz crystal (SOSC oscillator), or the LF-INTOSC
oscillator, which can be selected in CONFIG3L<0>.
If the external clock is used, the SOSC oscillator should
be enabled. If LF-INTOSC is providing the clock, the
INTOSC clock can be brought out to the RTCC pin by
the RTSECSEL<1:0> bits (PADCFG<2:1>).
17.2.3 DIGIT CARRY RULES
This section explains which timer values are affected
when there is a rollover:
• Time of Day: From 23:59:59 to 00:00:00 with a
carry to the Day field
• Month: From 12/31 to 01/01 with a carry to the
Year field
• Day of Week: From 6 to 0 with no carry (see
Table 17-1)
• Year Carry: From 99 to 00; this also surpasses the
use of the RTCC
For the day-to-month rollover schedule, see Table 17-2.
Because the following values are in BCD format, the
carry to the upper BCD digit occurs at the count of 10,
not 16 (SECONDS, MINUTES, HOURS, WEEKDAY,
DAYS and MONTHS).
TABLE 17-1: DAY OF WEEK SCHEDULE
Day of Week
Sunday
0
Monday
1
Tuesday
2
Wednesday
3
Thursday
4
Friday
5
Saturday
6
TABLE 17-2: DAY-TO-MONTH ROLLOVER
SCHEDULE
Month
Maximum Day Field
01 (January)
02 (February)
31
28 or 29⁽¹⁾
03 (March)
31
04 (April)
30
05 (May)
31
06 (June)
30
07 (July)
31
08 (August)
31
09 (September)
30
10 (October)
31
11 (November)
30
12 (December)
31
Note 1: See Section 17.2.4 “Leap Year”.
DS39957B-page 228
Preliminary
 2010 Microchip Technology Inc.