Real-Time Clock
The three alarm bytes can be used in two ways. First,
when the alarm time is written in the appropriate hours,
minutes, and seconds alarm locations, the alarm inter-
rupt is initiated at the specified time each day, if the
alarm-enable bit is high. In this mode, the “0” bits in the
alarm registers and the corresponding time registers
must always be written to 0 (Table 2A and 2B). Writing
the 0 bits in the alarm and/or time registers to 1 can
result in undefined operation.
The second use condition is to insert a “don’t care”
state in one or more of the three alarm bytes. The don’t-
care code is any hexadecimal value from C0 to FF. The
two most significant bits of each byte set the don’t-care
condition when at logic 1. An alarm is generated each
hour when the don’t-care bits are set in the hours byte.
Similarly, an alarm is generated every minute with
don’t-care codes in the hours and minute alarm bytes.
The don’t-care codes in all three alarm bytes create an
interrupt every second.
All 128 bytes can be directly written or read, except for
the following:
1) Registers C and D are read-only.
2) Bit 7 of register A is read-only.
3) The MSB of the seconds byte is read-only.
DS12885/DS12887/DS12887A/DS12C887/DS12C887A
Table 2A. Time, Calendar, and Alarm Data Modes—BCD Mode (DM = 0)
ADDRESS
00H
01H
02H
03H
04H
05H
06H
07H
08H
09H
0AH
0BH
0CH
0DH
0EH-31H
32H
33H-7FH
X
UIP
SET
IRQF
VRT
X
BIT 7
0
0
0
0
AM/PM
0
AM/PM
0
0
0
0
0
0
0
0
0
DV2
PIE
PF
0
X
X
0
10 Years
DV1
AIE
AF
0
X
X
DV0
UIE
UF
0
X
X
RS3
SQWE
0
0
X
X
BIT 6
BIT 5
BIT 4
10 Seconds
10 Seconds
10 Minutes
10 Minutes
0
0
0
10 Hours
10 Hours
10 Hours
10 Hours
0
10 Date
10 Months
0
Date
Month
Year
RS2
DM
0
0
X
Century
X
X
X
RS1
24/12
0
0
X
RS0
DSE
0
0
X
BIT 3
BIT 2
BIT 1
Seconds
Seconds
Minutes
Minutes
Hours
Hours
Day
BIT 0
FUNCTION
Seconds
Seconds Alarm
Minutes
Minutes Alarm
Hours
Hours Alarm
Day
Date
Month
Year
Control
Control
Control
Control
RAM
Century*
RAM
RANGE
00–59
00–59
00–59
00–59
1–12 +AM/PM
00–23
1–12 +AM/PM
00–23
01–07
01–31
01–12
00–99
—
—
—
—
—
00–99
—
10 Century
X = Read/Write Bit.
*DS12C887,
DS12C887A only. General-purpose RAM on DS12885, DS12887, and DS12887A.
Note:
Unless otherwise specified, the state of the registers is not defined when power is first applied. Except for the seconds regis-
ter, 0 bits in the time and date registers can be written to 1, but may be modified when the clock updates. 0 bits should always be
written to 0 except for alarm mask bits.
____________________________________________________________________
13
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