DS1385 Datasheet, PDF (5/20 Pages) Dallas Semiconductor – RAMified Real Time Clock 4K x 8

DS1385/DS1387
ADDRESS MAP DS1385/DS1387 Figure 2
0
00
14–BYTES
13
0D
14
0E
50–BYTES
USER RAM
63
3F
0
000
4K X 8
NV SRAM
4096
FFF
TIME, CALENDAR AND ALARM LOCATIONS
The time and calendar information is obtained by read-
ing the appropriate register bytes shown in Table 1. The
time, calendar and alarm are set or initialized by writing
the appropriate register bytes. The contents of the time,
calendar and alarm registers can be either Binary or
Binary–Coded Decimal (BCD) format. Table 1 shows
the binary and BCD formats of the twelve time, calendar
and alarm locations.
Before writing the internal time, calendar and alarm reg-
isters, the SET bit in Register B should be written to a
logic one to prevent updates from occurring while ac-
cess is being attempted. Also at this time, the data for-
mat (binary or BCD), should be set via the data mode bit
(DM) of Register B. All time, calendar and alarm regis-
ters must use the same data mode. The set bit in Regis-
ter B should be cleared after the data mode bit has been
written to allow the real–time clock to update the time
and calendar bytes.
Once initialized, the real–time clock makes all updates
in the selected mode. The data mode cannot be
changed without reinitializing the ten data bytes. The
24/12 bit cannot be changed without reinitializing the
hour locations. When the 12–hour format is selected,
0
SECONDS
1
SECONDS ALARM
2
MINUTES
3
MINUTES ALARM
4
HOURS
5
HOURS ALARM
6
DAY OF THE WEEK
7
DAY OF THE MONTH
8
MONTH
9
YEAR
10
REGISTER A
11
REGISTER B
12
REGISTER C
13
REGISTER D
the high order bit of the hours byte represents PM when
it is a logic one. The time, calendar and alarm bytes are
always accessible because they are double buffered.
Once per second the 10–bytes are advanced by one
second and checked for an alarm condition. If a read of
the time and calendar data occurs during an update, a
problem exists where seconds, minutes, hours, etc.
may not correlate. The probability of reading incorrect
time and calendar data is low. Several methods of
avoiding any possible incorrect time and calendar reads
are covered later in this text.
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. The second method 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 will be
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.
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