DS1495 Datasheet, PDF (4/19 Pages) Dallas Semiconductor – RAMified Real Time Clock

DS1495/DS1497
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 loca-
tions. When the 12-hour format is selected, the high or-
der bit of the hours byte represents PM when it is a logic
one. The time, calendar, and alarm bytes are always ac-
cessible because they are double buffered. Once per
second the ten 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.
USER NONVOLATILE RAM - RTC
The 50 user nonvolatile RAM bytes are not dedicated to
any special function within the DS1495/DS1497. They
can be used by the application program as nonvolatile
memory and are fully available during the update cycle.
This memory is directly accessible in the RTC section.
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