DS1395 datasheet(4/19 Pages) DALLAS

4 / 19 page

DS1395/DS1397

020794 4/19

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.

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