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TIME, CALENDAR AND ALARM LOCATIONS

The time and calendar information is obtained by reading the appropriate memory bytes. The time,

calendar, and alarm are set or initialized by writing the appropriate RAM bytes. The contents of the 10

time, calendar, and alarm bytes can be either Binary or Binary–Coded Decimal (BCD) format. Before

writing the internal time, calendar, and alarm registers, the SET bit in Register B should be written to a

logic 1 to prevent updates from occurring while access is being attempted. In addition to writing the 10

time, calendar, and alarm registers in a selected format (binary or BCD), the data mode bit (DM) of

Register B must be set to the appropriate logic level. All 10 time, calendar, and alarm bytes must use the

same data mode. The set bit in Register 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 10

data bytes. Table 2 shows the binary and BCD formats of the 10 time, calendar, and alarm locations. The

24–12 bit cannot be changed without reinitializing the hour locations.

When the 12–hour format is

selected, the high order bit of the hours byte represents PM when it is a logic 1. The time, calendar, and

alarm bytes are always accessible because they are double buffered. Once per second the 10 bytes are

advanced by 1 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 interrupt is initiated at the specified time each day

if the alarm enable bit is high. 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 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.

TIME, CALENDAR AND ALARM DATA MODES Table 2

RANGE

ADDRESS

LOCATION

FUNCTION

DECIMAL

RANGE

BINARY DATA MODE

BCD DATA MODE

0

Seconds

0-59

00-3B

00-59

1

Seconds Alarm

0-59

00-3B

00-59

2

Minutes

0-59

00-3B

00-59

3

Minutes Alarm

0-59

00-3B

00-59

Hours-12-hr Mode

1-12

01-0C AM, 81-8C PM

01-12AM, 81-92PM

4

Hours-24-hr Mode

0-23

00-17

00-23

Hours Alarm-12-hr

1-12

01-0C AM, 81-8C PM

01-12AM, 81-92PM

5

Hours Alarm-24-hr

0-23

00-17

00-23

6

Day of the Week

Sunday = 1

1-7

01-07

01-07

7

Date of the Month

1-31

01-1F

01-31

8

Month

1-12

01-0C

01-12

9

Year

0-99

00-63

00-99