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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 one. 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 3 alarm bytes. The “don’t care” code is any hexadecimal value from C0 to FF. The 2 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.

Table 1. Time, Calendar, and Alarm Data Modes

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