Rainbow Electronics DS14287 - Manual (Page 9)

DS14285/DS14287
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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.
TIME, CALENDAR AND ALARM DATA MODES Table 1
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