DS12887
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
ADDRESS
LOCATION
0
1
2
3
4
5
6
7
8
9
FUNCTION
Seconds
Seconds Alarm
Minutes
Minutes Alarm
Hours-12-hr Mode
Hours-24-hr Mode
Hours Alarm-12-hr
Hours Alarm-24-hr
Day of the Week
Sunday = 1
Date of the Month
Month
Year
DECIMAL
RANGE
0-59
0-59
0-59
0-59
1-12
0-23
1-12
0-23
1-7
1-31
1-12
0-99
RANGE
BINARY DATA MODE BCD DATA MODE
00-3B
00-59
00-3B
00-59
00-3B
00-59
00-3B
00-59
01-0C AM, 81-8C PM
01-12AM, 81-92PM
00-17
00-23
01-0C AM, 81-8C PM
01-12AM, 81-92PM
00-17
00-23
01-07
01-07
01-1F
01-0C
00-63
01-31
01-12
00-99
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DALLAS [ DALLAS SEMICONDUCTOR ]
