Rainbow Electronics DS1687 - Manual (Page 10)

DS1685/DS1687
10 of 38
Figure 2. DS1685 RTC ADDRESS MAP
0 00H 0 SECONDS
1 SECONDS ALARM
13
CLOCK/
CALENDAR
14 BYTES
0DH 2 MINUTES
14 0EH 3 MINUTES ALARM
63
50 BYTES
USER RAM
03FH 4 HOURS
64 040H 5 HOURS ALARM
6 DAY OF THE WEEK
7 DAY OF THE MONTH
8 MONTH
9 YEAR
BANK0,
BANK 1
REGISTERS,
RAM
10 REGISTER A
11 REGISTER B
12 REGISTER C
127 07FH 13 REGISTER D
BINARY OR BCD INPUTS
TIME, CALENDAR, AND ALARM LOCATIONS
The time and calendar information is obtained by reading 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 10 time, calendar, and alarm locations that reside in
both bank 0 and in bank 1, plus the two extended registers that reside in bank 1 only (bank 0 and bank 1
switching are explained later in this text).
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. Also at this time, the data
format (binary or BCD) should be set by the data mode bit (DM) of Register B. All time, calendar, and
alarm registers 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 RTC to update the time and calendar bytes.
Once initialized, the RTC makes all updates in the selected mode. The data mode cannot be changed
without reinitializing the 10 data bytes. 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 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., might 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.
The three time 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 time-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 is