55
Watchdog Timer
4
There are two techniques for reading the Time of Day from the Watchdog Timer. The
first is to halt the External Time of Day Registers from tracking the Internal Time of
Day Registers by setting the Te bit (Bit 7 of the Command Register) to a logic zero (0)
then reading the contents of the Time of Day Registers. Using this technique
eliminates the chance of the Time of Day changing while the read is taking place. At
the end of the read, the Te bit is set to a logic one (1) allowing the external Time of Day
Registers to resume tracking the internal Time of Day Registers. No time is lost as the
internal Time of Day Registers continue to keep time while the external Time of Day
Registers are halted. This is the recommended method.
The second technique for reading the Time of Day from the Watchdog Timer is to read
the external Time of Day Registers without halting the tracking of the Internal
Registers. This is not recommended as the registers may be updated while the reading
is taking place, resulting in erroneous data being read.
Time of Day Alarm Registers
Registers 3, 5, and 7 are the Time of Day Alarm Registers and are formatted similar to
Register 2, 4, and 6 respectively. Bit 7 of registers 3, 5, and 7 is a mask bit. The mask
bits, when active (logic one (1)), disable the use of the particular Time of Day Alarm
Register in the determination of the Time of Day Alarm (see Table 4-7). When all the
mask bits are low (0) an alarm will occur when Registers 2, 4, and 6 match the values
found in registers 3, 5, and 7. When Register 7’s mask bit is set to a logic one (1)
Register 6 will be disregarded in the determination of the Time of Day alarm and an
alarm will occur everyday. When Registers 7 and 5’s mask bit is set to a logic one (1),
Register 6 and 4 will be disregarded in the determination of the Time of Day alarm
and an alarm will occur every hour. When Registers 7, 5 and 3’s mask bit is set to a
logic one (1), Register 6, 4, and 2 will be disregarded in the determination of the Time
of Day alarm and an alarm will occur every minute (when Register 1’s seconds step
from 59 to 00).
The Time of Day Alarm registers are read and written to in the same format as the
Time of Day registers. The Time of Day alarm flag and interrupt are cleared when the
alarm registers are read or written.
Table 4-7 Time of Day Alarm Registers
Register
Comment
Minutes
Hours
Days
1
1
1
Alarm once per minute
0
1
1
Alarm when minutes match
0
0
1
Alarm when hours and minutes match
0
0
0
Alarm when hours, minutes, and days match
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