B&B Electronics PC Watchdog Timer Card ATRWDT - Manual (Page 11)

Documentation Number ATxWDT-1303 9
B&B Electronics Mfg Co – 707 Dayton Rd - PO Box 1040 - Ottawa IL 61350 - Ph 815-433-5100 - Fax 815-433-5104
B&B Electronics Ltd – Westlink Comm. Pk – Oranmore, Galway, Ireland – Ph +353 91-792444 – Fax +353 91-792445
Chapter 3: OPERATION
Communicating with the WDT
The WDT uses a four-bit latch to start and stop the timers and
set the timeout period.
Regardless of the mode of operation of the WDT, a "1" must be
written to the least significant bit of the latch to start the timers.
Writing a "0" to the least significant bit of the latch at any time will
stop the timers. Note that since the least significant bit is used to
start and stop the counters, any even number written to the latch will
result in the counters being turned off.
The remaining three bits of the latch are used to select one of
eight timeout periods. Table 3 shows the possible timeouts.
To use the WDT from your program, the latch value
corresponding to the desired timeout must be periodically written to
the latch. After the counters are started, they are reset every time
the WDT is addressed. As long as the WDT is addressed before the
end of the selected timeout period, no reset will occur. To do this
from a BASIC program, place an OUT <WDT address>, <latch
value> somewhere in a reoccurring loop. Each time the OUT
statement is executed, the WDT counters are reset and begin
counting again. To disable the WDT, place an OUT <WDT
address>, <0> in your program. Be certain that this statement is
executed whenever your program is terminated.
Table 3. WDT Timeouts
Binary
Latch
Value
Hex
Latch
Value
Approximate
Timeout
(seconds)
0001 1 0.5
0011 3 2
0101 5 6
0111 7 14
1001 9 30
1011 B 126
1101 D 254
1111 F 510