Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Counter/Timers (8254)
Read-Back Command
The third method of reading the 8254 is through use of the Read-Back
command. This command allows you to check the count value,
programmed mode, and current state of the OUT pin and Null Count
flag of the selected counters. The Read-Back command is selected by
writing to the Control Word register and has the format shown in
Figure 11-3 on page 11-7.
The Read-Back command may be used to latch multiple counter
output latches (OL) by setting the COUNT* bit D5 to zero and
selecting the desired counter(s) by setting the appropriate bits D1
through D3 to 1. This single command is functionally equivalent to
several Counter Latch commands, one for each counter latched. Each
counter’s latched count is held until it is read (or until the counter is
reprogrammed).
The counter is automatically unlatched when read, but other counters
remain latched until they are read. If multiple count Read-Back
commands are issued to the same counter without reading the count,
all but the first are ignored; the count that will be read is the count at
the time the first Read-Back command was issued.
The Read-Back command may also be used to latch status
information of selected counter(s) by setting STATUS* bit D4 = 0.
STATUS* must be latched to be read; status of a counter is accessed
by a read from that counter.
11-11