Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Counter/Timers (8254)
If multiple status latch operations of the counter(s) are performed
without reading the status, all but the first are ignored; the status that
is read is the status of the counter at the time the first status Read-
Back command was issued.
Both count and status of the selected counter(s) may be latched
simultaneously by setting both COUNT* and STATUS* bits D5 and
D4 equal to 0. This is functionally the same as issuing two separate
Read-Back commands at once, and the above discussions also apply
here.
Specifically, if multiple count and/or status Read-Back
commands are issued to the same counter(s) without any intervening
reads, all but the first are ignored. This is illustrated in Table 11-1.
If both count and status of a counter are latched, the first read
operation of that counter returns latched status, regardless of which
was latched first. The next one or two reads (depending on whether
the counter is programmed for one or two type counts) return latched
count.
11-14