Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Counter/Timers (8254)
Odd counts: OUT is initially high. The initial count minus one
(an even number) is loaded on one CLK pulse and
then is decremented by two on succeeding CLK
pulses. One CLK pulse after the count expires,
OUT goes low and the counter is reloaded with
the initial count minus one. Succeeding CLK
pulses decrement the count by two. When the
count expires, OUT goes high again and the
counter is reloaded with the initial count minus
one. The above process is repeated indefinitely.
So for odd counts, OUT is high for (N = 1)/2
counts and low for (N - 1)/2 counts.
Mode 4: Software Triggered Strobe
OUT is initially high. When the initial count expires, OUT goes low
for one CLK pulse and then goes high again. The counting sequence
is "triggered" by writing the initial count.
GATE = 1 enables counting
GATE = 0 disables counting
(GATE has no effect on OUT)
After writing a Control Word and initial count, the counter is loaded
on the next CLK pulse. This CLK pulse does not decrement the
count, so for an initial count of N, OUT does not strobe low until
N + 1 CLK pulses after the initial count is written.
If a new count is written during counting, it is loaded on the next CLK
pulse and counting continues from the new count. If a two-byte count
is written, the following happens:
1. Writing the first byte has no effect on counting.
2. Writing the second byte allows the new count to be loaded on
the next CLK pulse.
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