Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Counter/Timers (8254)
If a new count is written to the counter, it will be 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 disables counting. OUT is set low
immediately (no clock pulse required).
2. Writing the second byte allows the new count to be loaded on
the next CLK pulse.
This allows the counting sequence to be synchronized by software.
Again, OUT does not go high until N + 1 CLK pulses after the new
count of N is written.
If an initial count is written while GATE = 0, it will still be loaded on
the next clock pulse. When GATE goes high, OUT goes high N
pulses later. No clock pulse is needed to load the counter because this
has already been done.
Mode 1: Hardware Retriggerable One-Shot
OUT is initially high. OUT goes low on the CLK pulse following a
trigger to begin the one-shot pulse and remains low until the counter
reaches zero. OUT then goes high and remains high until the CLK
pulse after the next trigger.
After writing the Control Word and initial count, the counter is armed.
A trigger results in loading the counter and setting OUT low on the
next CLK pulse, thus starting the one-shot pulse. An initial count of N
results in a one-shot pulse N CLK cycles in duration. The one-shot is
retriggerable, hence OUT remains low for N CLK pulses after any
trigger. The one-shot pulse can be repeated without rewriting the
same count into the counter. GATE has no effect on OUT.
If a new count is written to the counter during a one-shot pulse, the
current one-shot is not affected unless the counter is retriggered. In
that case, the counter is loaded with the new count and the one-shot
pulse continues until the new count expires.
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