Ziatech Corporation ZT 8809A, Руководство по эксплуатации

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Counter/Timers (8254)
Mode 2: Rate Generator
This mode functions like a divide-by-N counter. It is typically used to
generate a real-time clock interrupt. OUT is initially high. When the
initial count has decremented to 1, OUT goes low for one CLK pulse.
OUT then goes high again, the counter reloads the initial count, and
the process is repeated. Mode 2 is periodic: the same sequence is
repeated indefinitely. For an initial count of N, the sequence repeats
every N CLK cycles.
GATE = 1 enables counting
GATE = 0 disables counting
If GATE goes low during an output pulse, OUT is set high
immediately. A trigger reloads the counter with the initial count on
the next CLK pulse. OUT goes low N CLK pulses after the trigger.
Thus the GATE input can be used to synchronize the counter.
After writing a Control Word and initial count, the counter will be
loaded on the next CLK pulse. OUT goes low N CLK pulses after the
initial count is written. This allows the counter to be synchronized by
software also.
Writing a new count while counting does not affect the current
counting sequence. If a trigger is received after writing a new count
but before the end of the current period, the counter will be loaded
with the new count on the next CLK pulse and counting continues
from the new count. Otherwise, the new count is loaded at the end of
the current counting cycle. In Mode 2, a count of 1 is illegal.
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