ADLINK Technology cPCI-7200, Руководство пользователя

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8254 Programmable Interval Time
Rewriting a counter register during counting results in the
following:
(1) Write 1st byte stops the current counting.
(2) Write 2nd byte starts the new count.
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Mode 1 : Programmable One-Shot.
The output will go low on the count following the rising edge
of the gate input. The output will go high on the terminal
count. If a new count value is loaded while the output is low it
will not affect the duration of the one-shot pulse until the
succeeding trigger. The current count can be read at anytime
without affecting the one-shot pulse.
The one-shot is re-triggerable, hence the output will remain
low for the full count after any rising edge of the gate input.
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Mode 2 : Rate Generator.
Divided by N counter. The output will be low for one period of
the input clock. The period from one output pulse to the next
equals the number of input counts in the count register. If the
count register is reloaded between output pulses the present
period will not be affected, but the subsequent period will
reflect the new value.
The gate input when low, will force the output high. When the
gate input goes high, the counter will start form the initial
count. Thus, the gate input can be used to synchronized by
software.
When this mode is set, the output will remain high until after
the count register is loaded. The output then can also be
synchronized by software.