P104-WDG-CSM User Manual
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Appendix A: Programmable Interval Timer
This Appendix includes basic information about the type 8254 Counter/Timer chip. For those
interested in more detailed information, a full description can be found in the manufacturer's
data sheets.
Please note: The information in this Appendix is for general reference, and may include
features or functions not applicable to this card.
Operation Modes
Modes of operation are described in the following paragraphs to familiarize you with the power
and versatility of this device. The following conventions apply in describing operation of type
8254 chips:
Clock: A positive pulse into the counter's clock input.
Trigger: A rising edge input to the counter's gate input.
Counter Loading: Programming a binary count into the counter.
Mode 0: Pulse on Terminal Count
After the counter is loaded, the output is set low and will remain low until the counter
decrements to zero. The output then goes high and remains high until a new count is loaded
into the counter. A trigger enables the counter to start decrementing. This mode is commonly
used for event counting with Counter #0.
Mode 1: Retriggerable One-Shot
The output goes low on the clock pulse following a trigger to begin the one-shot pulse and
goes high when the counter reaches zero. Additional triggers result in reloading the count and
starting the cycle over. If a trigger occurs before the counter decrements to zero, a new count
is loaded. Thus, this forms a re-triggerable one-shot. In mode 1, a low output pulse is provided
with a period equal to the counter count-down time.
Mode 2: Rate Generator
This mode provides a divide-by-N capability where N is the count loaded into the counter.
When triggered, the counter output goes low for one clock period after N counts, reloads the
initial count, and the cycle starts over. This mode is periodic, the same sequence is repeated
indefinitely until the gate input is brought low. This mode also works well as an alternative to
mode 0 for event counting.
Mode 3: Square Wave Generator
Like mode 2, this mode operates periodically. The output is high for half of the count and low
for the other half. If the count is even, then the output is a symmetrical square wave. If the
count is odd, then the output is high for (N+1)/2 counts and low for (N-1)/2 counts. Periodic
triggering or frequency synthesis are two possible applications for this mode. Note that in this
mode, to achieve the square wave, the counter decrements by two for the total loaded count,
then reloads and decrements by two for the second part of the waveform.
Mode 4: Software Triggered Strobe
This mode sets the output high and, when the count is loaded, the counter begins to count
down. When the counter reaches zero, the output will go low for one input period. The counter
must be reloaded to repeat the cycle. A low gate input will inhibit the counter.
(14 Seiten)
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