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National Instruments Corp.
15
FP-CTR-500 and cFP-CTR-500
count-input channel with the lower number is the prescaler
counter and the count-input channel with the higher number is
the pulse counter.
3. Set the Gate Source attribute of the prescaler counter to
Always Enabled
, and set Read Reset Mode to
Don’t Reset
On Read
.
4. Set the Count Source of the prescaler counter to the clock on
which you want to base your pulse train. This can be the
external counter input or one of the [c]FP-CTR-500 internal
references.
5. Subtract 1 from the value that you want to divide the input
clock by, and set the terminal count of the prescaler counter to
the result. For example, a terminal count of 4 divides the input
clock by 5. If you use the 1 kHz reference as the prescaler count
source, this setting generates a 200 Hz clock for the pulse
counter.
To determine the frequency of the prescaler output, use the
following formula:
where
f
pre
is the frequency of the prescaler counter output
f
src
is the count input frequency for the prescaler counter
term
pre
is the terminal count value for the prescaler
counter
Step 2. Set Up the Pulse Counter
To set up the pulse counter, complete the following steps:
1. Set the Count Source of the pulse counter (the count-input
channel with the higher number) to
Previous Channel
so that
it uses the output from the prescaler counter.
Note
If you are not using a prescaler counter, set the
Count Source to the clock on which you want to base
your pulse train.
2. Set the Read Reset Mode of the pulse counter to
Don’t Reset
On Read
and set the Gate Source to
Always Enabled
.
f
pre
f
src
term
pre
1
+
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=
