Chapter 8 Counters
NI USB-621x User Manual 8-4 ni.com
Controlling the Direction of Counting
In edge counting applications, the counter can count up or down. You can
configure the counter to do the following:
• Always cou nt up
• Always cou nt down
•Count up when the Counter n B input is high; count down when it
is low
For information about connecting counter signals, refer to the Default
Counter/Timer Pinouts section.
Pulse-Width Measurement
In pulse-width measurements, the counter measures the width of a pulse on
its Gate input signal. You can configure the counter to measure the width
of high pulses or low pulses on the Gate signal.
You can route an internal or external periodic clock signal (with a known
period) to the Source input of the counter. The counter counts the number
of rising (or falling) edges on the Source signal while the pulse on the Gate
signal is active.
You can calculate the pulse width by multiplying the period of the Source
signal by the number of edges returned by the counter.
A pulse-width measurement is accurate even if the counter is armed while
a pulse train is in progress. If a counter is armed while the pulse is in the
active state, it waits for the next transition to the active state to begin the
measurement.
Single Pulse-Width Measurement
With single pulse-width measurement, the counter counts the number of
edges on the Source input while the Gate input remains active. When the
Gate input goes inactive, the counter stores the count in a hardware save
register and ignores other edges on the Gate and Source inputs. Software
then reads the stored count.
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