National Instruments USB-621 Series, User Manual

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Chapter 8 Counters
NI USB-621x User Manual 8-4 ni.com
Controlling the Direction of Counting
In edge co u nting applications, the co u nter can co u nt u p or down. Yo u can
config u re the co u nter to do the following:
• Always co u nt u p
• Always co u nt down
• Co u nt u p when the Co u nter n B inp u t is high; co u nt down when it
is low
For information abo u t connecting co u nter signals, refer to the Default
Counter/Timer Pinouts section.
Pulse-Width Measurement
In p u lse-width meas u rements, the co u nter meas u res the width of a p u lse on
its Gate inp u t signal. Yo u can config u re the co u nter to meas u re the width
of high p u lses or low p u lses on the Gate signal.
Yo u can ro u te an internal or external periodic clock signal (with a known
period) to the So u rce inp u t of the co u nter. The co u nter co u nts the n u mber
of rising (or falling) edges on the So u rce signal while the p u lse on the Gate
signal is active.
Yo u can calc u late the p u lse width by m u ltiplying the period of the So u rce
signal by the n u mber of edges ret u rned by the co u nter.
A p u lse-width meas u rement is acc u rate even if the co u nter is armed while
a p u lse train is in progress. If a co u nter is armed while the p u lse is in the
active state, it waits for the next transition to the active state to begin the
meas
u rement.
Single Pulse-Width Measurement
With single p u lse-width meas u rement, the co u nter co u nts the n u mber of
edges on the So u rce inp u t while the Gate inp u t remains active. When the
Gate inp u t goes inactive, the co u nter stores the co u nt in a hardware save
register and ignores other edges on the Gate and So u rce inp u ts. Software
then reads the stored co u nt.