National Instruments Data Acquisition Device NI USB-621x, User Manual

Page: 99 / 185

Chapter 8 Counters
NI USB-621x User Manual 8-6 ni.com
condition is not met, consider u sing d u plicate co u nt prevention, described
in the Duplicate Count Prevention section.
For information abo u t connecting co u nter signals, refer to the Default
Counter/Timer Pinouts section.
Period Measurement
In period meas u rements, the co u nter meas u res a period on its Gate inp u t
signal after the co u nter is armed. Yo u can config u re the co u nter to meas u re
the period between two rising edges or two falling edges of the Gate inp u t
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 occ u rring on the So u rce inp u t between the
two active edges of the Gate signal.
Yo u can calc u late the period of the Gate inp u t 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.
Single Period Measurement
With single period meas u rement, the co u nter co u nts the n u mber of rising
(or falling) edges on the So u rce inp u t occ u rring between two active edges
of the Gate inp u t. On the second active edge of the Gate inp u t, 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.
Fig u re 8-7 shows an example of a single period meas u rement.
Figure 8-7. Single Period Measurement
S OURCE
GATE
Co u nter V a l u e
HW Sa ve Regi s ter
1 0 3
5
4 5 2