National Instruments Data Acquisition Device NI USB-621x - Manual (Page 101)

Chapter 8 Counters
NI USB-621x User Manual 8-8 ni.com
condition is not met, the counter returns a zero. Refer to the Duplicate
Count Prevention section for more information.
For information about connecting counter signals, refer to the Default
Counter/Timer Pinouts section.
Semi-Period Measurement
In semi-period measurements, the counter measures a semi-period on its
Gate input signal after the counter is armed. A semi-period is the time
between any two consecutive edges on the Gate input.
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 occurring on the Source input between two
edges of the Gate signal.
You can calculate the semi-period of the Gate input by multiplying the
period of the Source signal by the number of edges returned by the counter.
Single Semi-Period Measurement
Single semi-period measurement is equivalent to single pulse-width
measurement.
Buffered Semi-Period Measurement
In buffered semi-period measurement, on each edge of the Gate signal, the
counter stores the count in a hardware save register. A USB Signal Stream
transfers the stored values to host memory.
The counter begins counting on the first active edge of the Gate after it is
armed. The arm usually occurs between edges on the Gate input. The
counter does not store a value for this incomplete semi-period.