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.
Refer to the following sections for more information about semi-period measurement options:
• Single Semi-Period Measurement
• Implicit Buffered Semi-Period Measurement
Refer to the Pulse versus Semi-Period Measurements section for information about the
differences between semi-period measurement and pulse measurement.
Single Semi-Period Measurement
Single semi-period measurement is equivalent to single pulse-width measurement.
Implicit Buffered Semi-Period Measurement
In implicit buffered semi-period measurements, on each edge of the Gate signal, the counter
stores the count in the FIFO. The sampled values will be transferred to host memory using a
high-speed data stream.
The counter begins counting when it is armed. The arm usually occurs between edges on the
Gate input. You can select whether to read the first active low or active high semi-period using
the CI.SemiPeriod.StartingEdge property in NI-DAQmx.
The following figure shows an example of an implicit buffered semi-period measurement.
Figure 60. Implicit Buffered Semi-Period Measurement
1
2
3
1
3
3
SOURCE
GATE
Counter Value
Buffer
1 3
2
2
1 1
13
12
0
Counter
Armed
Starting
Edge
For information about connecting counter signals, refer to the Default Counter/Timer Routing
section.
Pulse versus Semi-Period Measurements
In hardware, pulse measurement and semi-period are the same measurement. Both measure the
high and low times of a pulse. The functional difference between the two measurements is
how the data is returned. In a semi-period measurement, each high or low time is considered
74 | ni.com | NI cRIO-905x User Manual
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