© National Instruments | 7-7
M Series User Manual
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.
Refer to the following sections for more information about M Series semi-period measurement
options:
•
Single Semi-Period Measurement
• Buffered Semi-Period Measurement
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 DMA controller transfers the stored values to host memory.
The counter begins counting when it is armed. The arm usually occurs between edges on the
Gate input. So the first value stored in the hardware save register does not reflect a full
semi-period of the Gate input. In most applications, this first point should be discarded.
Figure 7-9 shows an example of a buffered semi-period measurement.
Figure 7-9. Buffered Semi-Period Measurement
3
1
2
2
3
1
2
2
3
2
SOURCE
GATE
Counter Value
Buffer
1 3
2
2
11
132
121
0
2
Counter Armed
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