Chapter 7
Counters
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National Instruments Corporation
7-7
Buffered Period Measurement
Buffered period measurement is similar to single period measurement, but
buffered period measurement measures multiple periods.
The counter counts the number of rising (or falling) edges on the Source
input between each pair of active edges on the Gate input. At the end of
each period on 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 when it is armed. The arm usually occurs in the middle
of a period of the Gate input. So the first value stored in the hardware save
register does not reflect a full period of the Gate input. In most applications,
this first point should be discarded.
Figure 7-8 shows an example of a buffered period measurement.
Figure 7-8.
Buffered Period Measurement
Note that if you are using an external signal as the Source, at least one
Source pulse should occur between each active edge of the Gate signal.
This condition ensures that correct values are returned by the counter. If this
condition is not met, consider using duplicate count prevention, described
in the
section.
For information about connecting counter signals, refer to the
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.
S
OURCE
GATE
Co
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nter V
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l
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e
B
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ffer
1
1 2
3
3
2
3
3
1 1
2
2
2
2
3
3
2
(Di
s
c
a
rd) (Di
s
c
a
rd) (Di
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c
a
rd)
3
Co
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nter Armed
