Chapter 8 Counters
© National Instruments Corporation 8-9 NI USB-621x User Manual
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.
Figure 8-9 shows an example of a buffered semi-period measurement.
Figure 8-9. Buffered Semi-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, 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.
3
1
2
3
1
3
SOURCE
GATE
Counter Value
Buffer
1 3
2
2
1 1
1 3
1 2
0
Counter Armed
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