National Instruments NI 6238 用户手册

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Figure 7-9.  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.

For information on connecting counter signals, refer to the 

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 is equivalent to single pulse-width 
measurement.

SOURCE

GATE

Counter Value

Buffer

1

1

2

3

3

2

3

3

1

1

2

2

2

2
3
3

2

(Discard)

(Discard)

(Discard)

3

Counter Armed