Chapter 9

Counters

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.

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 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 9-10shows an example of a buffered semi-period measurement.

Counter Armed

 

 

 

 

GATE

 

 

 

 

SOURCE

 

 

 

 

Counter Value 0

1 2 3

1 1

2

1

Buffer

3 3

1 3

2

3

 

1

 

1

 

 

 

 

2

Figure 9-10.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.

© National Instruments Corporation

9-9

NI USB-621x User Manual