Chapter 9

Counters

Single Period Measurement

With single period measurement, the counter counts the number of rising (or falling) edges on the Source input occurring between two active edges of the Gate input. On the second active edge of the Gate input, the counter stores the count in a hardware save register and ignores other edges on the Gate and Source inputs. Software then reads the stored count.

Figure 9-8shows an example of a single period measurement.

GATE

 

 

 

 

 

SOURCE

 

 

 

 

 

Counter Value

0

1

2

3

4 5

HW Save Register

 

 

 

 

5

Figure 9-8.Single Period Measurement

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 USB Signal Stream transfers the stored values to host memory.

The counter begins on the first active edge of the Gate after it is armed. The arm usually occurs in the middle of a period of the Gate input. The counter does not store a measurement for this incomplete period.

Figure 9-9shows an example of a buffered period measurement. In this example, a period is defined by two consecutive rising edges.

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NI USB-621x User Manual