Chapter 9

Counters

Counter Armed

Pause Trigger

(Pause When Low)

SOURCE

Counter Value

0 0

1

2

3

4

5

Figure 9-3.Single Point (On-Demand) Edge Counting with Pause Trigger

Buffered (Sample Clock) Edge Counting

With buffered edge counting (edge counting using a sample clock), the counter counts the number of edges on the Source input after the counter is armed. The value of the counter is sampled on each active edge of a sample clock. A USB Signal Stream transfers the sampled values to host memory.

The count values returned are the cumulative counts since the counter armed event. That is, the sample clock does not reset the counter.

You can route the counter sample clock to the Gate input of the counter. You can configure the counter to sample on the rising or falling edge of the sample clock.

Figure 9-4shows an example of buffered edge counting. Notice that counting begins when the counter is armed, which occurs before the first active edge on Gate.

 

Counter Armed

 

 

 

 

Sample Clock

 

 

 

 

 

 

 

(Sample on Rising Edge)

 

 

 

 

 

 

 

SOURCE

 

 

 

 

 

 

 

Counter Value

0

1

2 3 4 5 6

7

 

 

 

 

 

 

 

 

 

 

 

 

 

3

 

3

Buffer

 

 

 

 

 

 

6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 9-4.Buffered (Sample Clock) Edge Counting

© National Instruments Corporation

9-3

NI USB-621x User Manual