National Instruments NI USB-621x manual 90

Chapter 9 Counters

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.

Frequency Measurement

You can use the counters to measure frequency in several different ways. You can choose one of the following methods depending on your application.

Method 1—Measure Low Frequency with One

Counter

In this method, you measure one period of your signal using a known timebase. This method is good for low frequency signals.

You can route the signal to measure (F1) to the Gate of a counter. You can route a known timebase (Ft) to the Source of the counter. The known timebase can be 80MHzTimebase. For signals that might be slower than 0.02 Hz, use a slower known timebase.

You can configure the counter to measure one period of the gate signal. The frequency of F1 is the inverse of the period. Figure 9-11illustrates this method.

					Interval Measured
F1		F1
	Gate
		1	2	3		…		…		N
Ft	Source	Ft
				Period of F1 =			N
	Single Period
	Measurement						Ft
				Frequency of F1 =					Ft
									N

Figure 9-11. Method 1

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