National Instruments NI USB-621x - Manual (Page 90)

Chapter 9 Counters
NI USB-621x User Manual 9-10 ni.com
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 9-11 illustrates this
method.
Figure 9-11. Method 1
F1
Ft
F1
Ft
Gate
Source
1 2 3 … N
Single Period
Measurement
…
Period of F1 =
N
Ft
Frequency of F1 =
N
Ft
Interval Measured