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Chapter 7 Counters
bandwidth; if such bandwidth is not available due to other measurements taking place, this
method may fail to transfer all the required samples to perform the measurement.
• Using two counters for high frequency measurements is accurate for high frequency
signals. However, the accuracy decreases as the frequency of the signal to measure
decreases. At very low frequencies, this method may be too inaccurate for your application.
Another disadvantage of this method is that it requires two counters (if you cannot provide
an external signal of known width). An advantage of high frequency measurements with
two counters is that the measurement completes in a known amount of time.
• Measuring a large range of frequencies with two counters measures high and low frequency
signals accurately. However, it requires two counters, and it has a variable sample time and
variable error percent dependent on the input signal.
Table 7-4 summarizes some of the differences in methods of measuring frequency.
For information about connecting counter signals, refer to the
Default Counter/Timer Pinouts
section.
Position Measurement
You can use the counters to perform position measurements with quadrature encoders or
two-pulse encoders. You can measure angular position with X1, X2, and X4 angular encoders.
Linear position can be measured with two-pulse encoders. You can choose to do either a single
point (on-demand) position measurement or a buffered (sample clock) position measurement.
You must arm a counter to begin position measurements.
Table 7-4. Frequency Measurement Method Comparison
Comparison
One Counter Two Counters
— Averaged
High
Frequency
Large Range
Number of
counters used
1 1 1 or 2 2
Number of
measurements
returned
1 Many 1 1
Measures high
frequency
signals
accurately
Poor Fair Good Good
Measures low
frequency
signals
accurately
Good Good Poor Good
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