Chapter 7 Counters
© National Instruments 7-51 X Series User Manual
Other Counter Features
The following sections list the other counter features available on X Series
devices.
Cascading Counters
You can internally route the Counter n Internal Output and Counter n TC
signals of each counter to the Gate inputs of the other counter. By cascading
two counters together, you can effectively create a 64-bit counter. By
cascading counters, you also can enable other applications. For example, to
improve the accuracy of frequency measurements, use reciprocal
frequency measurement, as described in the Large Range of Frequencies
with Two Counters section.
Prescaling
Prescaling allows the counter to count a signal that is faster than the
maximum timebase of the counter. X Series devices offer 8X and 2X
prescaling on each counter (prescaling can be disabled). Each prescaler
consists of a small, simple counter that counts to eight (or two) and rolls
over. This counter can run faster than the larger counters, which simply
count the rollovers of this smaller counter. Thus, the prescaler acts
as a frequency divider on the Source and puts out a frequency that is
one-eighth (or one-half) of what it is accepting.
Figure 7-39. Prescaling
Prescaling is intended to be used for frequency measurement where the
measurement is made on a continuous, repetitive signal. The prescaling
counter cannot be read; therefore, you cannot determine how many edges
have occurred since the previous rollover. Prescaling can be used for event
counting provided it is acceptable to have an error of up to seven (or one)
ticks. Prescaling can be used when the counter Source is an external signal.
Prescaling is not available if the counter Source is one of the internal
timebases (100MHzTimebase, 20MHzTimebase, or 100kHzTimebase).
External Signal
Counter Value
Prescaler Rollover
(Used as Source
by Counter)
0 1
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