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. The cRIO controller offers 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 as shown in the
figure below.
Figure 47. Prescaling
External Signal
Counter Value
Prescaler Rollover
(Used as Source
by Counter)
0 1
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 (80 MHz Timebase, 20 MHz
Timebase, or 100 kHz Timebase).
Synchronization Modes
The 32-bit counter counts up or down synchronously with the Source signal. The Gate signal
and other counter inputs are asynchronous to the Source signal, so the cRIO controller
synchronizes these signals before presenting them to the internal counter.
Depending on how you configure your controller, the cRIO controller uses one of two
synchronization methods:
• 80 MHz Source Mode
• External or Internal Source Less than 20 MHz
76 | ni.com | cRIO-904x User Manual
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