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:
•
•
External or Internal Source Less than 20 MHz
80 MHz Source Mode
In 80 MHz source mode, the controller synchronizes signals on the rising edge of the source,
and counts on the third rising edge of the source. Edges are pipelined so no counts are lost, as
shown in the figure below.
Figure 49. 80 MHz Source Mode
80 MHz Source
Synchronize
Count
External or Internal Source Less than 20 MHz
With an external or internal source less than 20 MHz, the module generates a delayed Source
signal by delaying the Source signal by several nanoseconds. The controller synchronizes
signals on the rising edge of the delayed Source signal, and counts on the following rising
edge of the source, as shown in the figure below.
Figure 50. External or Internal Source Less than 20 MHz
Source
Delayed Source
Synchronize
Count
NI cRIO-905x User Manual
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© National Instruments
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