© National Instruments | 5-39
NI cDAQ-9132/9133/9134/9135/9136/9137 User Manual
Prescaling
Prescaling allows the counter to count a signal that is faster than the maximum timebase of the
counter. The cDAQ 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 Figure 5-37.
Figure 5-37. 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 (80MHzTimebase, 20MHzTimebase, or
100kHzTimebase).
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 cDAQ controller synchronizes
these signals before presenting them to the internal counter.
Depending on how you configure your controller, the cDAQ controller uses one of
two synchronization methods:
• 80 MHz Source Mode
• 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 Figure 5-38.
Figure 5-38. 80 MHz Source Mode
External Signal
Counter Value
Prescaler Rollover
(Used as Source
by Counter)
01
80 MHz Source
Synchronize Count
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