Prescaling; Synchronization Modes - National Instruments X Series User Manual

Chapter 7 Counters

Prescaling

Prescaling allows the counter to count a signal that is faster than the maximum timebase of the
counter, as shown in Figure 7-39. 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.
Prescaler Rollover
(Used as Source
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).

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 X Series devices synchronize
these signals before presenting them to the internal counter.
Depending on how you configure your device, X Series devices use one of three synchronization
methods:
• 100 MHz Source Mode
• External Source Greater than 25 MHz
• External or Internal Source Less than 25 MHz
7-46 | ni.com
Figure 7-39. Prescaling
External Signal
by Counter)
Counter Value
0 1