7-32
|
ni.com
Chapter 7
Counters
Prescaling
Prescaling allows the counter to count a signal that is faster than the maximum timebase of the
counter. M 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 7-31.
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). 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).
Duplicate Count Prevention
Duplicate count prevention (or synchronous counting mode) ensures that a counter returns
correct data in applications that use a slow or non-periodic external source. Duplicate count
prevention applies only to buffered counter applications such as measuring frequency or period.
In such buffered applications, the counter should store the number of times an external Source
pulses between rising edges on the Gate signal.
Extern
a
l
S
ign
a
l
Co
u
nter V
a
l
u
e
Pre
s
c
a
ler Rollover
(U
s
ed
as
S
o
u
rce
b
y Co
u
nter)
0
1
