Chapter 7
Counters
©
National Instruments
7-51
X Series User Manual
Other Counter Features
The following sections list the other co
u
nter feat
u
res available on X Series
devices.
Cascading Counters
Yo
u
can internally ro
u
te the Co
u
nter
n
Internal O
u
tp
u
t and Co
u
nter
n
TC
signals of each co
u
nter to the Gate inp
u
ts of the other co
u
nter. By cascading
two co
u
nters together, yo
u
can effectively create a 64-bit co
u
nter. By
cascading co
u
nters, yo
u
also can enable other applications. For example, to
improve the acc
u
racy of freq
u
ency meas
u
rements,
u
se reciprocal
freq
u
ency meas
u
rement, as described in the
Large Range of Frequencies
with Two Counters
section.
Prescaling
Prescaling allows the co
u
nter to co
u
nt a signal that is faster than the
maxim
u
m timebase of the co
u
nter. X Series devices offer 8X and 2X
prescaling on each co
u
nter (prescaling can be disabled). Each prescaler
consists of a small, simple co
u
nter that co
u
nts to eight (or two) and rolls
over. This co
u
nter can r
u
n faster than the larger co
u
nters, which simply
co
u
nt the rollovers of this smaller co
u
nter. Th
u
s, the prescaler acts
as a freq
u
ency divider on the So
u
rce and p
u
ts o
u
t a freq
u
ency that is
one-eighth (or one-half) of what it is accepting.
Figure 7-39.
Prescaling
Prescaling is intended to be
u
sed for freq
u
ency meas
u
rement where the
meas
u
rement is made on a contin
u
o
u
s, repetitive signal. The prescaling
co
u
nter cannot be read; therefore, yo
u
cannot determine how many edges
have occ
u
rred since the previo
u
s rollover. Prescaling can be
u
sed for event
co
u
nting provided it is acceptable to have an error of
u
p to seven (or one)
ticks. Prescaling can be
u
sed when the co
u
nter So
u
rce is an external signal.
Prescaling is not available if the co
u
nter So
u
rce is one of the internal
timebases (100MHzTimebase, 20MHzTimebase, or 100kHzTimebase).
External Signal
Counter Value
Prescaler Rollover
(Used as Source
by Counter)
0 1
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