Chapter 8
Counters
8-34
ni.com
Figure 8-29.
Filter Example
Enabling filters introd
u
ces jitter on the inp
u
t signal. For the 125 ns and
6.425
μ
s filter settings, the jitter is
u
p to 25 ns. On the 2.56 ms setting, the
jitter is
u
p to 10.025
μ
s.
Refer to the KnowledgeBase doc
u
ment,
Digital Filtering with M Series
,
for more information abo
u
t digital filters and co
u
nters. To access this
KnowledgeBase, go to
ni.com/info
and enter the info code
rddfm
s
.
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. USB-621
x
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 8-30.
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).
Prescaling can be
u
sed when the co
u
nter So
u
rce is an external signal.
1 2
3
1 4
1
2
3
4 5
PFI Termin
a
l
Filter Clock
(40 MHz)
Filtered Inp
u
t
Filtered inp
u
t goe
s
high
when termin
a
l i
s
sa
mpled
high on five con
s
ec
u
tive
filter clock
s
.
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
