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Chapter 8
Counters
©
National Instruments Corporation
8-37
Prescaling is not available if the co
u
nter So
u
rce is one of the internal
timebases (80MHzTimebase, 20MHzTimebase, or 100kHzTimebase).
Duplicate Count Prevention
D
u
plicate co
u
nt prevention (or synchrono
u
s co
u
nting mode) is
u
sed when
taking freq
u
ency or period meas
u
rements on USB-621
x
devices, which
ens
u
res that a co
u
nter ret
u
rns correct data in applications that
u
se a slow or
non-periodic external so
u
rce. D
u
plicate co
u
nt prevention applies only to
b
u
ffered co
u
nter applications, s
u
ch as meas
u
ring freq
u
ency or period. In
s
u
ch b
u
ffered applications, the co
u
nter sho
u
ld store the n
u
mber of times an
external So
u
rce p
u
lses between rising edges on the Gate signal.
Example Application That Works Correctly
(No Duplicate Counting)
Fig
u
re 8-33 shows an external b
u
ffered signal as the period meas
u
rement
So
u
rce.
Figure 8-33.
Duplicate Count Prevention Example
On the first rising edge of Gate, the c
u
rrent co
u
nt of 7 is stored. On the next
rising edge of Gate, the co
u
nter stores a 2 since two So
u
rce p
u
lses occ
u
rred
after the previo
u
s rising edge of Gate.
The co
u
nter synchronizes or samples the Gate signal with the So
u
rce
signal, so the co
u
nter does not detect a rising edge in Gate
u
ntil the next
So
u
rce p
u
lse. In this example, the co
u
nter stores the val
u
es in the b
u
ffer on
G
a
te
2
7
7
6 7 1
2
1
Ri
s
ing Edge
of G
a
te
Co
u
nter detect
s
ri
s
ing edge
of G
a
te on the next ri
s
ing
edge of
S
o
u
rce.
S
o
u
rce
Co
u
nter V
a
l
u
e
B
u
ffer