Chapter 7
PFI
©
National Instruments Corporation
7-5
Ass
u
me that an inp
u
t terminal has been low for a long time. The inp
u
t
terminal then changes from low to high, b
u
t glitches several times. When
the filter clock has sampled the signal high on N consec
u
tive edges, the low
to high transition is propagated to the rest of the circ
u
it. The val
u
e of N
depends on the filter setting; refer to Table 7-1.
The filter setting for each inp
u
t can be config
u
red independently. On power
u
p, the filters are disabled. Fig
u
re 7-4 shows an example of a low to high
transition on an inp
u
t that has its filter set to 125 ns (N = 5).
Figure 7-4.
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
.
Table 7-1.
Filters
Filter Setting
N (Filter Clocks
Needed to
Pass Signal)
Pulse Width
Guaranteed to
Pass Filter
Pulse Width
Guaranteed to
Not Pass Filter
125 ns
5
125 ns
100 ns
6.425
μ
s
257
6.425
μ
s
6.400
μ
s
2.56 ms
~101,800
2.56 ms
2.54 ms
Disabled
—
—
—
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
.