PCI/PXI/PCIe-6509 User Guide and Specifications
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© National Instruments
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17
You can configure the digital input lines to pass through a digital filter and control the timing
interval the filter uses. The filter blocks pulses that are shorter than half of the specified timing
interval and passes pulses that are longer than the specified interval. Intermediate-length
pulses—pulses longer than half of the interval but less than the interval—may or may not pass
the filter.
Table 2 lists the pulse widths that the NI 6509 is certain to pass and block.
You can enable digital filtering on as many input lines as necessary for your application. All
filtered lines share the same timing interval, which ranges from 400 ns to 200 ms on the NI
PCI/PXI-6509. The PCIe-6509 supports only three timing intervals: 160 ns, 10.24
s, and
5.12 ms.
Internally, the digital filter uses two clocks: a sample clock and a filter clock. The filter clock is
generated by a counter and has a period equal to one half of the specified timing interval. The
NI 6509 samples the input signal on each rising edge of the sample clock, but the NI 6509
recognizes a change in the input signal only if the input signal maintains its new state for at least
two consecutive rising edges of the filter clock. The filter clock is programmable and allows you
to control how long a pulse must last to be recognized by the NI 6509.
Digital Filtering Example
Figure 14 shows an example of digital filtering.
Figure 14.
Digital Filtering Example
In periods A and B, the filter blocks the glitches because the external signal does not remain
steadily high from one rising edge of the filter clock to the next. In period C, the filter passes the
transition because the external signal remains steadily high. Depending on when the transition
occurs, the filter may require up to two filter clocks—one full filter interval—to pass a transition.
Table 2.
NI 6509 Digital Filtering
Filter Interval
Pulse Width Passed
Pulse Width Blocked
Low Pulse
High Pulse
Low Pulse
High Pulse
t
interval
t
interval
t
interval
t
interval
/2
t
interval
/2
Extern
a
l
S
ign
a
l
Sa
mpled
Extern
a
l
S
ign
a
l
Filter
Clock
Sa
mple Clock
Filtered
S
ign
a
l
H
H
H
H
H
H
L
L
H
H
H
L
L
H
H
A
B
C
