©
National Instruments Corporation
7
NI 6520/6521 User Guide
Table 1 lists the pulse widths guaranteed to be passed and blocked.
You can enable filtering on as many input lines as is necessary for your
application. All filtered lines share the same timing interval, which ranges
from 100
μ
s to 200 ms.
Internally, the filter uses two clocks: the sample clock and the filter clock.
The sample clock has a 100 ns period. The filter clock is generated by a
counter and has a period equal to one half of the specified timing interval.
The input signal is sampled on each rising edge of the sample clock, which
is every 100 ns. However, a change in the input signal is recognized only if
it 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. The sample clock provides a fast sample
rate to ensure that input pulses remain constant between filter clocks.
Digital Filtering Example
Figure 2 shows a filter configuration with an 800 ns filter interval (400 ns
filter clock).
Figure 2.
Digital Filtering Example
Table 1.
NI 6520/6521 Digital Filter Timing Intervals
Filter Interval
Pulse Width Passed
Pulse Width Blocked
Low Pulse
High Pulse
Low Pulse
High Pulse
t
interval
t
interval
+ 100
μ
s
t
interval
– 100
μ
s
(t
interval
/2) + 100
μ
s
(t
interval
/2) – 100
μ
s
H
H
H
H
H
H
L
L
H
H
H
L
L
H
H
A
B
C
External
Signal
Filter
Clock
Sample Clock (100 ns)
External
Signal
Sampled
Filtered
Signal