Digital Filtering Example - National Instruments NI 6520 User Manual

Table 1 lists the pulse widths guaranteed to be passed and blocked.
Table 1. NI 6520/6521 Digital Filter Timing Intervals
Pulse Width Passed Pulse Width Blocked
Filter Interval Low Pulse High Pulse Low Pulse High Pulse
+ 100 μs – 100 μs /2) + 100 μs /2) – 100 μs
t t t (t (t
interval interval interval interval interval
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 μs 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).
External
Signal
Filter
Clock
Sample Clock (100 ns)
H L L H H
External
Signal
B
H L L H H H H H H H
Sampled
A C
Filtered
Signal
Figure 2. Digital Filtering Example
© National Instruments Corporation 7 NI 6520/6521 User Guide