National Instruments NI 6602 User Manual page 25

Chapter 3 Signal Connections
Note The effect of filtering is that the signal transition is shifted by a minimum of
one filter clock and a maximum of two filter clocks.
NI 660x User Manual
consecutive rising edges of the filter clock. The frequency of the filter clock
timebase determines whether a transition in the signal may propagate or
not. The function of the internal sampling clock is to increase the sampling
rate and prevent aliasing. Figure 3-1 demonstrates the function of this filter.
External Signal
on PFI Line
Filter Clock
Maximum Timebase
External Signal Sample
by Maximum Timebase
Filtered PFI Line
In period A, 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 B, 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. The figure shows a rising (0 to 1) transition. The same filtering
applies to falling (1 to 0) transitions.
The filter is sensitive to the duration for which a digital signal transitions
from one state to another. If a square wave is applied to the filter, its
propagation will depend on its frequency and duty cycle.
There are four filter settings available in the TIO devices: 5 μs, 1 μs, 500 ns,
and 100 ns. The 5 μs filter will pass all pulse widths (high and low) that are
5 μs or longer. It will block all pulse widths that are 2.5 μs (one-half of
5 μs) or shorter. Pulse widths between 2.5 μs and 5 μs may or may not pass,
depending on the phase of the pulse with respect to the filter clock
timebase. The same relationship extends to all other filter clocks.
In addition to these hard-wired filter clocks, you can use any PFI, RTSI, or
internal signal as the source for the filter clock timebase. Use signals with
a duty cycle as close to 50 percent as possible.
If the period of the filter clock timebase is t
pass pulse widths that are 2*t
H L L H H
A
Figure 3-1. Digital Filtering
fltrclk
or longer and to block pulse widths that
fltrclk
3-2
H H H H H
B
, this filter guarantees to
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