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Table 8-1. filters, Figure 8-5. filter example – National Instruments Network Device DAQ S User Manual

Page 118

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Chapter 8

Programmable Function Interfaces (PFI)

© National Instruments Corporation

8-7

NI 6124/6154 User Manual

Assume that an input terminal has been low for a long time. The input
terminal then changes from low to high, but glitches several times. When
the filter clock has sampled the signal high on N consecutive edges, the low
to high transition is propagated to the rest of the circuit. The value of N
depends on the filter setting; refer to Table 8-1.

The filter setting for each input can be configured independently. On power
up, the filters are disabled. Figure 8-5 shows an example of a low to high
transition on an input that has its filter set to 125 ns (N = 5).

Figure 8-5. Filter Example

Enabling filters introduces jitter on the input signal. For the 125 ns and
6.425

μs filter settings, the jitter is up to 25 ns. On the 2.56 ms setting,

the jitter is up to 10.025

μs.

When a PFI input is routed directly to RTSI, or a RTSI input is routed
directly to PFI, the s Series device does not use the filtered version of the
input signal.

Refer to the KnowledgeBase document, Digital Filtering with M Series
and CompactDAQ
, for more information about digital filters and counters.
To access this KnowledgeBase, go to

ni.com/info

and enter the info

code

rddfms

.

Table 8-1. 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

μs

257

6.425

μs

6.400

μs

2.56 ms

~101,800

2.56 ms

2.54 ms

Disabled

1 2

3

1 4

1

2

3 4 5

RTSI, PFI, or

PXI_STAR Terminal

Filter Clock

(40 MHz)

Filtered Input

Filtered input goes
high when terminal
is sampled high on
five consecutive filter
clocks.