National Instruments NI USB-621x manual 123

Chapter 10 PFI

Table 10-1.

Filters

N (Filter Clocks

Pulse Width

Pulse Width

Needed to Pass

Guaranteed to Pass

Guaranteed to Not

Filter Setting

Signal)

Filter

Pass Filter

125 ns

5

125 ns

100 ns

6.425 µs

257

6.425 µs

6.400 µs

2.55 ms

~101,800

2.55 ms

2.54 ms

Disabled

—

—

—

The filter setting for each input can be configured independently. On power

up, the filters are disabled. Figure 10-4shows an example of a low to high

transition on an input that has its filter set to 125 ns (N = 5).

PFI Terminal

Filtered input goes high

when terminal is sampled

Filter Clock

1

1 2 3 4

1

2 3 4 5

high on five consecutive

(40 MHz)

filter clocks.

Filtered Input

Figure 10-4.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 2.55 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 M Series device does not use the filtered version of the input signal.

Refer to the KnowledgeBase document, Digital Filtering with M Series, for more information about digital filters and counters. To access this KnowledgeBase, go to ni.com/info and enter the info code rddfms.

© National Instruments Corporation 10-5 NI USB-621x User Manual