National Instruments PCI-6281, User Manual

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Chapter 8 PFI
PFI Filters
You can enable a programmable debouncing filter on each PFI, RTSI, or PXI_STAR signal.
When the filters are enabled, your device samples the input on each rising edge of a filter clock.
M Series devices use an onboard oscillator to generate the filter clock with a 40 MHz frequency.
Note NI-DAQmx
only
supports filters on counter inputs.
The following is an example of low to high transitions of the input signal. High to low transitions
work similarly.
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 8-3 shows an example of a low to high transition on an input that has its filter
set to 125 ns (N = 5).
Figure 8-3. 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
M Series device does not use the filtered version of the input signal.
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
RT S I, PFI, or
PXI_ S TAR Termin a l
Filtered inp u t goe s
high when termin a l
i s sa mpled high on
five con s ec u tive filter
clock s .
Filter Clock
(40 MHz)
Filtered Inp u t