© National Instruments | 9-9
M Series 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 9-3.
The filter setting for each input can be configured independently. On power up, the filters are
disabled. Figure 9-4 shows an example of a low to high transition on an input that has its filter
set to 125 ns (N = 5).
Figure 9-4. 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 2.56 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.
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 9-3. 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 — — —
12314123 45
RTSI, PFI, or
PXI_STAR Terminal
Filtered input goes
high when terminal
is sampled high on
five consecutive filter
clocks.
Filter Clock
(40 MHz)
Filtered Input
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