Chapter 9 Digital Routing and Clock Generation
© National Instruments Corporation 9-7 M Series User Manual
• Counter input signals for either counter—Source, Gate, Aux,
HW_Arm, A, B, or Z
• DI Sample Clock
• DO Sample Clock
Most functions allow you to configure the polarity of PFI inputs and
whether the input is edge or level sensitive.
RTSI 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 9-2.
The filter setting for each input can be configured independently. On power
up, the filters are disabled. Figure 9-3 shows an example of a low to high
transition on an input that has its filter set to 125 ns (N = 5).
Table 9-2. 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 — — —
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