© National Instruments
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2-17
The DIO change detection circuitry can be enabled to detect rising edges, falling edges, or either
edge individually on each DIO line. The device synchronizes each DI signal to the 100 MHz
Timebase, and then sends the signal to the change detectors. The circuitry ORs the output of all
enabled change detectors from every DI signal. The result of this OR is the Change Detection
Event signal.
Change detection performs bus correlation by considering all changes within a 50 ns window
one change detection event. This keeps signals on the same bus synchronized in samples and
prevents overruns.
The Change Detection Event signal can do the following:
•
Drive any RTSI<0..7>, PXI_Trig<0..7>, PFI<0..39>, or PXI_STAR signal
•
Drive the DO Sample Clock or DI Sample Clock
•
Generate an interrupt
The Change Detection Event signal also can be used to detect changes on digital output events.
DI Change Detection Applications
The DIO change detection circuitry can interrupt a user program when one of several DIO
signals changes state.
You also can use the output of the DIO change detection circuitry to trigger a DI or counter
acquisition on the logical OR of several digital signals. By routing the Change Detection Event
signal to a counter, the relative time between bus changes can be captured.
The Change Detection Event signal can be used to trigger DO or counter generations.
Digital Filtering
A programmable debouncing filter can be enabled on each digital line on Port 0. When the filters
are enabled, the device samples the input on each rising edge of a filter clock. The device divides
down the onboard 100 MHz or 100 kHz clocks to generate the filter clock. 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 two consecutive edges and the signal remained stable in between, the low-to-high transition
is propagated to the rest of the circuit.
