National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

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Chapter 8 Counters
© National Instruments Corporation 8-33 NI USB-621x User Manual
Cascading Counters
Yo u can internally ro u te the Co u nter n Internal O u tp u t and Co u nter n TC
signals of each co u nter to Gate inp u ts of the other co u nter. By cascading
two co u nters together, yo u can effectively create a 64-bit co u nter. By
cascading co u nters, yo u also can enable other applications. For example,
to improve the acc u racy of freq u ency meas u rements, u se reciprocal
freq u ency meas u rement, as described in the Method 3 b u llet of the
Frequency Measurement section.
Counter Filters
Yo u can enable a programmable debo u ncing filter on each PFI signal.
When the filters are enabled, yo u r device samples the inp u t on each rising
edge of a filter clock. USB-621 x devices u se an onboard oscillator to
generate the filter clock with a 40 MHz freq u ency.
Note NI-DAQmx only s u pports filters on co u nter inp u ts.
The following is an example of low to high transitions of the inp u t signal.
High to low transitions work similarly.
Ass u me that an inp u t terminal has been low for a long time. The inp u t
terminal then changes from low to high, b u t glitches several times. When
the filter clock has sampled the signal high on N consec u tive edges, the low
to high transition is propagated to the rest of the circ u it. The val u e of N
depends on the filter setting; refer to Table 8-5.
The filter setting for each inp u t can be config u red independently. On power
u p, the filters are disabled. Fig u re 8-29 shows an example of a low to high
transition on an inp u t that has its filter set to 125 ns (N = 5).
Table 8-5. 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
μ
s 257 6.425
μ
s 6.400
μ
s
2.56 ms ~101,800 2.56 ms 2.54 ms
Disabled — — —