National Instruments DAQ X Series, Руководство пользователя

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Chapter 7 Counters
X Series User Manual 7-12 ni.com
Also, p u lse meas u rements s u pport sample clock timing while semi-period
meas u rements do not.
Semi-Period Measurement
In semi-period meas u rements, the co u nter meas u res a semi-period on its
Gate inp u t signal after the co u nter is armed. A semi-period is the time
between any two consec u tive edges on the Gate inp u t.
Yo u can ro u te an internal or external periodic clock signal (with a known
period) to the So u rce inp u t of the co u nter. The co u nter co u nts the n u mber
of rising (or falling) edges occ u rring on the So u rce inp u t between
two edges of the Gate signal.
Yo u can calc u late the semi-period of the Gate inp u t by m u ltiplying the
period of the So u rce signal by the n u mber of edges ret u rned by the co u nter.
Refer to the following sections for more information abo u t X Series
semi-period meas u rement options:
• Single Semi-Period Measurement
• Implicit Buffered Semi-Period Measurement
Refer to the Pulse versus Semi-Period Measurements section for
information abo u t the differences between semi-period meas u rement and
p u lse meas u rement.
Single Semi-Period Measurement
Single semi-period meas u rement is eq u ivalent to single p u lse-width
meas u rement.
Implicit Buffered Semi-Period Measurement
In implicit b u ffered semi-period meas u rement, on each edge of the Gate
signal, the co u nter stores the co u nt in the FIFO. A DMA controller transfers
the stored val u es to host memory.
The co u nter begins co u nting when it is armed. The arm u s u ally occ u rs
between edges on the Gate inp u t. Yo u can select whether to read the first
active low or active high semi period u sing the CI.SemiPeriod.StartingEdge
property in NI-DAQmx.
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