National Instruments USB-621 Series, User Manual

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Chapter 8 Counters
NI USB-621x User Manual 8-26 ni.com
Freq u ency O u tp u t can be ro u ted o u t to any o u tp u t PFI terminal. All PFI
terminals are set to high-impedance at start u p. The FREQ OUT signal also
can be ro u ted to DO Sample Clock and DI Sample Clock.
In software, program the freq u ency generator as yo u wo u ld program one of
the co u nters for p u lse train generation.
For information abo u t connecting co u nter signals, refer to the Default
Counter/Timer Pinouts section.
Frequency Division
The co u nters can generate a signal with a freq u ency that is a fraction of
an inp u t signal. This f u nction is eq u ivalent to contin u o u s p u lse train
generation.
For information abo u t connecting co u nter signals, refer to the Default
Counter/Timer Pinouts section.
Pulse Generation for ETS
In the eq u ivalent time sampling (ETS) application, the co u nter prod u ces a
p u lse on the o u tp u t a specified delay after an active edge on Gate. After
each active edge on Gate, the co u nter c u m u latively increments the delay
between the Gate and the p u lse on the o u tp u t by a specified amo u nt. Th u s,
the delay between the Gate and the p u lse prod u ced s u ccessively increases.
The increase in the delay val u e can be between 0 and 255. For instance, if
yo u specify the increment to be 10, the delay between the active Gate edge
and the p u lse on the o u tp u t increases by 10 every time a new p u lse is
generated.
S u ppose yo u program yo u r co u nter to generate p u lses with a delay of 100
and p u lse width of 200 each time it receives a trigger. F u rthermore, s u ppose
yo u specify the delay increment to be 10. On the first trigger, yo u r p u lse
delay is 100, on the second it is 110, on the third it is 120; the process
repeats in this manner
u ntil the co u nter is disarmed. The co u nter ignores
any Gate edge that is received while the p u lse triggered by the previo u s
Gate edge is in progress.
The waveform th u s prod u ced at the co u nter’s o u tp u t can be u sed to provide
timing for u ndersampling applications where a digitizing system can
sample repetitive waveforms that are higher in freq u ency than the Nyq u ist