Chapter 9 Counters
NI USB-621x User Manual 9-24 ni.com
Frequency Output can be routed out to any PFI <4..7> or PFI <12..15>
terminal. All PFI terminals are set to high-impedance at startup. The FREQ
OUT signal also can be routed to DO Sample Clock and DI Sample Clock.
In software, program the frequency generator as you would program one of
the counters for pulse train generation.
For information about connecting counter signals, refer to the Default
Counter/Timer Pinouts section.
Frequency Division
The counters can generate a signal with a frequency that is a fraction of an
input signal. This function is equivalent to continuous pulse train
generation.
For information about connecting counter signals, refer to the Default
Counter/Timer Pinouts section.
Pulse Generation for ETS
In this application, the counter produces a pulse on the output a specified
delay after an active edge on Gate. After each active edge on Gate, the
counter cumulatively increments the delay between the Gate and the pulse
on the output by a specified amount. Thus, the delay between the Gate and
the pulse produced successively increases.
Note ETS = Equivalent Time Sampling.
The increase in the delay value can be between 0 and 255. For instance, if
you specify the increment to be 10, the delay between the active Gate edge
and the pulse on the output will increase by 10 every time a new pulse is
generated.
Suppose you program your counter to generate pulses with a delay of
100 and pulse width of 200 each time it receives a trigger. Furthermore,
suppose you specify the delay increment to be 10. On the first trigger, your
pulse delay will be 100, on the second it will be 110, on the third it will be
120; the process will repeat in this manner until the counter is disarmed.
The counter ignores any Gate edge that is received while the pulse triggered
by the previous Gate edge is in progress.
The waveform thus produced at the counter’s output can be used to provide
timing for undersampling applications where a digitizing system can
sample repetitive waveforms that are higher in frequency than the Nyquist
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