National Instruments Data Acquisition Device NI USB-621x - Manual (Page 117)

Chapter 8 Counters
NI USB-621x User Manual 8-24 ni.com
Frequency Output can be routed out to any output PFI 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 the equivalent time sampling (ETS) 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.
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 increases 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 is 100, on the second it is 110, on the third it is 120; the process
repeats 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
frequency of the system. Figure 8-27 shows an example of pulse generation