Chapter 8 CountersNI USB-621x User Manual 8-24 ni.comFrequency Output can be routed out to any output PFI terminal. All PFIterminals are set to high-impedance at startup. The FREQ OUT signal alsocan be routed to DO Sample Clock and DI Sample Clock.In software, program the frequency generator as you would program one ofthe counters for pulse train generation.For information about connecting counter signals, refer to the DefaultCounter/Timer Pinouts section.Frequency DivisionThe counters can generate a signal with a frequency that is a fractionof an input signal. This function is equivalent to continuous pulse traingeneration.For information about connecting counter signals, refer to the DefaultCounter/Timer Pinouts section.Pulse Generation for ETSIn the equivalent time sampling (ETS) application, the counter produces apulse on the output a specified delay after an active edge on Gate. Aftereach active edge on Gate, the counter cumulatively increments the delaybetween 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, ifyou specify the increment to be 10, the delay between the active Gate edgeand the pulse on the output increases by 10 every time a new pulse isgenerated.Suppose you program your counter to generate pulses with a delay of 100and pulse width of 200 each time it receives a trigger. Furthermore, supposeyou specify the delay increment to be 10. On the first trigger, your pulsedelay is 100, on the second it is 110, on the third it is 120; the processrepeats in this manner until the counter is disarmed. The counter ignoresany Gate edge that is received while the pulse triggered by the previousGate edge is in progress.The waveform thus produced at the counter's output can be used to providetiming for undersampling applications where a digitizing system cansample repetitive waveforms that are higher in frequency than the Nyquistfrequency of the system. Figure 8-27 shows an example of pulse generation
