National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

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Chapter 8 Counters
© National Instruments Corporation 8-23 NI USB-621x User Manual
Frequency Generation
Yo u can generate a freq u ency by u sing a co u nter in p u lse train generation
mode or by u sing the freq u ency generator circ u it.
Using the Frequency Generator
The freq u ency generator can o u tp u t a sq u are wave at many different
freq u encies. The freq u ency generator is independent of the two
general-p u rpose 32-bit co u nter/timer mod u les on USB-621 x devices.
Fig u re 8-25 shows a block diagram of the freq u ency generator.
Figure 8-25. Frequency Generator Block Diagram
The freq u ency generator generates the Freq u ency O u tp u t signal. The
Freq u ency O u tp u t signal is the Freq u ency O u tp u t Timebase divided by a
n u mber yo u select from 1 to 16. The Freq u ency O u tp u t Timebase can be
either the 20 MHz Timebase divided by 2 or the 100 kHz Timebase.
The d u ty cycle of Freq u ency O u tp u t is 50% if the divider is either 1 or
an even n u mber. For an odd divider, s u ppose the divider is set to D. In
this case, Freq u ency O u tp u t is low for (D + 1)/2 cycles and high for
(D – 1)/2 cycles of the Freq u ency O u tp u t Timebase.
Fig u re 8-26 shows the freq u ency generator o u tp u t waveform when the
divider is set to 5.
Figure 8-26. Frequency Generator Output Waveform
100 kHz Time bas e
20 MHz Time bas e
Freq u ency
O u tp u t
Time bas e
FREQ OUT
Divi s or
(1–16)
Freq u ency Gener a tor
÷
2
Freq u ency
O u tp u t
Time bas e
FREQ OUT
(Divi s or = 5)