Chapter 9 Counters

Frequency Generation

You can generate a frequency by using a counter in pulse train generation mode or by using the frequency generator circuit.

Using the Frequency Generator

The frequency generator can output a square wave at many different frequencies. The frequency generator is independent of the two general-purpose 32-bit counter/timer modules on M Series devices.

Figure 9-26shows a block diagram of the frequency generator.

 

 

Frequency

 

20 MHz Timebase

⎟ 2

Output

 

Timebase

 

 

 

Frequency Generator

FREQ OUT

100 kHz Timebase

Divisor (1–16)

Figure 9-26.Frequency Generator Block Diagram

The frequency generator generates the Frequency Output signal. The Frequency Output signal is the Frequency Output Timebase divided by a number you select from 1 to 16. The Frequency Output Timebase can be either the 20 MHz Timebase divided by 2 or the 100 kHz Timebase.

The duty cycle of Frequency Output is 50% if the divider is either 1 or an even number. For an odd divider, suppose the divider is set to D. In this case, Frequency Output is low for (D + 1)/2 cycles and high for (D – 1)/2 cycles of the Frequency Output Timebase.

Figure 9-27shows the output waveform of the frequency generator when the divider is set to 5.

Frequency

Output

Timebase

FREQ OUT (Divisor = 5)

Figure 9-27.Frequency Generator Output Waveform

© National Instruments Corporation

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NI USB-621x User Manual