EDN -- 12.21.95 FIFO makes cheap waveform generato

-December 21, 1995

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Design Ideas:December 21, 1995

FIFO makes cheap waveform generator

Todd Williams,
Napco Security Systems
Amityville, NY

The circuit in Figure 1 shows the basic configuration of a simple and inexpensive arbitrary waveform generator. IC1’s 82C54 produces the timebase for the wave table. This IC can generate a 152-Hz to 5-MHz clock under software control, which translates to a 1.52-Hz to 50-kHz, 100-point continuous wave of any type. The FIFO AM7201A (IC2) holds the wave information. When FIFO HALT is high, IC3A and IC3B control the loading of the wave into the FIFO by first halting the FIFO read clock at IC2’s pin 15 and deactivating the retransmit line at IC2’s pin 23, respectively. When you then provide a low-going pulse to the FIFO RESET input and a negative edge at FIFO WRITE, you can write the wave information to the input of the FIFO.

Once the FIFO loads the wave, you need to pull FIFO HALT low. Then, the empty flag at pin 21, which controls the retransmit line (pin 23) through IC3A, loops the internal pointer at the rate of the FIFO read clock. This looping generates a continuous uninterrupted wave when the circuit connects to a DAC. You can now go back and change or even sweep the wave frequency. The pointer in the FIFO just goes faster or slower to produce a higher or lower frequency.

Connecting an 8-bit DAC with a 5-MHz latch rate and an optional buffer with a 50 ohm impedance completes the function-generator design. This circuit can generate many simple waveforms using simple math trigonometric functions. You can create more complicated waveforms by using a wave editor. Using a 12-bit DAC would also extend the circuit’s capabilities. (DI#1804)

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