of Microprocessors

DIGITAL HARDWARE
599
MOST SIGNIFICANT
10 BITS OF N
REF. IN
RAW
SAWTOOTH
CONSTANT
>----.....- AMPLITUDE
OUTPUT
(AI
( B)
Fig. 11-8.
Digital divider waveshaping circuits. (A) Generating a sawtooth
wave. (B) Compensating a sawtooth from a divide-by-N oscillator.
increased at higher frequencies by deriving it from a DAC connected to the
most significant frequency control bits. For a divide-by-N frequency source,
a multiplying DAC could be placed in the feedback path of an op-amp to
provide reciprocal compensation.
Along these same lines, a square-wave output can be integrated into a
triangle wave, and a multiplying DAC can be used to raise the integrator
gain at high frequencies. Two variable integrators in a state-variable low-pass
configuration can filter a square wave into a reasonable sine wave as well. The
2040 integrated voltage-controlled filter can do both of these jobs quite well
and can be driven directly from a current output DAC.
Although these essentially analog methods do work, they seem to
defeat the whole purpose ofgoing digital in an oscillator module. Much more
flexible waveshaping can be had by implementing some of the techniques
discussed in Chapter 13. In the example of accumulator division given
earlier, the most significant 8 bits of the register were implemented as a
binary counter, which counted through everyone of its states every cycle. If
an 8-bit DAC is connected to the counter, a sawtooth wave with amplitude
independent of frequency would emerge. One could also construct logic to
implement all of the direct waveform computation methods detailed in
Chapter 13 such as triangle and sine conversion as well.
Another possibility is table lookup shaping followed by digital-toanalog
conversion and filtering to get an analog output. In hardware, this is
accomplished by placing a read-only memory (ROM) between the accumulator
divider and the DAC as in Fig. 17-9. The ROM functions exactly like a
MOST
SIGNIFICANT
B BITS OF
ACCUMULATOR
DIVIDER \
256-WORD
BY B-BIT
READ-ONLY
MEMORY
8-BIT
DAC
ANALOG
OUTPUT
"-------------J \
ADDRESS
INPUT
DATA
OUTPUT
Fig. 17-9. Digital waveshaping of digital oscillator