of Microprocessors

760 MUSICAL ApPLICATIONS OF MICROPROCESSORS
Obviously, the free-running type is much more suitable as a programmable
oscillator because a frequency control word can be written into the
interval register once and the timer will then generate pulses at that
frequency automatically without program intervention. Generally, the microsecond-wide
pulses produced must be shaped before they can effectively
operate a speaker. One possibility is to use the pulse to discharge a capacitor
that is being constantly charged through a resistor or current source. This
produces a sawtooth wave that' is useful over a range of two to three octaves.
Another possibility is to have the pulse trigger a single shot to produce a
much wider pulse and thus a rectangular waveform. Sophisticated multichannel
timer chips such as the Intel 8253 can be programmed so that one timer
determines the frequency in free-running mode and triggers another timer in
single-shot mo.de, thus producing a programmable duty cycle. The Motorola
6840 produces the same effect by splitting its 16-bit timer register into two
8-bir halves, one of which determines the waveform high time and the other
determines the low time. Of course, the pulse ourput can also toggle a flipflop
for a square wave, a function built into some timer ICs.
One particular peripheral interface chip that has been used extensively
for sound generation in low-cost personal computers is the 6522 "versatile
interface adapter" (VIA). This remarkable IC has two full 8-bit I/O ports,
two timers, and a multimode 8-bit shift register. In a typical computer, the
ports are used to interface to a printer and control internal functions, while
the timers and shift register are either not used or are connected to a small
amplifier and internal speaker. Timer 1 is a standard 16-bit unit that can
operate in free-running mode and produce a pulse or half-frequency squarewave
output. The other timer is more interesting, however, because it can
operate in conjunction with the shift register. Although its counter is also 16
bits, the interval register is only 8 bits, which limits frequency resolution
somewhat. In one of its operating modes, the internal 8-bit shift register is
rotated one position on each Timer 2 timeout. By writing different bit
patterns into the shift register, different eight-segment waveforms can be
produced on the shift register output pin for a number of different timbres (it
is instructive to determine how many audibly different patterns there are).
One can even set the shift register to shift at its maximum rate and then use
different bit patterns to simulate a pulse-width modulated nine-level D-to-A
converter for producing somewhat smoother waveforms. Also, since the two
timers are independent, two simultaneous tones are readily produced. In all,
the 6522 provides quite a lot of sound-generation function often for little or
no added cost.
IS
Timed Program Loops
Of course, the classic method of producing simple sounds by computer
the timed program loop. It is, in fact, so simple and fundamental (and