of Microprocessors

DIGITAL HARDWARE 619
effective frequency of that harmonic will be increased somewhat. Although
the magnitude of frequency shift is restricted to a few hertz, the technique is
useful in simulating a real vibrating string with slightly sharp upper harmonICS.
An Intelligent Oscillator?
The best method of interfacing either of the previously described oscillators
to a system is the use of a dedicated microprocessor. Actually operating
such an oscillator bank with frequency glides and dynamic waveform changes
to worry about, not to mention other modules in the system such as
amplifiers and filters, may pose a very heavy load on the control computer. It
would be much nicer if the oscillator bank could be given high-level com':
mands such as "increase frequency of channel 7 from C4 to G4 linearly over a
period of 300 msec starting at time point 1230," or "change the harmonic
content smoothly from what it is currently to the following specification over
the next 150 msec."
A dedicated processor to perform such functions would actually add
very little to the module cost. Two-thousand bytes of program ROM should
be sufficient for a moderately sophisticated command interpreter. Read/write
memory is only necessary for miscellaneous programming use and the storage
of parameters; thus, it can be 256 or at most lK bytes in size. Memory and
I/O interfacing would involve abour five additional ICs. A simple logic
replacement microprocessor such as a 6502 suffices quite well for the intelligence.
The net result is that less than $50 worth of extra parts can be added
to make an intelligent oscillator.
Speaking of the 6502 microprocessor, the observant reader may have
noticed that the 16-channel multiplexed oscillator timing diagram exactly
parallels the 6502's bus timing. In fact, one could drive the 6502 clock with
the internal/external address selector signal. Since the 6502 does nothing
during the first half of its I-JLsec bus cycle, that half would be the oscillator's
internal half cycle. The external half cycle would be devoted to the microprocessor,
which could then directly write into the frequency and
waveform memories. In fact, with some attention to detail, these memories
could appear to the micro as regular read/write memory and thereby eliminate
interface ports and registers altogether!
Communication between the control computer and the oscillator can
now be more casual. This allows techniques such as serial asychronous (RS­
232),3 which are normally useless in real-time control applications, to be
effectively utilized. In turn, this means that digital modules can be designed
3This is the method most often used to talk to "normal" peripherals such as terminals,
printers, etc. It is a serial by bit technique that is inherently slow (compared with
microprocessor speed) and usually implemented with little or no busy/done feedback
or error checking. Its main virtue is standardization and minimal wiring complexity
(3 wires are sufficient for bidirectional communication).