[sdiy] VCDO using PIC16C771

[jean-pierre.desrochers at ville.quebec.qc.ca]

Maybe this has been already found
on some listing but I thought it would be
useful for anybody to share..
To get rid of any temp problem on my new VCO design
I'll use a Microchip PIC16C771 to control an Exar XR2206
VCO's frequency.. The XR2206 has a VERY clean sine output
and other waveform already available.

Here is the trick:
CV voltage 0-5VDC going to the 12 bits Pic ADC,
The Pic reads the AD value and goes to a serial EEPROM
lookup table at that AD address to fetch a 16bits 'period value'.
The XR2206 freq (square wave) is sent back to one of the PIC I/O pin.
If the XR2206 freq is too low or too high according to the lookup data
(at the specific CV input) the Pic send a PWM square wave
filtered with a low pass to compensate the drift and bring
back the CV/freq desired to the XR2206.
Of course the low pass filter is fast enough to
not create any portamento effect but enough eficient
to have a nice DC feedback voltage.
The serial EEPROM is a 4K X 16bits so it gives
32 steps between each semitones.
32steps X 12semitones X 8octaves = 3072 words (16bits)

This is an embrio idea but I'll give it a try..
J-P Desrochers
Quebec City