[sdiy] VCDO using PIC16C771

[Andre Majorel]

On 2007-02-21 15:49 -0500, jean-pierre.desrochers at ville.quebec.qc.ca wrote:
> 
> 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.

It seems to me that, for this application, 4 kB of RAM would be
more useful than 4 kB of EEPROM. You will have to update the
note-number-to-DAC-value table continuously because of temperature
drift and EEPROMs can only take so many cycles.

Yes, with RAM, you'll need to initialise the table at power on,
but you would have the same problem with an EEPROM ; chances are
the data in the EEPROM is for a warm device.

I wouldn't mind a VCO that did an auto-calibration sweep at power
on (the VCAs are off anyway). An auto-tune button would probably
be a good idea.

-- 
André Majorel <URL:http://www.teaser.fr/~amajorel/>
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