[sdiy] VCO autotune for poly analog modular

[Ingo Debus]

Am 03.04.2007 um 19:33 schrieb Jeff Farr:

> I can't add much in technical terms here, but I've been thinking about
> similar things myself lately for a XR2206 based sweep generator.  I
> was thinking about using a MCU to control the frequency input to the
> chip and the lookup table idea is how I figured an autotune feature is
> best implemented.  However, I don't know the best way for the MCU to
> measure the actual pitch of the oscillator.  The simplest way is
> probably to feed the squarewave out from the osc to the MCU and count
> the peaks, but IIRC I read something about using a reference
> oscillator/phase cancellation and some sort of RMS measurement as
> well.. sounds complicated..

Many MCUs have one or more timers with capture mode, i.e. when a  
rising falling edge (or falling edge is possible as well) on an input  
pin is detected the timer count is copied into a register. The timer  
itself run continuously with a high frequency (MHz range) at wraps  
around when it reaches its maximum count. Thus the time between two  
rising edges can be measured.

Ingo