You could be right; but I thought it was because the MCU was a more accurate source than the XR2206.

How can a digital device, which has a quantized output, possibly be more accurate than an analog device, which is not constrained by those finite step quantizing limitations???

 

The MCU will be as accurate as its crystal, about 20 parts per million, the XR2206 will be as good as its timing capacitor, about 5%. The MCU wins by a factor of 2500. It's disadvantage is that its only possible output frequencies are factors of its crystal frequency.

I read the spec again:
Given that his frequency range is only C3 (130 Hz) to C7 (2097 Hz), why are we faffing with XR2206 and phase locked loop when there's a MCU? Add an audio DAC and it could be directly synthesised from a wavetable to make a sinewave a thousand times better than the XR2206 can manage, with an accuracy of below 1Hz and no stability problems.

 

You probably know that we can discuss what the TS wants until we are blue in the face, but until we understand from the TS what he needs we are just amusing ourselves.

 

Yes, but his lack of clarity is a perfect excuse to waffle on about any number on vaguely related topics. You probably know that we don't do this just to help the TS!

 

You probably know that we can discuss what the TS wants until we are blue in the face, but until we understand from the TS what he needs we are just amusing ourselves.

DC is totally correct, and like so many times folks are guessing. And my guess is that the MCU only produces one frequency at a time, and only for a short time. THAT is why I did suggest an output latch and a D/A converter feeding some VCO to produce frequencies. But notice in the first post there is a mention of a PWM output, which we have no clue how that relates to anything.
Exactly like I have repeated until many are tired of hearing about t, we really do need more information. Probably the champion of that is the one about creating an intense electric field and high voltage. How intense and how high? I did provide some good information but still I could have helped more if I had known more. The person undoubtedly has a fantastic concept in mind that might change the world, but I am stuck with the terrible handicap o not being able to read minds and understand what exactly needs to be done.
I need to develop enhanced mind reading skills!!!