[sdiy] 4046 VCO overclocking a PIC

Richie Burnett rburnett at richieburnett.co.uk
Sun Aug 23 20:12:33 CEST 2015

What you are doing sounds reasonable enough for hobby tinkering. However, I wouldn't over-clock a PIC in a product that I intended to sell, unless I was prepared to screen candidate parts myself for correct operation across the anticipated temperature and voltage ranges at the overclocked speed.

I've heard stories of 20MHz pics being clocked at hundreds of MHz with the supply voltage increased out of spec and special measures taken to cool the part. But it is anyone's guess what that will do to lifetime and reliability of code execution, flash storage endurance, etc.  Once you go beyond the chip's maximum ratings, you're really on your own, and the rest of the datasheet spec is void.

One thing to watch out for is the behaviour of the PLL on devices that multiply up the clock frequency. Obviously division down of the clock input with sequential logic will be fine with rapid frequency variations. But a phase locked loop is a closed loop system with a finite response time and limits to its locking range. So don't change the input frequency too far, or too quickly, otherwise expect the PLL to drop out of lock momentarily.

I haven't personally clocked a micro from an RF VCO but I have clocked them from DDS chips before, not musical application though. Same idea, just with digital control instead of voltage control.


Sent from my Xperia SP on O2

---- Tom Wiltshire wrote ----

>Hi All,
>I've recently been doing some experiments using a 74HC4046 VCO as an external clock for a PIC microprocessor.
>The nice thing about the PIC is that it doesn't care what rate its clock goes. You can run it down into 10s of hertz if you want, then speed it back up, no problems. I've also discovered that I can overclock the chip significantly. I'm using a 16F1828, which is supposed to go to 32MHz. I've set up the internal PLL at x4, so this equates to an external clock from the VCO of 8MHz. Pushing the 74HC4046 up to 14MHz (as fast as it goes for the chip I've got) doesn't crash the PIC, implying that it's successfully running code at 14 x 4 = 56MHz!
>For VCO modulation, I used a exponential current sink instead of the usual R1 on pin 11, as  done by Thomas Henry, and described here by Scott Stites:
>	http://www.birthofasynth.com/Thomas_Henry/Pages/X-4046.html
>I've tweaked it for my situation (+/-12V, MHz output frequencies not audio, etc etc). I also simplified it a bit, leaving out the linear FM input and the high-frequency trim. I was thinking that this combination of VCO->PIC could form the basis of some kind of top-octave-divider, so I was only really looking for a bit of vibrato or a pitch bend over a few semitones. I'd vaguely thought I might get an octave up and down, but that would mean getting the VCO up to 16MHz, which it doesn't seem keen to do, so I'll have to settle for 8 semitones either way or so. Since this is a fairly limited range, I'm not demanding much of the exponential convertor. Alternatively, I could lower all the pitches by an octave so I don't have to push the VCO and PIC so hard, and then I might be able to use a wider modulation range.
>Has anyone else tried anything like this? Are there other musical circuits using the 4046 as a high frequency VCO? Any other thoughts or comments?
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