[sdiy] 4046 VCO overclocking a PIC

Tom Wiltshire tom at electricdruid.net
Sun Aug 23 18:36:14 CEST 2015


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?

Thanks,
Tom





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