[sdiy] 4046 VCO overclocking a PIC

Tim Ressel timr at circuitabbey.com
Sun Aug 23 20:57:24 CEST 2015


Honestly I've never understood this need to overclock devices. I was 
taught that the datasheet is to be followed as closely as possible. If 
you intentionally violate the specs then you are on your own and you 
don't get to call the manufacturer for help

People who are way cleverer than me came up with the specs, not to 
mention the device itself. Who am I to disregard them?

Quick story:  One fine day a production product stared failing the 
production test. It was found that an op amp was getting toasted. 
Further investigation revealed the circuit was, and had always been, 
violating the spec sheet. The Nat Semi amps originally used could handle 
the violation, but they switched to a 3rd party amp of the same part 
number and it could not handle it, and fried. A simple change fixed the 
violation.

If you need more processor power, get a bigger processor. I don't know 
about Microchip, but my fav Atmel has processors that can do 300 MHz or 
more. Just in case I need it...

--TimR

On 8/23/2015 9:36 AM, 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?
>
> Thanks,
> Tom
>
>
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-- 
--Tim Ressel
Circuit Abbey
timr at circuitabbey.com




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