[sdiy] 4046 VCO overclocking a PIC

Tim Ressel timr at circuitabbey.com
Fri Aug 28 20:47:25 CEST 2015

I need to add that this is a clever idea -- using a VCO to clock a 
processor to get an effect. Kudos for coming up with it. I was only 
taking issue with  the over clocking bit.


On 8/23/2015 7:09 PM, Tim Ressel wrote:
> Good point, Tom. The specs can be conservative. But you can't count on 
> it. The op amp failures I spoke of were caused by an input going more 
> than 0.3V lower than the negative pin. In that case, for that 
> manufacturer, there was no wiggle room in the spec at all. Perhaps I 
> am too conservative -- I do make stuff to sell these days -- but 
> better safe than sorry.
> There is a filter called a Bi-n-Tic that needs a V/Oct expo VCO that 
> is 8 times higher than normal. That design could benefit from your 
> nifty VCO.
> --TimR
> On 8/23/2015 2:54 PM, Tom Wiltshire wrote:
>> I've got no particular desire to overclock the chip, except that it 
>> enables me push the modulation range further upwards than might have 
>> appeared possible from the datasheet specs. I was reporting on a 
>> discovery, rather than trying to promote an idea.
>> As to "people way cleverer" - well, mostly those people have bosses 
>> who aren't as sharp as they are telling them that they better make 
>> sure that 99.99% of all the chips go out the door! So the specs tend 
>> to be pretty conservative ;-)
>> I agree, though. If I was making a product where I expected 10K units 
>> to go out of the door, I'd be damn sure to stay within the specs. 
>> Then it's Microchip's problem , not mine, if it doesn't work. But as 
>> I said at the beginning, I've been doing some experiments. It is at 
>> least *interesting* that you can run a Pic at nearly twice it's rated 
>> speed with no short-term il-effects. I hadn't realised their margin 
>> for error was quite so large. Ok, I' guessed you could over clock 
>> them 10%, maybe 20%, or 25% pushing it a bit. But nearly x2? Nope, 
>> wouldn't have considered it possible. But it is.
>> So, are there other musical circuits using the 4046 as a high 
>> frequency VCO? What can i do with a uP with a variable frequency 
>> clock? Wavetable oscillator? The uP would only have to deal with 
>> outputting waveforms, and the VCO could handle the frequency, for 
>> example. What else?
>> Thanks,
>> Tom
>> PS, Tim: I had a similar situation, years ago. A particular product 
>> started coming back for "repair" since it was out of spec with 
>> depressing regularity. It turned out the spec used the same error as 
>> the pressure sensor used in the product. This gave absolutely zero 
>> leeway for any error whatsoever from any other source. Only sensors 
>> that actually surpassed their spec by a reasonable margin would make 
>> products that met the spec we'd (or the marketing department, at 
>> least) claimed. Unfortunately in this case, there was no simple fix, 
>> and we finished up hand selecting sensors to match the rest of the 
>> errors in the circuit. At least it taught one young guy something…
>> On 23 Aug 2015, at 19:57, Tim Ressel <timr at circuitabbey.com> wrote:
>>> 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
>>>> _______________________________________________
>>>> Synth-diy mailing list
>>>> Synth-diy at dropmix.xs4all.nl
>>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>> -- 
>>> --Tim Ressel
>>> Circuit Abbey
>>> timr at circuitabbey.com
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy

--Tim Ressel
Circuit Abbey
timr at circuitabbey.com

More information about the Synth-diy mailing list