[sdiy] 4046 VCO overclocking a PIC

Tom Wiltshire tom at electricdruid.net
Sun Aug 23 23:54:44 CEST 2015


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
>> 
>> 
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> 
> -- 
> --Tim Ressel
> Circuit Abbey
> timr at circuitabbey.com
> 
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