On Thu, 2005-06-23 at 10:57 +0200, Wouter van Ooijen wrote:
> > I've built and tested the circuit show at
> > http://www.btinternet.com/~Peter.Onion/PIC/mclr.png . Driving it from
> > one of the pump outputs on a wisp628 rather than the PWM 
> > output it gives
> > 4.65V (run) and 12.9V (prog) on !MCLR pin.
> 
> It might be just a little bit low for a worst-case 16C84. Nearly all
> other (newer) PICs should be OK. The impedance of the 12.9V might also
> be a bit higher than with the 2-pin pump, so you will have trouble at
> even higher values of the MCLR pull-up. 

I think running the charge pump from a higher frequency source reduce
the source impedance and minimise these sorts of problems ?

> And you might be in trouble a
> bit faster when the Vcc is way lower than 5.0V. In general: all the
> points where a Wisp628 must be trated with care will be a little bit
> more 'probelematic' with this design.

I'm only doing this for fun, so margin testing isn't something I'm
worried about at the moment.  But now I know who to ask if I ever do
need to consider it ;-)

> PS: there is also a comparator in your chip, which you can use to create
> the second pin of the pump, without software intervention. Not my idea,
> but I forgot where I heard it first.

I was going to use a 16F870 (since I have some sitting doing nothing),
but as you point out the 16F628 has PWM and a comparator, so the "two
pin pump" circuit can be implemented with no software intervention...

Thanks for the helpful advice.

Peter


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