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'[EE] Programming voltage generator'
2005\06\22@175154 by Peter Onion

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Folks,

I've been looking a the charge pump that Wouter uses on the wisp628
( http://www.voti.nl/wisp628/index.html ), and it got me thinking.  I'm
planning to use a 16F870 in my "wisp628" clone, which means there will
be an otherwise unused PWM module, which could be used to drive a charge
pump with no software overhead (other than turning it on and off).

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.

What do you think ?  Can you foresee any problems with this idea ?

Peter


2005\06\22@213759 by Chen Xiao Fan

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Look at PICkit 1 and you can use a boost converter
to do that. You can port Microchip's implementation
to the 16F870. A boost converter is much better than
a charge pump and will not really cost more (you
need an inductor but less diode and capacitors).

Regards,
Xiaofan

2005\06\23@044201 by Peter Onion

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On Thu, 2005-06-23 at 09:37 +0800, Chen Xiao Fan wrote:
> Look at PICkit 1 and you can use a boost converter
> to do that. You can port Microchip's implementation
> to the 16F870. A boost converter is much better than
> a charge pump and will not really cost more (you
> need an inductor but less diode and capacitors).

Interesting.  In this situation I think charge pumps are "better" as
they don't need the feed back and control to keep them from over-volting
the Vpp pin.

And the circuit in AN258 seems to be a recipe for the smoke to escape
from the transistor in the boot circuit if the PWM output stops in the
high state (Note I don't write bug free code so anything is possible).

Peter.



2005\06\23@045725 by Alan B. Pearce

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>> Look at PICkit 1 and you can use a boost converter
>> to do that. You can port Microchip's implementation
>> to the 16F870. A boost converter is much better than
>> a charge pump and will not really cost more (you
>> need an inductor but less diode and capacitors).
>
>Interesting.  In this situation I think charge pumps are "better" as
>they don't need the feed back and control to keep them from over-volting
>the Vpp pin.

The original ICD used PWM to drive a boost converter, but I don't believe it
has a feedback loop, just detection to know when it is turned on, and if the
voltage is too low it will not go into programming mode.

2005\06\23@045911 by Wouter van Ooijen

face picon face
> Look at PICkit 1 and you can use a boost converter
> to do that. You can port Microchip's implementation
> to the 16F870. A boost converter is much better than
> a charge pump and will not really cost more (you
> need an inductor but less diode and capacitors).

A switched converter is much better at generating a specified Vpp,
independent of the input Vcc, with a lower impedance. But it also needs
an extra switcher transistor, a (less common) inductor, and it needs
software attention.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\06\23@045911 by Wouter van Ooijen

face picon face
> 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. 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.

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.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\06\23@052717 by Peter Onion

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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


2005\06\23@072643 by Wouter van Ooijen

face picon face
> > 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 ;-)

Not just margining. Also for instance running on USB power, or an 7805
which just happens to have a bad day.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu


2005\06\23@075218 by Michael Rigby-Jones

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>-----Original Message-----
>From: spam_OUTpiclist-bouncesTakeThisOuTspammit.edu [.....piclist-bouncesKILLspamspam@spam@mit.edu]
>Sent: 23 June 2005 09:42
>To: Microcontroller discussion list - Public.
>Subject: RE: [EE] Programming voltage generator
>
>
>On Thu, 2005-06-23 at 09:37 +0800, Chen Xiao Fan wrote:
>> Look at PICkit 1 and you can use a boost converter
>> to do that. You can port Microchip's implementation
>> to the 16F870. A boost converter is much better than
>> a charge pump and will not really cost more (you
>> need an inductor but less diode and capacitors).
>
>Interesting.  In this situation I think charge pumps are
>"better" as they don't need the feed back and control to keep
>them from over-volting the Vpp pin.

That's true, the charge pump is inherently voltage limited.  However, you can make a simple inductor based switcher that has no feedback and just uses a zener to clamp the output.  Not suitable for higher powers, and efficiency is terrible of course, but would be fine for a Vpp generator.

Regards

Mike

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2005\06\23@080309 by Peter Onion

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On Thu, 2005-06-23 at 13:24 +0200, Wouter van Ooijen wrote:

> > 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 ;-)
>
> Not just margining. Also for instance running on USB power, or an 7805
> which just happens to have a bad day.

I've just ordered a few schottky diodes to see if using them in place of
the silicon ones will up the voltage a bit.

Peter



> Wouter van Ooijen
>
> -- -------------------------------------------
> Van Ooijen Technische Informatica: http://www.voti.nl
> consultancy, development, PICmicro products
> docent Hogeschool van Utrecht: http://www.voti.nl/hvu
>  
>

2005\06\23@141937 by Peter Onion

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On Thu, 2005-06-23 at 13:03 +0100, Peter Onion wrote:
>
> I've just ordered a few schottky diodes to see if using them in place of
> the silicon ones will up the voltage a bit.
>

s/1N4148/BAT42/   12.9V -> 14.0V   :-)

In other words....  I took out the 1N4148s in my charge pump, and put
BAT42 Schottky diodes in their place.  The output voltage has gone up
from 12.9V to 14.0V

Peter


2005\06\23@151434 by Wouter van Ooijen

face picon face
> s/1N4148/BAT42/   12.9V -> 14.0V   :-)
>
> In other words....  I took out the 1N4148s in my charge pump, and put
> BAT42 Schottky diodes in their place.  The output voltage has gone up
> from 12.9V to 14.0V

I think you should not compare the open (unloaded) voltages but with a
reasonable load (10..50 kohm). I dunno whether that will show a
different picture, probably not.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu





2005\06\23@154151 by Peter Onion

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On Thu, 2005-06-23 at 21:12 +0200, Wouter van Ooijen wrote:
> > s/1N4148/BAT42/   12.9V -> 14.0V   :-)
> >
> > In other words....  I took out the 1N4148s in my charge pump, and put
> > BAT42 Schottky diodes in their place.  The output voltage has gone up
> > from 12.9V to 14.0V
>
> I think you should not compare the open (unloaded) voltages but with a
> reasonable load (10..50 kohm). I dunno whether that will show a
> different picture, probably not.

With the schottky diodes I'm getting 13.3V with a 22 k ohm load
resistor.   I think your 2 pump circuit with silicon diodes should
theoretically give 3 x 5.0  - 3 x 0.6 = 13.2V off load.

A BAT42 reads 0.26V Vf on my test meter compared with 0.6V for the
1N4148, so the theoretical output should be 3 x 5.0 - 4 x 0.26 = 13.96V,
which is within experimental error.

So I think I'm "close enough for jazz"  ;-)  What say you ?

Plus it programmed a 16F828A with the extra load on.

Peter.


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