I get around Flash programming problems by simply diode-isolating the PIC
only as well as its MCLR line. I use a  low-drop diode such as SD101, etc.
I make sure that the isolated VCC has proper bypassing, and that the MCLR
line is pulled up OK, but is easy to pull down by the programmer.

At the low current that the PIC normally uses, the diode voltage drop is
about 100mV max, which makes NO difference in any design I've worked, even
at 24Mhz (I've overdriven a few PICS before, for video work).

Neither I nor any client has had trouble programming with anything, even
David Tait's design works great.

If the board has room, I make the programming connector an RJ12-6 (6-pin
telephone jack). Sure makes programming easy in a poorly-lit room or auto,
back of van, or whatever.

I have made a PDF schematic of the setup for those who want it, just send
an email.

--Bob Axtell

At 02:44 PM 5/28/2003 -0400, you wrote:
>On Tue, May 27, 2003 at 08:34:14PM -0500, Picdude wrote:
> > On Tuesday 27 May 2003 15:16, Byron A Jeff scribbled:
> > > > I have never been able to use LVP for one reason ... the LVP pin is
> very
> > > > inconveniently located on RB3, since I use ICSP almost exclusively
> > > > nowadays.
> > >
> > > The LVP wasn't the issue. In fact I have a THVP design here:
> > >
> > > http://www.finitesite.com/d3jsys/proghvp.html
> >
> > I've had one question w.r.t. most HVP ICSP's, and perhaps you might
> know ...
> > with the target PIC in-circuit, it's usually directly connected to it's own
> > on-board PS, usually a 7805.  So how is the programmer connected to the Vdd
> > line?
>
>Three possible answers:
>
>1) They don't have to be connected. You can just share a ground. This one is
>    only a problem if you have a part where MCLR can become an input and a
>    power cycle is the only way to reset. This is possible for me because I
>    always power the programmer from the PC, and never from the target.
>
>2) You can route board power through the ICSP connector so that the programmer
>    can control Vdd when connected and a jumper connects straight through when
>    the programmer isn't. My ICSP version of the TLVP here:
>
>    http://www.finitesite.com/d3jsys/proglvp-icsp.gif
>
>    can be wired that way. And BTW you can rewire the THVP same way.
>
>3) Do what Wouter does: short the power on the target briefly
>
> >  A diode on the programmer Vdd line only protects the programmer when
> > the app circuit switches on.  I won't want to connect a diode
> permanently in
> > the app-circuit's Vdd line for other reasons.
>
>
>Note that this particular HVP circuit was designed for ICSP, so none of these
>issues exist. And instead of putting the diode on the target, it would be
>better placed on the programmer, since that's what would need the protection.
>
> > Is the programmer Vdd line really piggybacked/paralleled with the app
> power?
>
>No! That would be a disaster.
>
> > I assume the app power will be off right?
>
>Not necessarily. That can cause parasitic power issues where the programmer is
>in fact trying to power the whole target with unknown power requirements.
>
> >  Most articles/docs on ICSP address issues with MCLR connections, but
> not Vdd.
> >
> > I notice however, that your design uses app-ckt power during
> programming, and
> > does not control the Vdd line.  But how is the PIC reset during
> programming?
>
>The MCLR line resets the PIC. You can do that under power. The only exception
>is when MCLR is turned into an input. Of course there's still the issue of
>resetting everything else on the target.
>
> > I've tried with the Tait, and it does not work if I leave the app-ckt power
> > on only.
>
>So the right solution is pretty clear to me: Route app-ckt power through the
>ICSP connector and run it through a P-channel MOSFET that's on the programmer
>which can then be controlled by the programmer. You need a P channel so that
>you can get the it to turn on by dropping the gate low.
>
>Hope this helps,
>
>BAJ
>
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