On Tue, May 10, 2005 at 10:01:43AM +0200, Paul van der Linden wrote:
> Olin Lathrop wrote:
> 
> >Paul van der Linden wrote:
> > 
> >

[SNIP]

> Ok, thank you. But how to make sure that the external circuit can 
> tolerate it? How do I seperate it while programming?
> 
> And repost for the people who don't get the attachments:
> http://members.lycos.nl/pvdl1984/micro.html

That bottom programming circuit is bad news. It depends on the clamping
diodes of the PIC to limit the current. In addition it depends on a 
particular style of RS-232 port to function. 

It's a disaster waiting to happen. Definitely not recommended.

If you really feel you have to go that route, at least sprinkle a 4.7V
zener between each of the PIC pins connected to the RS-232 port and
ground. This will limit the voltages so that the protection diodes are
not used.

Don't expect that circuit to work with USB serial cables either as there
is no guarantee of the ability of the driver to properly drive the
serial pins.

It's just not a good idea all around.

Wouter discusses the issues with this type of circuit in the El-Cheapo
section of hiw WLoader interface here:

http://www.voti.nl/wloader

Wouter solves the problem by implementing a MAX232 interface.

BTW IMHO WLoader and others like it point to the right way to do it.
There is nothing about any type of external programmer, including ICD, 
that would make me interested in using it. I developed my own line of
simple programmers for the express purpose of code dumping bootloaders.
Removing the traditional programming interface alleviates the need to
follow Microchip's pin and timing constraints. You can pick both the
hardware and data link interface when you have a bootloader.

If I were doing it (and I hope to be back into PIC work very soon) I'd
dump a bootloader onto the chip, attach a MAX232 based serial interface to 
it (probably Wouter's half duplex one shown on the WLoader page), and
have at it.

BAJ
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