Hej Caisson. Tack fšr ditt meddelande 10:09 1999-10-11 +0200 enligt nedan:
> > Van: Russell McMahon <apptech@CLEAR.NET.NZ>
> > Aan: PICLIST@MITVMA.MIT.EDU
> > Onderwerp: More on (ab)use of PIC protection diodes & bad design
> > Datum: zaterdag 9 oktober 1999 1:38
>
>Hello Russel,
>
>   The story below tell's me that the protection-diodes should be protected
>..
>
>Normally I would discard such a story as an hoax.  But you seem to be
>serious about it.  Could you try to explain ? (in as simple as terms as
>possible please ...)
>
>What's so specific of those internal protection-diodes that they can't be
>used like their external counterparts ?
>
>Regards,
>   Rudy Wieser

I am not an expert but I«ll try to explain from what I have read somewhere:

Often the "on chip protection diodes" are implemented in a "low budget way".
That way of doing it, the diodes when conducting leak charges (electrons/holes) into the rest of the chip silicon, possibly causing all sorts of misbehaviour.
Actually the diodes are often bipolar transistors, I've read, but maybe not always.

The same effect also causes the latch up effect: a transient into a chip pin can turn on a transistor connected to Vcc, and at the same tima a one connected to Vdd, and then thay are conducting the process continues, effectively making a triggerted SCR!  The only way to get out of it is to shut down power.

Ten-twnenty years ago chip was much more sensitive top latch up tnan nowadays, but we shall still take care.

Possibly it is the guaranteed limit current before latch up that is the 20 mA stated in the datasheets?

There are ways of creating charge barriers around separate protection diodes on the chip, but that would use more silicon, so the PIC would maybe be 0,0001 USD more expensive... ;)

...But it is of course be much cheaper than external protection or product havoc...

FWIW
/Morgan
Morgans Reglerteknik, HŠllekŒs, 277 35 KIVIK, SWEDEN
   tel +46(0)414-446620, fax -70331,   mrt@iname.com