On Wed, 10 Sep 1997, Tjaart van der Walt wrote:

> Steve Smith wrote:
> >
> > The pic and most other cmos have intrinsic diodes in them the pull up
> > resistor gives forard bias to these and the chip works but some times in a
> > very strange manner. I have fallen into this hole more than once just when
> > you thaught it worked properly suddenly it didnt.
> >
> > The only pin that hasnt got the diode is RA4 on '84 or equivelent pin on
> > other devices the one that doubles as Vpp
> >
> > Cheers Steve.....
>
> You can actually power the PIC (not recommended though) through an IO
> pin.
PTM: Another aspect:
If you try to use in 16c84 a port B pin as a input, you got remember that
this pull-up is about 50..100kOhm.
I had a proto where I tried to give input '0' by connecting input pin to
ground thru 10kOhm resistor. It didn't work, because the voltage at the
pin vas about 1.66V. With a 1kOhm resistor it worked fine.
Another method is to connect the pull-ups off, but then you need an
external pull-up or something else. Else the (CMOS) input will run amok.
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