Roman,

Would you find a schottky diode wired with its anode to the I/O pin and its
cathode to the Vdd rail an acceptable alternative to a 5.1V Zener?

If not, why not?

If so, what's wrong with putting that diode inside the chip.

That is, of course, what mChip has done.

Bob Ammerman
RAm Systems
(contract development of high performance, high function, low-level
software)

----- Original Message -----
From: Roman Black <fastvid@EZY.NET.AU>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, February 11, 2001 8:20 AM
Subject: Re: [PIC]: Input clamp current (was Schmitt Trigger (RB0)


> Bob Ammerman wrote:
> >
> > Roman,
> >
> > Microchip have an application note (AN251? or AN521?) that talks about
using
> > just an R to high-voltage. In that note that state the diodes have a
500uA
> > rating, which is presumably a continuous rating.
> >
>
>
> I'm familiar with the appnote and it makes me
> shudder. Their attempt to prove that you only need
> one PIC and very few extra parts to build the
> circuit doesn't make it a good design. No
> commercial products that I have repaired use
> that system. Even the cheapest TVs use two
> resistors AND a lower resistor (voltage
> divider), so there is a finite max voltage
> expected at the semi.
>
> It's very common for one of the two pass resistors
> to fail, generally arcing and taking the semi
> with it. And the TV semi is usually a transistor
> which is substantially more rugged than the
> silicon inside a PIC. The better TVs use three
> or four pass resistors, a zener and cap and sometimes
> a lower resistor too. These pass resistors also
> fail, but with less catastrophic effects on the
> rest of the set.
>
> We can all argue clamp diodes the same way we
> argue commenting code, yes it probably will
> work but is not the way a professional does
> things. I won't expose my PIC pins or logic
> chip gates to more than 5v. :o)
> -Roman
>
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>

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