----- Original Message -----
From: "wouter van ooijen & floortje hanneman" <wf@XS4ALL.NL>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, August 05, 2001 2:21 PM
Subject: Re: [PIC]: 100 really obscure factoids about PICs


> > For the '876 line at least, there are specs in the 'absolute maximum
> > ratings' section that state:
>
> IMHO the 'absolute maxima ratings' are not the specs under which the
device
> will function according to all other specs, rather the specs under which
the
> device will not die (in other words: function according to all its specs
> once you apply the 'rated conditions').
>
> Wouter
>

Wouter,

You are, of course, right.

I would interpret a 20ma absolute maximum rating on clamp current to
indicate that such a current for a brief interval would not permanently
damage the chip (ie: no SCR lock-up), but that there would be no guarantee
of proper operation during the 20ma pulse.

However, the reality of it is that there is nothing magic about the internal
clamping diodes. mChip has given us information that 20ma will not
permanently damage them, so I'd feel comfortable depending on them to
continuously clamp perhaps 1/100 or even 1/10 as much current, or 0.2 ma or
2 ma.

It would be a trade off. For a high quantity, highly cost sensitive
application, I'd be more inclined to go with the minimum possible parts
count and to depend on the diodes.

Also, note that many of the circuits offered for more extensive (expensive)
clamping are little better than the single resistor at modest voltages.

A typical circuit uses two resistors in series and adds silicon diodes to
clamp the junction of the resistors to Gnd and Vdd, thus limiting that
junction to the range Vss-0.6 to Vdd+0.6 or so. This is _still_ out of
specification for the PIC pin! Saying that 0.6 volts above the rail is only
a little bit out of spec is like a woman saying she is just a little bit
pregnant. You either live strictly within the spec or you accept the fact
that you are not a virgin.

Using a 5.1V 5% zener to clamp the high side is another fallacy. The actual
zener voltage could be as high as 5.36 and Vdd (if also 5%) could be as low
as 4.75. The differece is 0.6V, again well above the specified 0.3V.

As yet another example: imagine a typical keypad interface circuit
consisting of a pullup resistor (or the built in weak pullup) and a series R
to the keypad button. In many cases the voltage waveform induced at the pin
would well exceed the clamping limits if it were not for the clamping
diodes. This particular application is almost always considered acceptable.

Finally, I am sure there is some safe clamping current that will work in
normal operation. I'd love to see mChip document it (even if just as a
'design guidance' type of number).

When I started this discussion I mentioned that talking about depending on
the internal clamping diode would restart a religious war. It looks like I
was at least partly right. :-)

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

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads