> > > The problem is that many (most) CMOS chips are now specified
> > > for Vdd-0.3 to Vcc+0.3 maximum limits *operating*.
> > 
> > AFAIK not for PICs, the Vcc+ and GND- specs are in the 
> absolute maxima 
> > section (= chip will survive, not chip will work). Has been so for 
> > ages, so for normal operation you could never depend on 
> these diodes.
> 
> Yes, could you please indicate a reset circuit that 
> guarantees a reset voltage of exactly zero volts, as required 
> by some pics, to 'ensure proper reset', also a circuit that 
> will clamp input voltages to 1/2 LSB of the used AD converter 
> from a rail (preferrably both), and is inexpensive ?

If you designed your old circuits by the book (datasheet) they would do
this, so why not re-use? If not you were working outside specs anyway,
and you were just lucky to get away with it.

I don't think I ever used an A/D in its extremes, so I never had that
problem. When you use only 1..4V input on 5V supply a simple opamp will
do in most cases.

> can still buy 
> 12C508 that does not have these problems (at least not with 
> high sensitivity to negative voltage on pins).

Maybe, or maybe uChip has silently changed the fab so now you must realy
obey the specs?

But I do agree with you that the specs are very conservative (on uChips
side!). Compare the output currents that are spec'ed for normal
opartaion with the figure on p1 of most datasheets.

> And, by the 
> way, when Vil is specced as Vss to 0.15 Vdd then any 
> pull-down resistor on an input pin should be referenced to 
> 0.075 Vdd, not to Vss, as with a resistor referenced to 
> ground, ANY noise will be out of spec (since voltages below 
> Vss are forbidden according to what you say - and even then, 
> noise immunity will be only 0.15 Vdd pkpk or about 0.053 Vdd 
> eff - about 5% Vdd and require a 2% regulator - thus any 
> ground bounce or noise coupled into the supply and appearing 
> on decoupling caps on other pins will put your circuit - 
> temporarily I hope - out of spec).

By the book that is all correct, so for realy high reliability in noisy
environments you might need a strong pull down / weak pull up combo.
Never seen that, but by the book it seems unavoidable.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: www.voti.nl/hvu
 
 

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