wouter van ooijen <wouter <at> voti.nl> writes:

> 
> > 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 ?

Ref: pic16f5x datasheet table 11.2 param D003 -> Vpor = Vss. (typ). Really ?
maybe Vss -0/+0.3V would be a little more reasonable, don't you think ?

Then, a BAT54 leaks about 100 nA at room temperature and 5V. So if you have 10
protected pins leakage is 1 uA (and becomes 10 uA at 60-70 deg C). This almost
doubles the current consumption of the chip in XT mode.

Then, Vil and Vih are specced as 0.15 Vdd and Vss resp. for Vdd < 4.5V, Iow, at
3V  Vil must be < 0.45 V which is < Vf (Si). So any switch matrix and such must
also use Schottky diodes (more cost, more leakage).

I am not complaining here, I am looking for solutions. As to what you said about
its 'being so', please. I can still buy 12C508 that does not have these problems
(at least not with high sensitivity to negative voltage on pins). 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).

Peter P.


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