to Olin L.:

I do not have an axe to grind. If I am dramatic then it's because that's what it
appears to take nowadays to get some sane people to (re-)read the datasheets. I
would also like to see what others do/did/will do. Even if I am dramatically
wrong I would like that to be pointed out based on the facts.

I was told (by Wouter) that 'I should design to the specs'. Here I shall try to:

I would like to read your comments about some parameters in the datasheet I
referred to (pic16f5x DS41213C), to stay on topic and off the dramatical stage:

a0) (this is how it started): p59: table 11.0: 'Voltage on all other pins with
respect to VSS: -0.6V to (VDD + 0.6V)': the chips I tested (about 10) all did
not function when clamped with 1N4148, reading 0.5x volts on the clamp (clamp
current was about 1mA), and needed clamping to 0.3x volts, using a Schottky
diode. In the same circuit, drop in compatible older chips worked flawlessly
(without any clamp). When those were forced into fault mode (that was almost
impossible since the specified pin clamp current would have been exceeded), a
1N4148 or TVS fixed the problem as external clamp.

a) p63. table 11.1: D003: Vpor = Vss (typ)

I take it that Vss is specified with .0 precision so: Vss <= Vpor <= 0.05*Vss
from this, lacking other info. Thus at Vss = 3V Vpor max is 0.15 V. Thus I need
to provide a low Vol driver for it (f.ex. a 2N7000 or bipolar). Also if a cap is
attached and a diode provides Vdd through a resistor to allow icsp, then the cap
will in theory 'never' discharge to 0.15 V, even if Vss and Vdd are shorted. In
practice it could take minutes. This is something that I have in fact noticed to
happen, in despite of providing a discharge resistor between Vdd and Vss.
Temporarily disconnecting power would rapidly drain Vdd but Vmclr would not drop
low enough due to the diode. Then the next power-up could be a warm start
(without por).

b) p64. table 11.2: D030: Vil = Vss to 0.15*Vdd = 0 to 0.45 V @ 3V. Thus here
only Schottky pulldowns will work.

c) p64. table 11.2: D031: Vih = Vdd to Vdd - 0.45 V. Again Schottky required.

d) same, D080, D081: The part outputs 8.5/3.0 mA only at high saturation drop. A
16C54 would provide 20.0/15.0 mA here (quoted from memory). The 16f54 is
supposed to be drop-in compatible. The migration document has no word on this.

I think that PICs are great chips. I also think that this particular batch is
not so. I also think that the 0.3V from rails requirement for low cost parts
without A/D is not reasonable. There is no point in having $ 0.5 chips that
require a 70 cent reset chip, a $ 1.0 2% regulator and 5 cents/pin in Schottky
clamps, plus a 2-sided board with plated vias to hold the resulting jungle, and
all this can double the parts count in a small project.

Maybe I had a freak batch but I don't think so. Vasile said that he determined
the susceptibility threshold to be 0.3-0.4 V. So what's the inexpensive,
obtainable, and well-known fix for this (besides adding 20 BAT54S diodes - hey,
at least one gets a quantity discount when buying so many). Yes I am a little
sarcastic. No, I will not go on.

Peter P.


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