Hej Jim Hartmann. Tack fšr ditt meddelande 11:36 1999-10-06 -0500 enligt nedan:
>Dear list,
>
>A while back I reported that very small currents (<10uA) "into" the upper
>protection diode on an input caused the PIC to draw more current into VDD.
>Recently I have found that current "out of" the lower protection diode
>causes more severe problems.  Currents of about 2mA on the 16C622 can cause
>registers within the chip to change or can cause a chip reset (possibly
>watchdog).  When pulling current out of a PORT B input, I observed
>spontaneous changes to TRISB and OPTION registers as well as reset.
>So I guess the moral is to be *very* careful about using the input
>protection diodes as clamps for inputs.
>
>-Jim 

?!  - AUGH!!!

Examine real-life behaviour of bus lines:
Seen as transmission lines; theoretically; if a strong driver (zero output impedance) changes it«s output from 5V to 0V, then at the end of a line (high impedance input) the voltage will peak to -5V, capable of lots of mA.

Peaking time is dependant on line length, current dependant on driver.

Practically drivers are not zero impedance, but HC drivers can do some mA, and even TTL, do especially pulldown some mA peak!

It seems that we should need at least series input resistors, and/or driver output resistors, if not impedance matched lines to protect if PIC«s are such wimpy devices as the tests described above indicate.

(seem strange that a controller do not tolerate practical digital life - not? ;) )

Especially with long bus lines such as a SPI network on a large PCB, this might be critical.
(I have one prototype with PIC16f877 running SPI, HC driven, line length 8cm, and it works, but I might add a resistor in production...)

This seem to me that there really do not exist "real" protection diodes in PIC, just the parasitic diodes of other cirquitry.

VeryVery very bad!!

Does anybody know wether there are similar problems with all PIC versions, or

/going into bunker/

i.e the AVR ?

/staying in bunker a while/


Thanks for the report Jim!
I«ll be careful.

/Morgan


Morgans Reglerteknik, HŠllekŒs, 277 35 KIVIK, SWEDEN
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