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'[PIC]: PIC16F84 EEPROM problem'
2000\11\24@094159 by Bardos Csaba

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Dear Piclisters,

First of all thanx for the replies to my previous question.
I have a problem with f84 eeprom. I made gadgets with f84s
that sit on a rs485 line. The interface between the line
and the PIC is a Maxim MAX485. The PIC EEPROM is used to
hold its address, so I can distinguish them based on this
address. The cable runs about 800m and is situated in cable
tunnel. They work fine. But sometimes something happens and
they don't do their job. I inspected them, and the only thing
that has changed their addresses. When I leave the gadget in
my office they work for months without problems. At its place
they last about one-two weeks. This eeprom content change is
more often when there is stormy weather. Is it imaginable that
the eeprom change is occured by some overvoltage ?
It is probably that this overvoltage appears on the cable.
Since the max485 has clapming diodes inside toward the +5v,
these spikes can appear on the +5v. (The +5v is produced
by a normal 7805 or 78L05). Can these spikes cause the
eeprom change ? (Only, since the program flash never
changes).

Thank you in advance,
Cs.

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2000\11\24@153059 by w. v. ooijen / f. hanneman

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You did not ask for a solution, but: you could put three (or five, or ...)
copies of the address in the data eeprom, and decide by majority vote on
the correct address, and write that back to the minority.
Wouter

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2000\11\25@060129 by Peter L. Peres

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>EEPROM data loss

It has never happened to me, but then I always put a 5V6 1W zener across
the PIC power lines when I have any reason to believe that the protection
diodes in the PIC inputs (or other chips) may conduct. You see, most
regulators don't sink any current, and anything rectified by the
protection diodes will raise the +5V rail. I once saw such a circuit that
caught so much current through an incorrectly designed 'resistor only'
digital input that it ran without a power supply and still had 6.5V on the
power rail until the PIC sank some current. It was an automotive circuit
at 12V.

hope this helps,

Peter

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2000\11\25@115013 by J Nagy

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Bardos Csaba wrote:
<snip>

>address. The cable runs about 800m and is situated in cable
>tunnel. They work fine. But sometimes something happens and
>they don't do their job. I inspected them, and the only thing
>that has changed their addresses. When I leave the gadget in
>my office they work for months without problems. At its place
>they last about one-two weeks. This eeprom content change is
>more often when there is stormy weather. Is it imaginable that
>the eeprom change is occured by some overvoltage ?

       In one of my previous positions, we were responsible for several
distributed 'loggers' around a very large electrical power substation.
Typically the units were separated by about 200m-500m, but were linked by
RS485 (for the satellite clock). Every time there was an electrical storm
we blew out at least one of the comm cards.
       I insisted that it was ground potential rise (current flow in the
earth that develops voltages), but the experts insisted that the systems
were so strongly bonded with huge copper busses that appreciable potential
rises could not occur. I finally bought opto-isolated RS485 interfaces and
the problems went away.

       Jim

 Elm Electronics
 ICs for Experimenters
http://www.elmelectronics.com/

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