Next week or so I will begin making a simple controller for a 
unit that is planned to work for 30 years.

I am a bit worried about how for many years the EPROM 
in PIC will hold the program without errors.
Cosmic radiation etc will some time make EPROM to read erroneously.

(The power is not on all the time, but the EPROM will age anyway.)

I will only make two units so ROM-version is no option.

What will be best type of PIC; EPROM, EEPROM or "flash"?
I can use the «84
Probably the old C84 is safer than F84 due to larger cells?

I can«t even supply more replacement chips as they will age the same.
And who knows where I am in 20 years.  I can leave a diskette with hex file
but will anybody know how to do with it, will there be chips to program (OK
i can supply them with it) will there be programmers, will there be a
working PC, and will the OS of Bill the then all mighty (?) be able to run
MPLAB, etc... 
Probably the magnetic of the diskette will also age, and also CD«s age as
oxugen migrates thru plastic and oxydizes the alumina layer...
Well, i can hammer a flow-chart into a stones surface?
The stone-age methods of rock writing suddenly seems advanced!

If I build it with a PIC, I should use the 17-series so I can let the
program browse the program to check it with a check-sum, right?
(Or the 6805 that I use sometimes)
Is there a method for that on a  14-bit core?

Only that checking routine resets the watchdog. 
(unless the error forms a erroneous goto to that part of the routine, or
forms another CLRWDT... or something...

Use two PIC that check each other somehow?

Probably the most practical is to use the old logic cirquits... right? ;)
Or what?


Example from my home:
I have retrieved my 21-year-old TV-game "LUXOR video game computer" from
the basement (and built new joysticks... the old ones was mended several
times) 

Now there are problems with 20% of the games.  The game cassetes are built
of eproms, I think: two or more chips mounted chip-on-board (already
then!), and covered by *removeable* black plastic half-boxes with clip-pins.

Fortunaltely the old tennis and icehockey games works, and it is fun to
bring the old machine up un parties to remember old times.  It has 4
colors, and a pixel is a half centimeter on a 14" TV.  And it can beep a
few tones!

Really much more fun then the super-duper Mario.

/Morgan
/  Morgan Olsson, MORGANS REGLERTEKNIK, SE-277 35 KIVIK, Sweden \
\  mrt@iname.com, ph: +46 (0)414 70741; fax +46 (0)414 70331    /