Matthew Rhys-Roberts wrote:
> Will someone correct me if I'm wrong about this?
>
> As far as I can tell, writing (i.e. erase-writing) to any PIC EEPROM 
> address will disrupt the stored charge in neighboring memory cells. 
> Eventually, if written enough times, EEPROM data corruption may result. 
> Microchip recommend using an EEPROM refresh routine periodically, in 
> cases where erase-write cycles are likely to approach 1- to 10-million 
> times ("specification D124").
>
> I'm only asking so I can make a sensible choice between EEPROM and Flash 
> program memory, for storing some infrequently changed non-volatile 
> constants.
>
> Thanks
>
> Matt
>   
During a lengthy test three years ago, I noticed those failures, too.

Microchip's approach of overwriting the entire array seems a bit like 
owning an elephant to kill
roaches in your parlor by stepping on them; its clumsy.

A sleeker approach is to simply write the variable 3 times, rather than 
just once. When you read it back, read
all three copies; if one doesn't match, then rewrite all three (using 
the two that match). In 3 years of use, I've not
seen another failure. This concept is "best 2 of 3". I used "best 3 of 
5" in a military application.

BTW, in the Nanowatt series of PICs, it is strange to me that the 
program memory  is more reliable than the
EEPROM memory, but it _IS_. I have stored variables in the program 
memory of the  PIC16F87 and '88,
never had a single failure, and to do so took less code space than "best 
2 of 3" EEPROM method.

--Bob A

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