Bob Axtell wrote:
> David Duffy (AVD) wrote:
>   
>> I have a design that uses a 16F88. The code is very similar to a few
>> other products but they use a 16F628A or 16F648A.
>>
>> I'm having problems with this one design where in the field the EE is
>> being erased. (I think) When I read the offending EE back (via MPLAB),
>> the data is FF instead of its usual 00 .. 07 range.
>>
>> I've gone over the code numerous times and checked the write sequence. I
>> can't even reproduce the fault back here. It can take days or months to
>> show up in the field.
>>
>> I don't know if more than one location is being erased as I'm only
>> storing and reading one byte. (the first one) Has anyone come across
>> this before?
>> David...
>>
>>   
>>     
> David, this is pretty common in the "nanowatt" series of PICs. This is 
> caused by weaknesses in the EEPROM
> manufactrung process. It turns out that, unlike the older PIC16C/18C, 
> there are now severe limitations on the
> number of times the EEPROM cell can be read before being refreshed. 
> Unfortunately, frequent refreshes reduce
> the useful lifetime of the EEPROM cell.
>
> My solution is to write critical values 3-5 times, then read with a 
> "best 3 of 5" algorithm. The idea is that if all 5
> match, use the first value found, but if any of the 5 are different, 
> rewrite all 5, using the value that at least 3 of
> the 5 match, before proceeding. This limits over-refreshing, and only 
> refreshes the cells that NEED to be refreshed.
>   

I wasn't aware of this problem until now. It's odd that I've never 
encountered this with the 16F628A / 648A devices.

I'll have to check more carefully, but I don't think the read/write 
ratio of the EE cell concerned is all that high. Maybe < 10 reads 
between every 1 write to the same cell at a guess.

For now I've done away with the EE, and will just read / write the 
current status to RAM. It doesn't change very often - maybe only a few 
times each day.

The main reason the EE was used was to persist the unit's status if it 
was power cycled, but this shouldn't happen very often anyway. I have 
now coded it so that it defaults back to the "closed mode" (it's a 
window controller) upon power-up to lessen security issues.

I will try a different EE cell on a spare unit, and try to leave that 
running for as long as I can (weeks / months) so see what happens.
David...

-- 
___________________________________________
David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: www.audiovisualdevices.com.au
___________________________________________

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