Just thought this might interest somebody... haven't seen anything along
these lines before. But then, it may be trivial... :)

I needed to count some 20M+ incidents, stored in nonvolatile storage
(EEPROM). Microchip's EEPROMs have 100k write cycles endurance spec'ed. A
simple 4-byte counter in EEPROM doesn't do it, as its low byte would get
written too often.

What I did is create a structure with an index and a moving 3-byte counter.
The 3-byte counter counts up to 0x18000 which is just a bit below 100k
(could count up to almost 100k without violating the spec). When it has
reached the max, the index gets bumped and the 3-byte counter gets
restarted from 0, but now one byte offset. Looks like this in EEPROM:

Index Data0 Data1 Data2 Data3 Data4 ...

The first counter is Data0..2 (with Index=0). When it is full, Index gets
incremented, and the next counter Data1..3 reset to 0. And so on, until the
max is reached (at which point I don't increment anymore, indicating an
overflow). On an increment, I write back only the byte(s) that got changed.
This way, no cell gets written more than 100k. 

The total EEPROM storage required for up to Nmax counts is (Nmax/100k)+1
(if Nmax is smaller than 25.5M; above that it's one more). 

Gerhard

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist