Richard Prosser wrote: > On 14/12/2007, Bob Axtell wrote: > >> 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 >> > > > Bob, > Why "best 3 of 5"? I would have thought "Best 3 or 4 of 5" would have been a > better schene? > > RP > Most of the time it is "5 of 5". But to keep the algorithm as small as possible, it takes the first 3 that match and uses it, but it examines all 5; if one doesn't match it overwrites all 5. I guess I was vague...sorry. --Bob -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist