[U-Boot-Users] Flash environment vs EEPROM environment
Jerry Van Baren
gerald.vanbaren at smiths-aerospace.com
Tue Feb 1 20:06:10 CET 2005
Wolfgang Denk wrote:
> Dear Thomas,
> in message <D9F0B2AD4531B0449D51C1F09199D484050E72 at mail.kom-saarbruecken.com> you wrote:
>>Yes, I know that this has been discussed recently and the recommendation
>>is to store the environment in flash, but:
>>- Are there conditions known to cause similar effects with flash chips
>>as described for EEPROM devices? Could power loss or similar conditions
>>when writing environment sectors cause a flash device to destroy other
>>sectors than the just written one?
> In theory yes. You could assume a system without power monitoring
> where the power is failing slowly so that at some point during the
> brownout the CPU migth start executing bogus insructions, or that
> some bus driver corrupts the addresses or data, or... In theory
> anything can happen.
> Best regards,
> Wolfgang Denk
Flash corruption is more than in theory: proper hardware design is to
have a power fail warning sufficient to allow a flash write cycle to
complete before power completely fails and your software should not
write to the flash when the power fail warning is active.
If you do a lot of flash erase/writing and glitch the power rapidly and
repeatedly during the flash activity, you _will_ have corrupted flash at
_unpredictable_ (i.e. not necessarily the block you were intending to
write/erase) locations. Guaranteed. Want to see the scars :-)?
Fortunately, most people (a) don't write rapidly and repeatedly to
flash, (b) have stable power supplies that don't glitch rapidly and
repeatedly (large output filter capacitors and power supervisory chips
are your friends!) and (c) flash operations are relatively fast making
the window of vulnerability very short.
Thus people get away with not using a power fail warning because the
probability of corruption is extremely small, not necessarily by design
but rather by happy coincidence.
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