Hi Andy,

An EEPROM cell (that is: one bit) is basically a MOSFET, which is programmed by storing charge onto a floating gate. Over time this charge will leak away until it is no longer strong enough to drive the FET open. The data retention time is the time it takes for this to happen. So your first guess is the right answer (bonus point!).

Be aware however that the leakage increases with temperature! Manufacturers use this to predict the data retention time by operating the devices at high temperature and collecting data (or did you really think they sit around and wait for ten years ;o) ?). Then they extrapolate this data to (usually) 25 deg. C.

An empty EEPROM cell is at logic '1' level. Programming it sets it to '0'. Since each cell has a certain *probability* to fail after so many years, it can not be predicted what a whole byte will contain. However, the actual failure you will observe is that a bit that was '0' will become '1'.

Hope this makes things clear.

Best regards,
Rob Klein.

PS I am aware that my explanation is somewhat simplified. Whole books have been written on the physics involved! But for basic understanding I think this is good enough.