from data sheet:

Code:
The erased state of an EEPROM bit is 1. During a read operation, bit lines are
precharged to 1. The floating gate devices of programmed bits conduct and pull the
bit lines to 0. Unprogrammed bits remain at the precharged level and are read as
ones. Programming a bit to 1 causes no change. Programming a bit to 0 changes
the bit so that subsequent reads return 0.
When appropriate bits in the BPROT register are cleared, the PPROG register
controls programming and erasing the EEPROM. The PPROG register can be read
or written at any time, but logic enforces defined programming and erasing
sequences to prevent unintentional changes to EEPROM data. When the EELAT
bit in the PPROG register is cleared, the EEPROM can be read as if it were a ROM.
The on-chip charge pump that generates the EEPROM programming voltage from
VDD uses MOS capacitors, which are relatively small in value. The efficiency of this
charge pump and its drive capability are affected by the level of VDD and the
frequency of the driving clock. The load depends on the number of bits being
programmed or erased and capacitances in the EEPROM array.