The PIC microcontroller is fitted with an area of EEPROM (Electrically Erasable Programmable Read Only Memory). This memory is special in that it retains it's contents even when the power is switched off. Normal variables vanish when the PICmicro is switched off, but values placed in EEPROM remain for ever.

EEPROM is more fiddly to use than ordinary memory and there are limits to the number of times you can change the contents of EEPROM before it "wears out". These limits are quite high, but they mean that you can't use EEPROM as an extension to main memory, where a variable could be altered many hundreds of thousands of times as a program runs. Remember that there is no limit on the number of times you can read an EEPROM location, only the number of times you can change it.

In a situation where you want to store configuration information EEPROM is very useful. We are going to use it to store the number which will unlock our combination lock, in other projects you could use it to replace switches to set the configuration of a device.