Read Only Memory

ROM type devices retain their stored data even after losing power. The data is either hard coded into the device during manufacture, or programmed in later (PROM) via a special programming device. Some types of ROM can be programmed, erased, and re-programmed with new data. (EPROM, EEPROM, FLASH).

PROM Programmable Read Only Memory
EPROM Erasable Programmable Read Only Memory
EEPROM Electrically Erasable Programmable Read Only Memory
FLASH A type of EEPROM based on blocks of data, which is typically must less expensive, and faster.

The key thing to know about any EPROM, OTP, EEPROM, flash memory, etc. is the operation principle.

All of the above mentioned types work by building up a charge on a MOSFET gate structure. Once the charge is built up during programming, it is stranded there by electrically isolating the gate. This is one "bit", represented by either some charge or no charge, depending on the design.

Since all these types use the same storage principle, they are theoretically about equal in terms of robustness.

The main difference between the technologies:

1) EEPROM and flash let you bleed off the charge via electrical erasure. EPROM requires UV light to bleed off the charge.

2) EEPROM bytes are individually programmable/erasable, whereas flash is programmable/erasable only in bulk or in blocks.

3) OTP is just EPROM without the UV window, so you can't erase it.