EPROM

Also called: erasable PROM, UV EPROM, erasable programmable read-only memory, UV-erasable PROM

Erasable programmable ROM: a ROM programmed electrically and erased by shining ultraviolet light through a window in the package, so it can be reused.

An EPROM stores each bit as charge trapped on an insulated ("floating") gate inside a transistor. The trapped charge stays for years without power, so the chip is non-volatile.

Programming is electrical, with a device programmer.
Erasing needs ultraviolet light. The package has a small quartz window over the chip; minutes under a UV lamp give the trapped charge enough energy to escape, wiping every bit at once.

That made EPROMs popular with developers in the 1980s and 1990s: program, test, erase, fix, program again. The cost is inconvenience. The chip has to come out of the circuit to be erased, and you can't erase just part of it.

An EEPROM removed both drawbacks by erasing electrically, in place, and flash later made electrical erasing dense and cheap. Plastic packages without a window, called OTP EPROMs, behave like a PROM: once programmed, they can't be erased.

Worked example

Example

Updating an EPROM

A bug is found in the program stored in an EPROM on a circuit board.

1.
Remove the chip from its socket.
2.
Expose its window to ultraviolet light until every bit is erased.
3.
Program the corrected contents with a device programmer.
4.
Put the chip back. Compare an EEPROM, which could be rewritten without leaving the board.

Common mistakes

Thinking an EPROM erases electrically. That's the EEPROM; the EPROM needs ultraviolet light.
Expecting to erase a single byte. UV erasing clears the whole chip.

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