PROM

Also called: programmable ROM, programmable read-only memory, OTP ROM, one-time programmable ROM, fuse ROM

Programmable ROM: bought blank and programmed once by the user, typically by blowing tiny fuses. After that its contents can never be changed.

A PROM leaves the factory blank, with every connection in place. The user programs it once with a device programmer, which sends pulses that blow tiny fuses wherever a bit should change. A blown fuse can't be repaired, so programming is one-time: such parts are often called OTP (one-time programmable).

Compared with a mask ROM, a PROM lets small companies and prototypes set their own contents without paying for a custom mask. Compared with later types, it can't be corrected: a mistake means throwing the chip away.

Like every ROM, it's non-volatile.

Memory aid for the family: each extra letter adds a freedom.

P: you can program it (PROM)
E: you can also erase it (EPROM)
EE: you can erase it electrically (EEPROM)

Worked example

Example

Why can't a PROM be reprogrammed?

A PROM bit has been programmed by blowing its fuse.

1.
Blowing the fuse physically breaks a tiny link.
2.
There's no way to put the link back, electrically or otherwise.
3.
So that bit is fixed for good. Unprogrammed bits can still be blown later, but nothing can be undone.

Common mistakes

Thinking a PROM can be erased. Only the E types (EPROM, EEPROM, flash) can be erased.
Thinking a PROM comes programmed from the factory. That's a mask ROM; a PROM is programmed by the user.

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