Understanding ROM, PROM, EPROM, and EEPROM: Design, Programming, and Identification

Read‑only memory (ROM) shares many design principles with RAM, but its latching mechanism is engineered for a single, irreversible write cycle. The most basic ROM uses tiny fuses that can be selectively blown to encode binary information; once a fuse is ruptured, the data cannot be altered, making these circuits inherently one‑time programmable.

Because of this one‑time write capability, such devices are known as Programmable Read‑Only Memories (PROMs). Some programmable memories, however, can be reset with considerable effort, creating a hybrid between RAM and ROM. These are called Erasable Programmable Read‑Only Memories (EPROMs) – a name that reflects their unique ability to be erased and reprogrammed.

EPROMs are available in two main variants: UV‑erasable EPROMs (UV‑EPROM) and electrically erasable EPROMs (EEPROM). Both types rely on MOSFETs with capacitive charge latches. UV‑EPROMs are cleared by exposing the chip to ultraviolet light, while EEPROMs can be erased and rewritten electrically.

EPROMS are easily identifiable by a transparent glass window that exposes the silicon die to light. After programming, the window is sealed with tape to protect the data from ambient illumination. Programming typically requires higher voltages than those used during read‑only operation.

RELATED WORKSHEETS:

• Memory Devices Worksheet