Erasable Programmable Read Only Memory (EPROM)

Erasable Programmable Read Only Memory (EPROM) is a non-volatile memory chip that retains data even when power is switched off. Each EPROM is individually programmed by an electronic device, and the stored data can be erased by exposing the chip to strong ultraviolet light for reprogramming.

An EPROM contains a transparent fused quartz window at the top of the package which allows exposure to ultraviolet light. The silicon chip is visible through this window, making it easily identifiable among other memory types.

How EPROM Works

Each storage location in EPROM consists of a floating-gate field-effect transistor. This transistor contains a channel with source and drain contacts at its ends. An insulated oxide layer covers the channel, followed by a floating gate electrode, then another oxide layer, and finally a control gate electrode on top.

Data is read by decoding the address at the address pins and connecting it to output buffers. Programming involves applying high voltage to trap electrons in the floating gate. The programming process is not electrically reversible − ultraviolet light causes ionization within the oxide, allowing stored charge to dissipate and erasing the memory.

Programming and Erasing Process

Applications

Initially considered too expensive for mass production, EPROM proved economical for small-volume production runs. Microcontrollers such as Intel 8048, Freescale 68HC11, and PIC microcontroller (C version) used on-chip EPROM. Windowed versions were primarily used for program development and debugging, allowing developers to modify firmware during the development cycle.

Advantages

• Non-volatile − Retains data without power, eliminating need for external backup.

• Reprogrammable − Data can be erased and reprogrammed multiple times.

• Cost-effective for small production volumes and prototyping.

• Reliable − Proven technology with good data retention characteristics.

Disadvantages

• UV light requirement − Cannot be erased using electrical signals alone.

• Bulk erase only − Cannot erase individual bytes; entire chip must be erased.

• Slow erase process − Takes 15-20 minutes compared to instant electrical erasure.

• Higher resistance transistors result in increased static power consumption.

• Physical access required for erasing, making in-circuit updates impossible.

Conclusion

EPROM served as an important stepping stone in memory technology, bridging the gap between mask ROM and electrically erasable memories. While largely superseded by EEPROM and flash memory, EPROM's principles laid the foundation for modern non-volatile memory technologies used in embedded systems today.