Explain the difference between Programmable ROM (PROM), Erasable Programmable ROM (EPROM) and Electrically Erasable Programmable ROM (EEPROM)

Published by Patrick Mutisya · 14 days ago

Computers and Their Components – ROM Types

3.1 Computers and Their Components

Objective

Explain the difference between Programmable ROM (PROM), Erasable Programmable ROM (EPROM) and Electrically Erasable Programmable ROM (EEPROM).

Key Concepts

  • ROM – Read‑Only Memory, non‑volatile storage used to retain data when power is removed.

  • Programmability – The ability to write data to the memory after manufacture.

  • Erase mechanisms – How the stored data can be removed or re‑programmed.

Types of Programmable ROM

FeaturePROMEPROMEEPROM
Initial stateBlank (all bits = 1)Blank (all bits = 1)Blank (all bits = 1)
Programming methodFuse burning with a high‑current pulse (via a PROM programmer)U \cdot light exposure through a quartz window after charge injectionElectrical voltage applied to individual cells via the data bus
Erase methodNot erasable – once programmed, data is permanentWhole chip erased by exposing to intense U \cdot light (≈ 15 s–5 min)Selective cell‑by‑cell erase using electrical signals
Re‑programmingImpossiblePossible after a full erase; limited number of cycles (≈ 10³–10⁴)Possible many times; typical endurance 10⁵–10⁶ cycles per cell
Typical applicationsFirmware that never changes (e.g., boot ROM in early computers)BIOS chips in older PCs, where occasional updates were neededModern BIOS/UEFI, microcontroller configuration bits, EEPROM data loggers
Physical characteristicsOpaque package; no windowClear quartz window on top of the package for U \cdot exposureStandard opaque package; no window required

How Each Type Works

Programmable ROM (PROM)

PROM is manufactured with an array of fusible links. Each link represents a memory cell that stores a binary ‘0’ when the link is blown and a ‘1’ when it remains intact. Programming is performed once by sending a high‑current pulse through selected links, permanently breaking them. Because the process is irreversible, PROM cannot be erased or re‑programmed.

Erasable Programmable ROM (EPROM)

EPROM uses a floating‑gate MOSFET structure. During programming, a high voltage (≈ 20 V) is applied to the control gate, causing electrons to tunnel onto the floating gate, representing a stored ‘0’. To erase the data, the chip is placed under a strong ultraviolet (UV) light source (≈ 253.7 nm). The U \cdot photons provide enough energy to discharge the floating gate, returning all cells to the ‘1’ state. Erasure affects the entire chip at once.

Electrically Erasable Programmable ROM (EEPROM)

EEPROM also uses floating‑gate MOSFETs, but the erase operation is performed electrically. By applying a specific voltage pattern to the control gate, electrons can be removed from or injected into the floating gate without U \cdot exposure. This allows individual bytes or even single bits to be erased and re‑programmed, making EEPROM suitable for data that changes frequently.

Comparison Summary

  1. Re‑usability: PROM – single use; EPROM – reusable after full‑chip erase; EEPROM – reusable after selective erase.

  2. Erase method: PROM – none; EPROM – U \cdot light; EEPROM – electrical voltage.

  3. Typical endurance: PROM – N/A; EPROM – low (10³–10⁴ cycles); EEPROM – high (10⁵–10⁶ cycles).

  4. Physical packaging: EPROM packages include a quartz window; PROM and EEPROM do not.

Suggested diagram: Cross‑section of a floating‑gate MOSFET showing electron storage on the floating gate for EPROM and EEPROM.

Key Take‑aways

  • PROM provides a permanent, one‑time programmable memory.

  • EPROM adds the ability to erase the entire chip using U \cdot light, allowing re‑programming.

  • EEPROM further enhances flexibility by enabling electrical, selective erasure and re‑programming, making it the most versatile of the three.