Memory technologies like EPROM and EEPROM are in demand in the evolution of digital systems. Both are types of non-volatile memory, designed to retain information even when power is removed, but they differ significantly in how they store, erase, and update data. Understanding these differences is needed for anyone working with embedded systems. This article explains how EPROM and EEPROM work, compares their features, and explores their advantages, limitations, and applications.
What is EEPROM?
EEPROM stands for Electrically Erasable Programmable Read-Only Memory. It is a type of non-volatile memory, meaning it retains stored information even when the device is powered off.
The main advantage of EEPROM is its ability to be electrically reprogrammed. Data can be erased and rewritten directly on the circuit board using controlled voltage signals, eliminating the need to physically remove the chip. Unlike earlier ROM types that required full erasure, EEPROM supports byte-level erasure, so specific bytes can be updated without disturbing the rest of the memory.
This makes EEPROM highly suitable for storing small but important data such as configuration settings, calibration values, or firmware parameters that may need to be modified multiple times during a system’s lifecycle.
What is EPROM?
EPROM stands for Erasable Programmable Read-Only Memory. Like EEPROM, it is non-volatile memory, meaning the stored data remains intact even when power is turned off. However, it uses a different erasure method compared to electrically erasable types.
An EPROM chip is packaged with a quartz glass window that exposes the silicon inside. When subjected to ultraviolet (UV) light, the stored charge in the memory cells is discharged, effectively erasing the data. This process typically takes 15–20 minutes of UV exposure. To update or rewrite data, the chip must first be removed from the circuit, erased under UV light, and then placed in a special programmed that uses relatively high programming voltages (12–24 V). After erasure, all memory cells return to their initial state, and new data can be written.
EPROM vs. EEPROM: Characteristics Comparison
| Aspect | EPROM | EEPROM |
|---|---|---|
| Erasure Method | UV light through quartz window | Electrical voltage pulses |
| Reprogramming | Requires removal + external programmer | In-circuit, no removal needed |
| Granularity | Entire chip erased at once | Byte-level erasure possible |
| Data Retention | 10–20 years | 10+ years |
| Ease of Use | Slow, external hardware required | Faster, simpler, no extra device |
Internal Structure and Working Principle of EPROM & EEPROM
Both EPROM and EEPROM are built on floating-gate MOSFET transistors, which use an insulated gate to trap or release electrons. The presence or absence of stored charge determines whether a memory cell represents a logic “0” or “1.”
• EPROM: Programming is achieved by applying a high voltage that forces electrons into the floating gate through hot-carrier injection. Once trapped, these electrons remain for years, making the data non-volatile. To erase the memory, the chip is exposed to ultraviolet (UV) light, which provides the energy needed to release the trapped electrons through the quartz window. This resets all cells simultaneously.
• EEPROM: Instead of UV light, EEPROM relies on Fowler–Nordheim tunneling, a quantum tunneling effect that allows electrons to move in or out of the floating gate under controlled electrical fields. This mechanism supports electrical erasure directly on the circuit board, enabling selective, byte-level updates and faster reprogramming without physically removing the chip.
Pros and Cons of EEPROM and EPROM
| Aspect | EEPROM | EPROM |
|---|---|---|
| Pros | • Supports in-circuit programming (no removal required) • Byte-level erasure for selective updates • Available in serial (I²C, SPI) and parallel versions • High endurance (\~1 million write/erase cycles) • Reliable data retention (10–20 years) | •Non-volatile with long data retention (10–20 years) • Reusable, unlike one-time PROM • Cost-effective during its prime era • Suitable for early prototyping and development |
| Cons | •More expensive than EPROM • Endurance limited compared to modern Flash• Write operations slower than reads • Typically smaller capacity than Flash | •Full-chip erasure only (no selective editing) • Requires UV light and quartz window for erasure • Slow erase time (15–20 minutes) • Needs external high-voltage programmer • Vulnerable to accidental UV exposure |
Applications of EPROM and EEPROM in Electronics
EPROM
• Firmware storage in early microcontrollers: Provided a reliable way to store embedded code before EEPROM and Flash became standard.
• Program memory in personal computers and calculators: Commonly used to hold system software and logic programs.
• Digital instruments: Found in oscilloscopes, test equipment, and measurement devices that required stable program storage.
• Prototyping and training kits: Favored in educational and development environments because data could be erased and rewritten multiple times for testing.
EEPROM
• BIOS/UEFI storage in computers: Holds important system startup instructions and can be updated without replacing hardware.
• Sensor calibration data: Used in automotive and industrial systems to store fine-tuned calibration values that need occasional updates.
• Telecommunication devices: Enables field reconfiguration of modems, routers, and base stations without chip replacement.
• Smartcards and RFID tags: Provides secure, non-volatile memory for authentication, identity management, and transaction data.
Medical devices: Stores patient-specific parameters and configuration data in instruments like glucose monitors or pacemakers.
PROM vs. EPROM vs. EEPROM
| Feature | PROM | EPROM | EEPROM |
|---|---|---|---|
| Programming | One-time only: Data is permanently written during initial programming. | Rewritable with UV light: Requires removal and reprogramming with high voltage. | Electrically rewritable: Supports reprogramming directly on the circuit board. |
| Erasure | Not possible: Once written, data cannot be changed or removed. | Chip-wide erasure: Entire memory must be erased using UV exposure through a quartz window. | Selective erasure: Can erase at the byte level or the entire chip as needed. |
| Reusability | No: Cannot be reused once programmed. | Yes: Erased and rewritten multiple times (but limited). | Yes: High flexibility with frequent updates. |
| Endurance | 1 cycle (write once). | Around 100–1,000 cycles before device wear-out. | Around 1,000,000 cycles, much higher than EPROM. |
| In-Circuit Use | No: Must be programmed before installation. | No: Must be removed for UV erasure and reprogramming. | Yes: Supports in-circuit updates, making it ideal for modern systems. |
| Cost | Low: Very cheap per bit. | Moderate: More expensive than PROM but affordable in its era. | Higher per bit: Costlier than PROM/EPROM, but offers superior flexibility. |
EPROM vs. EEPROM vs. Flash Memory
| Feature | EPROM | EEPROM | Flash Memory |
|---|---|---|---|
| Erasure Method | UV light through quartz window | Electrical, byte-level | Electrical, block/page-level |
| Programming | Requires removal + high-voltage programmer | In-circuit, electrical reprogramming | In-circuit, electrical reprogramming |
| Reusability | Yes, but slow and inconvenient | Yes, frequent updates possible | Yes, optimized for large-scale rewrites |
| Endurance | \~100–1,000 cycles | \~1,000,000 cycles | \~10,000–100,000 cycles (depends on type) |
| Speed | Very slow (UV erase: 15–20 min) | Moderate (slower writes than reads) | Fast (block operations, higher throughput) |
| Capacity | Small (KB–MB range) | Small to medium (KB–MB range) | Very high (MB–TB range) |
| Cost per Bit | Moderate (historic) | Higher | Low (mass storage standard) |
| Typical Use | Legacy systems, prototyping, education | BIOS, calibration data, secure devices | USB drives, SSDs, SD cards, smartphones, microcontrollers |
Conclusion
EPROM and EEPROM were milestones in memory technology, each serving as a bridge to more advanced storage solutions like Flash. EPROM offered a practical way to reprogram devices in its era, while EEPROM introduced greater flexibility with in-circuit and selective updates. Today, EEPROM remains relevant for storing small but critical data, while Flash dominates large-scale storage needs. By comparing these memory types, you gain a clear picture of how technology has advanced, and why EEPROM still finds its place in modern electronics.
Frequently Asked Questions [FAQ]
Why is EEPROM better than EPROM?
EEPROM is better because it allows electrical reprogramming in-circuit, supports byte-level erasure, and eliminates the need for UV light or chip removal. This makes it more flexible and convenient than EPROM.
Is Flash memory the same as EEPROM?
No. Flash memory is based on EEPROM technology but optimized for high density and block/page-level erasure. EEPROM allows byte-level erasure, while Flash is faster and cheaper per bit, making it ideal for mass storage.
How long can EEPROM and EPROM retain data?
Both can typically retain data for 10–20 years, though EPROM endurance is limited to ~100–1,000 cycles, while EEPROM can last up to ~1,000,000 cycles.
Why does EPROM need a quartz window?
The quartz window lets UV light penetrate the chip to erase stored charges from the floating gate. Without this transparent window, erasure would not be possible.
Where is EEPROM still used today?
EEPROM is widely used in BIOS/UEFI firmware, sensor calibration, RFID tags, smartcards, medical devices, and industrial equipment where selective updates are necessary.