Side-by-side comparison
| Parameter | EPROM | EEPROM |
|---|---|---|
| Full Form | Erasable Programmable ROM | Electrically Erasable Programmable ROM |
| Erase Method | UV light through quartz window, ~20–30 minutes | Electrical voltage (~12 V internally generated) |
| Erase Granularity | Whole chip only | Byte-by-byte or page |
| In-Circuit Erase | No — chip must be removed | Yes — erase and reprogram on board |
| Typical IC | 2764 (8Kx8), 27C256 (32Kx8) | 28C64 (8Kx8), AT28C256 (32Kx8) |
| Programming Voltage | 12.5 V on Vpp pin | 5 V supply; internal charge pump to ~12 V |
| Endurance | 100–1000 erase cycles | 100,000–1,000,000 cycles (AT28C256: 100K) |
| Data Retention | 10–20 years after programming | 10–100 years (AT28C256: 100 years) |
| Quartz Window | Required for UV erase | Not present — ceramic or plastic package |
| Cost | Higher (quartz window adds cost) | Lower for equivalent capacity today |
Key differences
The fundamental difference is the erase mechanism. EPROM (2764) uses UV light at 253.7 nm wavelength for 15–30 minutes to clear the floating gate charge — the chip must leave the PCB, and without UV exposure it retains data for 20+ years. EEPROM (AT28C256) generates ~12 V internally via a charge pump and erases individual bytes electrically in under 10 ms without desoldering. EEPROM endures 100,000 write cycles versus EPROM's 100–1000. However, EPROM offers higher density per die at lower cost for read-heavy, rarely-changed firmware scenarios.
When to use EPROM
Use EPROM (27C256) in high-volume, cost-sensitive products where firmware never changes after factory programming, such as arcade game cartridges or legacy industrial controller ROMs where UV erasure at the factory is acceptable.
When to use EEPROM
Use EEPROM (AT28C256 or I2C EEPROM like 24LC256) when the device must store calibration data or user settings that are updated in the field without removing the chip from the board.
Recommendation
For modern designs, choose EEPROM or Flash over EPROM — in-circuit programmability is non-negotiable for any product that may need firmware updates. For exam questions, know that EPROM uses UV, EEPROM uses electrical erase, and Flash is a block-erase variant of EEPROM.
Exam tip: Examiners ask you to compare programming voltages — state that both require ~12.5 V during write, but EPROM needs an external Vpp pin while EEPROM generates it internally via a charge pump.
Interview tip: Interviewers distinguish candidates who know that EEPROM supports byte-level erase (AT28C256) while NOR Flash (W25Q128) uses sector-level erase — state this difference clearly when asked about non-volatile memory options.