Image Credit: Allied Electronics, Inc. | Digi-Key Corporation
Erasable programmable read-only memory (EPROM) chips are programmable, reusable computer chips that can be erased using ultraviolet light and reprogrammed with a PROM programmer or PROM burner. Programming or burning an EPROM chip injects electrons with an elevated voltage into the floating gate a field-effect transistor, trapping the electrons and forcing a reading of zero. Erasing an EPROM chip bombards the chip with ultraviolet radiation through a quartz window to provide the trapped electrons with enough energy to escape the floating gate. To prevent slow erasure over a period of years from sunlight and fluorescent lights, the quartz window is covered with an opaque label during normal use.
EPROM is used widely in personal computers because it enables manufacturers to change the contents of programmable read-only memory before the computer is shipped. Since EPROM chips do not require power to retain their data, they are commonly used to store BIOS information and basic software for modems, video cards, and other peripherals. Flash EPROM or FEPROM incorporates FLASH technology and is available from some chip manufacturers. Other architectural considerations include density, which is the capacity of the memory chip expressed in bits, as well as the number of rows and columns. With EPROM, each row stores a memory word and connects to a word line for addressing purposes. The number of columns equals the bits per word. Each column connects to a sense / write circuit.
Selecting EPROM chips requires the selection of a serial or parallel data bus and the analysis of several performance specifications.
- Operating current is the minimum current needed for active chip operation.
- Standby current is the minimum current needed during inactivity.
- Power dissipation, which is generally expressed in watts or milliwatts, is the total power consumption of the device.
- Measured in nanoseconds (ns), access time indicates the speed of memory and represents a cycle that begins when the CPU sends a request to memory and ends when the CPU receives the data requested.
- Some EPROM chips support a specific temperature range and feature mechanical and electrical specifications that are suitable for commercial or industrial applications.
- Other EPROM chips meet screening levels for military specifications (MIL-SPEC).
EPROM chips vary in terms of supply voltage, logic family, and package type.
- Common logic families include standard, fast, high-speed and advanced CMOS; emitter coupled logic (ECL); TTL and Fairchild advanced Schottky TTL (FAST); gunning technology; and crossbar switch technology (CBT).
- Common package types include ball grid array (BGA), quad flat package (QFP), single in-line package (SIP), and dual in-line package (DIP). Many packaging variants are available for EPROM chips.
- Common packaging materials include plastic, ceramic, metal, and glass.