Introduction to Programmable Read-Only Memory (PROM)

　　What is a PROM?

　　PROM is the abbreviation of Programmable read-only memory. PROM is a simple programmable logic device (PLD). Early programmable logic devices only had three types: programmable read-only memory (PROM), ultraviolet erasable read-only memory (EPROM) and electrically erasable read-only memory (EEPROM). Due to structural limitations, they can only perform simple digital logic functions.

　　The invention of PROM

　　PROM was invented in 1956 by Wen-Chun Chou, who worked at the American Powder Emma Corporation in Gatton, New York. The invention was proposed by the United States Air Force to improve the flexibility and security of Air Force computers and the Atlas E/F band missile.

　　Classification of PROMs

　　PROM is mainly divided into three categories: ROM, EPROM and E2PROM.

　　1. One-time Programmable Read-Only Memory (ROM): Programmable read-only memory is only allowed to be written once, so it is also called "One Time Programming ROM" (OTP-ROM). When PROM leaves the factory, the stored contents are all 1. Users can write data 0 to some of the cells as needed (some PROMs have all data 0 when leaving the factory, so users can write 1 to some of the cells) to achieve the purpose of "programming" it. The typical product of PROM is the "bipolar fuse structure". If we want to rewrite certain cells, we can pass a sufficiently large current through these cells and maintain it for a certain period of time, and the original fuse will be blown, thus achieving the effect of rewriting certain bits. Another classic PROM is the PROM using "Schottky diodes". When leaving the factory, the diodes are in the reverse cutoff state. It is still necessary to use a large current method to add a reverse voltage to the "Schottky diode" to cause it to permanently break down.

　　2. Ultraviolet Erasable Programmable Read-Only Memory (EPROM): A computer storage chip that retains data after power is removed - that is, non-volatile. It is a group of floating-gate transistors that are individually programmed by an electronic device that provides a higher voltage than is commonly used in electronic circuits. Once programmed, EPROMs can only be erased by strong ultraviolet light. EPROMs are easily identified by the transparent window on the top of the package that allows the silicon chip to be seen, and this window is also used for ultraviolet erasure.

　　3. Electrically Erasable Programmable Read-Only Memory (EEPROM): A memory chip that does not lose data after power failure. EEPROM can erase existing information and reprogram it on a computer or dedicated device. It is generally used in plug-and-play applications.