Programmable ROM

A programmable read-only memory (PROM) is a form of digital memory where the contents are set after the device is manufactured. Once set, the contents are then permanent. A PROM is one type of read-only memory (ROM). PROMs are usually used in digital electronic devices to store low level programs such as firmware or microcode. PROMs may be used during development of a system that will ultimately be converted to ROMs in a mass produced version. These types of memories are used in microcontrollers, video game consoles, mobile phones, radio-frequency identification (RFID) tags, implantable medical devices, high-definition multimedia interfaces (HDMI), and in many other consumer and automotive products.

PROMs are manufactured blank and, depending on the technology, can be programmed at the wafer, final test, or system stage. Blank PROM chips are programmed by plugging them into a device called a PROM programmer. A typical PROM device has an array of memory cells. The bipolar transistors in the cells have an emitter that is connected to a fuse called a polyfuse. To program a PROM is to strategically blow the polyfuses.