Programming Embedded Systems in C and C++

60 Chapter 6: Memory

word at a time, by applying an electrical charge to the input pins of the chip.

Once a PROM has been programmed in this way, its contents can never be

changed. If the code or data stored in the PROM must be changed, the current

device must be discarded. As a result, PROMs are also known as one-time pro-

grammable (OTP) devices.

An EPROM (erasable-and-programmable ROM) is programmed in exactly the same

manner as a PROM. However, EPROMs can be erased and reprogrammed repeat-

edly. To erase an EPROM, you simply expose the device to a strong source of

ultraviolet light. (There is a “window” in the top of the device to let the ultraviolet

light reach the silicon.) By doing this, you essentially reset the entire chip to its ini-

tial—unprogrammed—state. Though more expensive than PROMs, their ability to

be reprogrammed makes EPROMs an essential part of the software development

and testing process.

Hybrid Types

As memory technology has matured in recent years, the line between RAM and

ROM devices has blurred. There are now several types of memory that combine

the best features of both. These devices do not belong to either group and can be

collectively referred to as hybrid memory devices. Hybrid memories can be read

and written as desired, like RAM, but maintain their contents without electrical

power, just like ROM. Two of the hybrid devices, EEPROM and Flash, are descen-

dants of ...