Page 143 - Programming Microcontrollers in C

128 Chapter 3 What Are Microcontrollers?

Packages that contain EPROM have a quartz glass window
through which the ultraviolet light can pass with minimum attenuation.
These packages are quite expensive, and therefore, microcontrollers
with EPROM are usually too expensive to use. EPROM was used for
development purposes in the past, but it is just too expensive in light
of more recent developments to be used for that purpose today.
For limited production purposes, a less expensive version of the
EPROM chip is available. This is the one time programmable (OTP)
chip. An OTP chip has the exact same silicon component as an
EPROM, but it is packaged in a standard plastic package. This package
is much less expensive than the windowed package discussed
previously. However, once the chip has been programmed, the
program contents cannot be changed.
There is yet another means of storing programs or, in some
instances, data in a microcontroller. This technique is called
electrically erasable programmable read-only memory (EEPROM).
EEPROM is programmable from instructions within the
microcontroller. EEPROM also requires a high programming voltage.
If there are large blocks of EEPROM on the chip, the programming
voltage is usually applied during the programming cycle through a
pin connected to an external voltage source. In cases where the amount
of EEPROM to be programmed is relatively small, a charge pump on
the microcontroller chip will allow the EEPROM to be programmed
with no externally applied voltage. The amount of EEPROM that
can be programmed with an on-board charge pump is usually so
small that it is not useful for storing program instructions. But on-
board EEPROM can be quite useful in the storage of data generated
by the program that must be saved through a power-down cycle.
Sometimes in the execution of the program, some data are generated
that must be saved for later use. These data are called volatile data or
variables, and are usually stored in random access memory (RAM).
Careful design of a program will usually result in the need for much
less RAM than ROM. In most microcontrollers, the amount of RAM
is usually 60 to at most a few hundred bytes. The amount of ROM,
EPROM or EEPROM usually runs from 1000 bytes upwards to a
few tens of thousands of bytes.
EEPROM is quite expensive, and has been replaced by a newer
technology called FLASH memory. FLASH programs in a manner