Page 142 - Programming Microcontrollers in C

Microcontroller Memory 127

communications is a common example of asynchronous data transfer.
Here, an input line can have two states: mark and space. A line is
held at the mark state whenever no data are being transferred. When
data are to be transferred, the data line is transitioned to the space
state and held there for a specified time. This period is called the
start bit. From that time onward, the data bits are placed on the line
a bit at a time so that at the specified time intervals the receiving
device can examine the data line and determine the bit sequence.
Asynchronous operation means that there is no computer clock-
related specification as to the time that events will occur. Another
example of asynchronous transfer occurs within the computer.
Generally, events and data transfers that are initiated by interrupts
are considered to be asynchronous. Most of the peripheral devices
that are found on Motorola microcontrollers will allow either
synchronous or asynchronous notification of the program that the
peripheral business is completed.
The following sections contain brief discussions of microcontroller
memory and several of the standard peripherals found on these devices.
These discussions are intended to be qualitative and provide a broad
overview of these parts of the microcontroller. Detailed descriptions
of how to access and use these several peripherals will be found in
later chapters.

Microcontroller Memory

In a microcontroller, the program instructions are usually stored
in a memory type called read-only memory (ROM). ROM is usually
programmed by a special mask during the manufacture of the
microcontroller and is called masked ROM. ROM is the least expensive
means of storing a program in a microcontroller, especially for high-
volume manufacturing.
There are at least two means for the end user of the microcontroller
to place the program memory into the chip. The first is called erasable
programmable read-only memory (EPROM). EPROM is a memory
technology that can be erased by exposing it to high-energy ultraviolet
light. The EPROM requires the application of a high voltage to be
programmed. The memory can be programmed with either a
development system or a special programming board designed
specifically to program the microcontroller.