M68HC05 Family — Understanding Small Microcontrollers

Freescale Semiconductor, Inc...
What is a Microcontroller?
Computer System Outputs
Central Processor Unit (CPU)
Freescale Semiconductor, Inc.
Output devices are used to communicate information or actions from the
computer system to the outside world. In a personal computer system,
the most common output device is the CRT (cathode ray tube) display.
Microcontroller systems often use much simpler output devices such as
individual indicator lamps or beepers.
Translation circuits (sometimes built into the same integrated circuit as
the microcomputer) can convert digital signals into analog voltage levels.
If necessary, other circuits can translate VDD and VSS levels that are
native to an MCU into other voltage levels.
The “controller” in microcontroller comes from the fact that these small
computer systems usually control something as compared to a personal
computer that usually processes information. In the case of the personal
computer, most output is information (either displayed on a CRT screen
or printed on paper). In contrast, in a microcontroller system, most
outputs are logic level digital signals that are used to drive display LEDs
(light-emitting diodes) or electrical devices such as relays or motors.
The CPU is at the center of every computer system. The job of the CPU
is to obediently execute the program of instructions that was supplied by
the programmer. A computer program instructs the CPU to read
information from inputs, to read information from and write information to
working memory, and to write information to outputs. Some program
instructions involve simple decisions that cause the program to either
continue with the next instruction or to skip to a new place in the
program. In a later chapter, we will look closely at the set of available
instructions for a particular microcontroller.
In mainframe and personal computers, there are actual layers of
programs, starting with internal programs, that control the most basic
operations of the computer. Another layer includes user programs that
are loaded into the computer system memory when they are about to be
M68HC05 Family — Understanding Small Microcontrollers — Rev. 2.0
20 What is a Microcontroller?
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