Page 398 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

12.8 Summary 375

landing gear, and cockpit to control the plane. The distributed computer system saved
wire and thus weight. Offices are using personal computers so that each person has a
microcomputer dedicated to his or her work rather than time-sharing a large computer.
Small jobs or small parts of a larger job should be put on small computers.
The main criterion for selecting a microprocessor (within a family such as the
Motorola family described here) is the size of the program stored in it. The
microcomputer should be able to store the programs and data, with a little to spare to
allow for correcting errors or adding features. That is, as the program and data approach
the maximum size of the microprocessor, the cost of programming rises very sharply,
because squeezing a few extra instructions in will require moving subroutines around and
cause errors to propagate as assumptions are forgotten and violated. The 6805 and its
successor, the 6808, are the best choices when the program size is about 4K bytes. The
6811, and its successor, the 6812, are better when the size is above that but less than
about 32K bytes. Generally the successors, the 6808 and 6812, should be used on new
designs, but the older 6805 and 6811 may be less costly and fully adequate. The 68300
series, 500 series, and MCORE are the best choices when the program size exceeds 64K
bytes,
Other criteria include the requirements for I/O and speed. All the microcontrollers
have some peripherals in the MPU chip. If the application needs more than the chip has,
the advantage of that chip relative to memory size may be overshadowed by the extra cost
of peripherals needed for the application. Speed can be a factor. Especially in
communication systems and control of electronic systems, the fastest microprocessor
may be needed. However, speed is often overrated. In most systems having I/O, the
microprocessor will spend much, if not most, of its time waiting for I/O. The faster
microprocessor will spend more time waiting. If you can select faster I/O (such as a
Winchester disk in place of a floppy disk) the overall performance of the system will be
much better than if you spend a great deal more money on the microprocessor.
A final and often overwhelming criterion is available software. Your company may
have been using the 6805 for years and may therefore have millions of lines of code for
it. This may force you to select the 6805 or the upward compatible 6808, even though
the 6811 or 6812 may be indicated due to memory size, I/O, or speed requirements.
Often, the availability of operating systems and high-level languages selects the
microprocessor. The Z80 microprocessor from Zilog and the 8080-8085 microprocessors
from Intel run the popular Microsoft operating system, which will support a very wide
range of languages and other programs for business data processing.
This section has pointed to the need to consider different microprocessors. You
should be able to select a microprocessor for an application and defend your selection.
You should extend your understanding and appreciation of microprocessors made by other
manufacturers.

12.8. Summary

This chapter has examined other microcomputers related to the 6812. There are smaller
microcomputers, such as the 6805 and the 6811, that are ideal for controlling appliances
and small systems; and there are larger microcomputers, such as the 68300 series, that