Page 397 - Introduction to Microcontrollers Architecture, Programming, and Interfacing of The Motorola 68HC12

374 Chapter 12 Other Microcontrollers

run, and DOZE causes the processor to enter low-power doze mode in which some
peripherals continue to ran. STOP causes the processor to enter low-power stop mode.
The MMC2001, a first implementation of the M-CORE family, uses 40 milliamps at
3.3 volts. Both wait and doze modes have current drain of 3 milliamps, and the stop
mode has current drain of only 60 microamps. SYNC causes the processor to suspend,
fetching new instructions until all previously fetched instructions complete execution.
To conclude this section, in Figure 12.14, we illustrate the overused inner product
subroutine again, passing the parameters in registers by value. Upon input, GPR rl has
v[0], r2 has v[l], r3 has w[0], and r4 has w[l], and upon exit, rl contains the result,

12.7 Selecting a Microcontroller for an Application

Suppose you are designing a product that will have a microprocessor in it. Which one
should you use? You have to look at many different alternatives, such as the ones we
looked at in this chapter and similar microcomputers made by other companies. You
should not select one with which you are very familiar, such as the 6812, or one that
you are overwhelmed with, such as the 500 series, unless you have good reason to select
it. You have to analyze the needs of the application to pick the most suitable
microcomputer. Smaller computers are less costly, and larger computers make it easier to
write large programs. However, many of the techniques are the same as those you have
already learned, passing parameters, handling local variables, writing clear programs, and
testing them. You are prepared to learn to read the 68300 series, 500 series, or M-CORE
family programs. However, the greater size and complexity of these microcomputers
requires longer to master all of their peculiarities than smaller microcomputers, to enable
you to fluently read their programs.
Generally, the larger the microprocessor, the easier it is to write large programs, The
68300 series, 500 series, and M-CORE family have more capabilities to handle high-
level languages, such as C or C++, and have an instruction set that allows assembly-
language programs to be written that can handle fairly complex operations in short
fashion (such as the LINK instruction). It is easy to say that the larger the
microprocessor, the better, and to select the largest one you can get. But consider some
other aspects.
The smaller microcomputers such as the 6805 are very inexpensive. A version of
this 6805 sells for only 50 cents. You can build a fully functioning microcomputer
using just the 6805 and a couple of resistors and capacitors. The 68300 series requires
external SRAM and ROM to make a working computer. The cost of the integrated
circuits, the printed circuit board, and the testing needed to get the board working make
the 68300 series system more than an order of magnitude more expensive than the 6805
system. An M-CORE microcontroller, running with a 32 MHz clock, can require more
than a two-layer printed circuit board. Multilayer boards are significantly more expensive
than one- or two-layer boards. This can make a big difference to the cost of your product,
especially if you intend to make thousands of copies of the product.
The trend toward networking should be observed. If you divide your problem in half,
each half may fit on a smaller microcomputer. We once read a news article that claimed
the Boeing 767 jet had over a 1000 microcomputers scattered throughout the wing tip,