Page 302 - Programming Microcontrollers in C

Chapter 6

Large Microcontrollers

In this chapter we’ll examine the programming of systems em­
ploying large microcontrollers. The realm of the large-microcontroller
system is not all that different from the 8-bit systems. One of the
main advantages of the use of a high-level language is that it keeps
the nasty details of the underlying computer hidden from the pro­
grammer. Usually, the programmer will not see much difference
between the code for different types of computers. The fallacy to this
idea is that when programming any microcontroller, the program­
mer must know about and use all of the on-board peripherals found
on the microcontroller. These peripherals will vary from machine to
machine, and how they are accessed will differ from device to de­
vice. In this chapter, however, we are going to see an application
where a substantial amount of assembly language is required. The
chip that we are going to use here has a Digital Signal Processor
section. This processor is accessed through special core chip regis­
ters and the core condition code register. The abstract machine that
the compiler creates code for contains no registers. Therefore, the
only access to these features is through assembly language. We will
show how to create assembly language functions that can be accessed
from your C program.
The part that we will use for the 16-bit discussions is the Motorola
MC68HC16 family of components. These are similar to the MC68HC11
in many ways, but there are important differences. First, there are some
new registers that must be programmed directly to make the
MC68HC16 work as desired. (The only register in the MC68HC11
that must receive special assembly instructions is the condition code
register.) Also, the MC68HC16 does not have automatic stacking of its
registers when an exception occurs, unlike the MC68HC11. There­
fore, all interrupt service routines must begin with code that saves the

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