Page 303 - Programming Microcontrollers in C

288 Chapter 6 Large Microcontrollers

status of the machine before the normal interrupt operations can pro­
ceed. We will see a few other differences between these devices, but
the main difference is in the way peripherals are handled.
On the MC68HC16, the core processor is called the CPU16. This
central computer is interfaced to an internal bus called the
inter-modual bus (IMB). The IMB is very much like the bus a hard­
ware designer would put onto an external circuit board. It has address
and data busses and all of the necessary control signals to control any
peripheral that the microcontroller might have applied. In the stan­
dard device, the MC68HC16Z1, there are five built-in peripherals:
the system integration module (SIM), the analog-to-digital converter
(ADC), the queued serial peripheral interface module (QSPI), the
general-purpose timer module (GPT), and the static random access
memory module (SRAM). These modules and others can be added or
deleted in future components as the customer needs dictate.
The several internal peripheral modules are each interfaced to the
IMB. Each module has a specific set of registers that are located at an
address relative to the base address of the module. These base ad­
dresses are set at design time. This is interesting because the same
modules are used on the MC68300 series of microcontrollers, which
are 32-bit microcontrollers. Therefore, the material presented in this
chapter is directly applicable to the MC68300 series of microcontrollers.
The only difference that the programmer will see is that the base
memory locations of the various peripheral modules are different, but
even these differences disappear because of the careful design of the
microcontrollers. We therefore can consider this chapter to be on large
microcontrollers rather than on the 16-bit systems alone.

The MC68HC16

The MC68HC16 is a truly complicated device. However, for our
purposes, it can be divided into its several components, and each
component is somewhat as one would expect from a programming
standpoint. Therefore, we will examine this family of parts as a col­
lection of modules. Each module is rather straightforward. The core
processor, which is known as the CPU16, is indeed a complete and
competent microcomputer. The bulk of its complexity is hidden by
the fact that the program is written in C. This section contains a brief
description of the MC68HC16Z1 microcontroller that can be used to