Page 304 - Programming Microcontrollers in C

The MC68HC16 289

aid in writing programs for the component in C. There are several
Motorola manuals that are useful adjuncts to this chapter. 1 2 3 4 5 6
Copies of these manuals are all to be found on the attached CD­
ROM. It is recommended that these manuals be reviewed prior to
any attempt to write code for this family.

CPU16 Core Processor

The CPU16 represents an attempt to bridge the difference be­
tween the large 8-bit microcontrollers and the high-end components
embodied in the MC68300 family. This processor is a 16-bit proces­
sor. As such, its instructions are each 16 bits wide, instructions are
read from memory 16 bits at time, and the instructions are processed
16 bits at a time. On this particular part, a 20-bit address bus with
additional control allows the program to access two individual one-
megabyte address maps. The controls available distinguish between
program memory and data memory.
It has been stated several times that the programmer’s model for
a microcontroller is not too important when writing code in C. To be
able to access all of the features of a CPU16, you will have to use
assembly language-based functions. There are certain features that
are simply outside the concept of a high-level language. In these
cases, it is recommended that functions which access and control
these features be written and then you can call these functions from
the C program. The main properties of the CPU16 that are not avail­
able to the C programmer are the DSP type registers found in the
part. Programming of these registers is the subject of a later section
of this chapter.
The 20-bit address space of the CPU16 is a significant deviation
from the normal 16-bit address space of the MC68HC11. This change
has been handled by the addition of several 4-bit extension registers
for those registers that deal specifically with addresses. These regis­
ters are the program counter (PC), the stack pointer (SP), the three

1 M68HC16 Family MC68HC16Z1 Users Manual MC68HC16Z1UM/AD
2 M68HC16 Family CPU16 Central Processor Unit Reference Manual CPU16RM/AD
3 Modular Microcontroller Family GPT General Purpose Timer Reference Manual GPTRM/AD
4 Modular Microcontroller Family ADC Analog-to-Digital Converter Reference Manual ADCRM/AD
5 Modular Microcontroller Family QSM Queued Serial Module Reference Manual QSMRM/AD
6 Modular Microcontroller Family SIM System Integration Module Reference Manual SIMRM/AD