Page 165 - Programming Microcontrollers in C

150 Chapter 4 Small 8-Bit Systems

16-bit machine and its peripheral components are nearly all different
from those found on the 8-bit machines. The M68HC16 is a superset
of the M68HC11; it will execute M68HC11 code, but the hardware
computer extensions and new peripheral components are significant.
To successfully program a microcontroller using a high-level lan­
guage, the programmer must be able to access various control and status
registers in the computer. The program must force the language to place
both program and data memory addresses in the proper locations in the
memory map. Vectors associated with interrupt service routines, and the
service routines themselves, must be handled directly by the program.
These tasks are difficult to accomplish with most high-level languages,
but C allows access to these things without extensions. However, most C
compilers for microcomputers have extensions that allow such special
features to be easily treated.
1
The compiler used in this chapter is called C6805 . It was written to
support the M68HC05 family of devices. Be forewarned: some M68HC05
microcontroller instructions have no counterpart in the standard C lan­
guage. Special directives identify unique microcontroller characteristics
to the compiler. Listed in Table 4-1 below are nine assembly instructions
available to the 68HC05. These instructions have no equivalent C call.
They can be accessed as either a single instruction (all uppercase) or as
a function call as shown. The function call requires a pair of closed
parentheses to follow the name of the instruction.

Function Operation
CLC or CLC() clear carry bit
SEC or SEC() set carry bit
CLI or CLI() clear interrupt flag (interrupts on)
SEI or SEI() set interrupt flag (interrupts off)
NOP or NOP() no operation
RSP or RSP() reset stack pointer
STOP or STOP() STOP instruction
SWI or SWI() software interrupt
WAIT or WAIT() WAIT instruction

Table 4-1: Assembly Codes Directly Callable By C6805

1 Byte Craft Limited, 421 King Street North, Waterloo, Ontario, Canada N2J 4E4