Page 307 - Programming Microcontrollers in C

292 Chapter 6 Large Microcontrollers

tor number. When programming the interrupt vectors in the several
peripheral modules of the MC68HC16, the programmer will select
the interrupt vector. When it comes time to place the interrupt ser­
vice routine address in the proper memory location, it is to the
vector address—NOT the vector number—that this value must be
assigned.
When setting up the vector table, a wise programmer will fill all
of the possible unused vector addresses with the address of a dummy
function that provides an orderly return to the program in the event
of an unexpected interrupt. Usually the first 0x18 or 24 vectors should
be filled with this address. An example function that can be used for
this type of operation is
static @port void _init_vector(void)
{ }
This program will compile to a single RTI (return from interrupt)
that will return the program control to the location when the interrupt
occurred. A static function is not used often. When a function is
declared static, it can be seen only in the file in which it is defined.
Following is a listing of the routine vector.c. This program
is modeled closely after that provided with the Cosmic MC68HC16
C compiler.

extern@far@portvoid_stext(void);/*startup routine*/
extern @port void OC3_Isr(void); /* ISR address */
static @port void _init_vector(void);

static const struct reset {
@far @port void (*rst)(void); /* reset + code
extension */
unsigned short isp; /* initial stack pointer */
unsigned short dpp; /* direct page pointer */
@port void (*vector[252])(void); /* interrupt vectors */
}_reset = {
_stext, /* 1-start address */
0x03fe, /* 2-stack pointer */
0x0000, /* 3-page pointer */
_init_vector, /* 4-Breakpoint */
_init_vector, /* 5-Bus Error */