Page 234 - Programming Microcontrollers in C

Header File 219

extern void IC1_Isr(), OC3_Isr(),_stext();
void (* const vector[])()={0,0,0,0,0,0,0,OC3_Isr,0,
0,0,0,IC1_Isr, 0,0,0,0,0,0,0,_stext};
This two-line sequence identifies three functions, each of which
returns nothing. The first two are interrupt service routines that will
process interrupts from input capture 1 and output compare 3,
respectively. The third entry is the name of the entry point in the
start-up routine that is linked to the C program. The second line of
code indicates that vectoris an array of constpointers to functions
of the type void . There is one entry in this array for each interrupt
vector and the reset vector for the micro-controller. This array is
initialized with either zeros or the addresses of the interrupt service
routines in the proper locations. The address of the start-up routine is
placed in the last location in the array. This little program will be
compiled and linked to the final program. The name of the file that
contains this file is interrup.c , and it must be modified for each
program in which it is used. At link time, the address of vector[ ]
will be forced to 0xffd6 which is the beginning of the vector table
in the MC68HC11 family.
Some programmers will initialize the vector table with a known
address rather than 0. In the event that a spurious interrupt occurs and
takes the processor to an unused vector location, the processor would
certainly get lost if the vector table were filled with zeros. Placing a
known program into all unused vector locations will prevent this problem.
Another problem can occur in the operation of unattended
microcontrollers. It is possible that control of the microcontroller
could be diverted to unused ROM locations by serious noise spikes.
Such a loss of control will not be devastating if the system uses a
computer operating properly (COP) system. However, another safety
back-up that the programmer can incorporate into the program is to
fill all unused memory with the one-byte instruction swi (software
interrupt). This instruction causes the program to save the machine
status and pass control to the function addressed in the SWI vector
location. If this value contains the address of the start-up program,
the system will be restarted immediately if control is accidentally
moved to unused ROM.