Page 442 - Programming Microcontrollers in C

A Clock Program 427

/* in this isr, all that must be done is to turn off the
pit interrupt request bit and increment the global
parameter count */

void pit_isr(void)
{
ITCSR.ITIF=ON; /* ON clears the interrupt bit in these
registers */
count++;
}
The interrupt handler takes care of restoring the status prior to
returning to the interrupted program.
The next logical function that we will need is the function
main(). Main is broken into two parts, the initialization portion of
the program and the applications portion of the program. The first
three instructions are the initialization portion of the program. Here,
the serial I/O system is initialized and the baud rate is set to the value
BAUD_RATE. The pit_init() routine written above is executed
and then the system fast interrupts are enabled with the assembly
language macro function defined in the header file mmc2001.h.
#define BAUD_RATE 38400
#define FAST_AUTOVECTOR 0x3000002c

main()
{
inituart(BAUD_RATE); /* initialize the UART */
pit_init(); /* initialize the PIT */
vector(handler,FAST_AUTOVECTOR); /* put handler address
in FAST_AUTOVECTOR. */
Enable_Fast_Interrupts();
FOREVER
{
keep_time(); /* keep track of the time */
if(kbhit())
reset_time();
}
}
The application portion of the program is quite simple. It is simply
a FOREVER loop that repeatedly executes keep_time(). Also in
the loop, a test is made to determine if a key has been touched on the
terminal attached to the serial port. If it has, the function
reset_time() is executed where the clock time can be set.