Page 416 - Programming Microcontrollers in C

Delays Revisited 401

Delays Revisited

With semaphores available, we can improve on the delay function
by removing the wait loop from within the function to the calling
function, where its use can be dictated by the program. In this case,
the calling function will create a semaphore and send the semaphore
to the delay function. The proposed delay function is

include “mmc2001.h”
#include “timer.h”
#include “intctl.h”

/***********************************************************
Delay by t milliseconds. This delay controls a semaphore
that will be examined by the calling program. When the
semaphore is released, the delay is completed. It is
assumed that fast interrupts are enabled when this func­
tion is called and that the interrupt handler has been
appropriately set up.
*****************************************************************/

static int sem; /* this variable needs to be file global */

void delay(WORD t, int semaphore)
{
long count;

sem=semaphore; /* save semaphore so isr can see it */
ITCSR.EN = ON; /* enable the pit */
ITCSR.OVW=ON; /* enable write through */
/* an interrupt will occur in t milliseconds */
count =(long)t*1000; /* make the delay in us */
ITDR =count/122; /* 122 us per tick */
ITCSR.ITIE = ON; /* enable pit interrupt */
FIER.EF8=ON; /* enable the fast interrupts */
} /* return to the calling program, we are done here */

void pit_isr(void) /* interrupt occurs when delay time expires */
{
ITCSR.ITIF = ON; /* turn interrupt flag off */
ITCSR.ITIE = OFF; /* disable the PIT interrupt */
ITCSR.EN = OFF; /* disable the PIT */
FIER.EF8 = OFF; /* disable the pit fast interrupt */
release_semaphore(sem); /* release semaphore to calling program */
}
Listing 8-3: Alternate Delay Routine