Page 436 - Programming Microcontrollers in C

A Clock Program 421

of a loop. There are four external variables, hours, seconds, minutes,
and count accessed by keep_time(). The variable count is
incremented in an interrupt service routine that is executed each
1.953125 milliseconds. The constant TIME_COUNT will have a value
of 511. When count becomes greater than TIME_COUNT, count
is reset to zero and the parameter seconds are incremented. When
seconds exceeds the value 59, seconds is reset to zero and
minutes is incremented. When minutes exceeds 59, it is reset to
zero and hours is incremented. Finally, when hours becomes
greater than 12, it is reset to 1 which corresponds to one o’clock.
Once each second, the function output_time() is executed.

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

#define TIME_COUNT 511
#define MAX_SECONDS 59
#define MAX_MINUTES MAX_SECONDS
#define MAX_HOURS 12
#define MIN_HOURS 1

/* function prototypes */
void output_time(void);
void keep_time(void);

/* external—global variables */
WORD seconds,minutes,hours,count;

/* the main applications program. count is incremented in
the isr 512 times each second. Therefore, this routine
must be executed at least once every two milliseconds. */

void keep_time(void)
{
if(count>TIME_COUNT)
{
count=0;
if(++seconds>MAX_SECONDS)
{
seconds=0;
if(++minutes>MAX_MINUTES)
{
minutes=0;
if(++hours>MAX_HOURS)
hours=MIN_HOURS;