I can use the display as above, but it is rather difficult. I have to remember to "refresh" the digits all the time my program is running, otherwise the user will notice them flicker or even disappear. If I was serious about working this way I would ensure that whenever I am waiting for a user input (for example in the key function we wrote earlier) I call the display refresh to keep it going.

What I would really like is an automatic way of calling a "display refresh" function every now and then.

The code on the right re-arranges the original so that the refresh is taken away from the drawing action. Each time the refresh function is called it will re-draw the segment pattern for one of the digits.

Note that I am using an array to hold the actual segments which are to be displayed on the LEDs, when I want to display a pattern I load it into the array and then let the refresh take care of the rest.

/* segment patterns for our LEDs */
unsigned char
          segments [DISPLAY_SIZE] ;

The refresh function also updates the display of only one of the LEDs. It uses the variable led_counter to keep track of the LED it is to work on. If I want to update all of the LEDs I have to call the refresh function four times. If you run Exercise 4.3 you will see the same nice, solid display.

/* number of LEDs in our display*/
#define DISPLAY_SIZE 4

/* segment patterns for our LEDs */
unsigned char
            segments[DISPLAY_SIZE];

/* counter used by our refresh  */
unsigned char led_counter = 0 ;

/* each time refresh is called it*/
/* sets the display for a led and*/
/* moves on to the next          */
void refresh ( void )
{
  /* turn off all the LEDs     */
PORTA = 0 ;
  /* set segments for the led  */
PORTB = segments [led_counter] ;
  /* turn the led on           */
PORTA = enable [led_counter ] ;
  /* move on to the next led   */
  led_counter = led_counter + 1 ;
  /* see if we fell off the end */
    if ( led_counter==DISPLAY_SIZE )
  {
    led_counter = 0 ;
  }
}

/* display just loads the pattern*/
/* into the segment. refresh will*/
/* read it later                 */
void display ( unsigned char digit,
               unsigned char pos )
{
  segments [pos] = patterns [digit];
}

void main ( void )
{

  setup_hardware () ;

  /*  display the values  */

  display ( 0, 0 ) ;
  display ( 1, 1 ) ;
  display ( 2, 2 ) ;
  display ( 3, 3 ) ;

  while (1)
  {
    /* refresh the display */
    refresh () ;
    /* delay                */
    delay ( 10 ) ;
  }
}