di2954.txt
;***************************************************************************************
;
; LISTING 1 - DELAY-LOOP ROUTINE
;
; "Maintain precise timing with PC's speaker logic," EDN, July 11, 2002, pg 106
;
;***************************************************************************************

/**** 	Program Generates Precise Delays 	   ****/
/**** 	Written By: D.S.Oberoi & Harinder Dhingra  ****/

#include <conio.h>
#include <dos.h>
#define counter_count_time 0.0008380958  /* Counter2's single count period(mS) */
#define read_port 0x378	  		 /* Address of Read Port 	       */

unsigned counter_data, count_elapsed, required_count;
float desired_delay_time;

void delay_loop(unsigned delay_count);
main()
{
	desired_delay_time=0.9;	     /* Delay Time in milli second */
    /* Find how many  counts are required for the  desired delay */
	required_count = desired_delay_time/counter_count_time;
	for (;;)
	{
	  ; /*  Place the desired statements here, before delay */
	  ;

	  delay_loop(required_count); /* Call The Delay Routine */

	  ; /*  Place the desired processing statements here, after delay */
	  ;
	}
}

/* Delay Routine starts here */
void delay_loop(unsigned delay_count)
{
	count_elapsed = 0;
	asm mov al, 0b4h     /* Set the counter2 operation */
	asm out 043h,al
	asm mov al, 0ffh     /* Load the intial count as ffffh 	 */
	asm out 042h, al     /* completed in two cycles 	 */
	asm out 042h,al
	asm mov al, 01h	     /* Enable the  counter and Start the ON period */
	asm out 061h, al

/* Counter data is read till,  desired counts are not  obtained 	 */
	while (count_elapsed < required_count)
	{
	    asm in al, 042h          /*  Get LSB       */
	    asm mov cl, al
	    asm in al, 042h	     /*  Get the MSB    */
	    asm mov ah, al
	    asm mov al, cl   	     /* Two bytes, combined to get 16 bit data*/
	    asm mov counter_data , ax
	    count_elapsed = (0xffff-  counter_data);  /* Calculate the elapsed counts  */
	}
/* Disable the counter2 */
	    asm mov al, 00h
	    asm out 061h, al
}