di2500.txt
;**************************************************************************************************
; LISTING 1 - ROUTINE FOR GENERATING ARBITRARY FREQUENCIES
; 
; "Generate frequencies with arbitrary relationships," EDN, March 16, 2000, pg 152
;
; http://www.ednmag.com/ednmag/reg/2000/031600/designideas.htm#06di6
;***************************************************************************************************

C1 EQU 40   ; Count for 500Hz
C2 EQU 28   ; Count for 700Hz
C3 EQU 20   ; Count for 1000Hz
C4 EQU 19   ; Count for 1050Hz
C5 EQU 18   ; Count for 1100Hz
C6 EQU 14   ; Count for 1500Hz
C7 EQU 12   ; Count for 1700Hz
C8 EQU 10   ; Count for 2000Hz

F1 EQU 21   ; Internal RAM locations for storing the 
F2 EQU 22   ; above count values
F3 EQU 23
F4 EQU 24
F5 EQU 25
F6 EQU 26
F7 EQU 27
F8 EQU 28

org 0
ajmp main;

;----------Timer T0 service routine --------------

org 00BH           ; Timer 0 to service vector

cpl p3.4
djnz F1,CF2 ; 24 osc. periods
cpl p1.7    ; 12 osc. periods
mov F1,#C1  ; 24 osc. periods; 
	    ; Total Osc.cycles = 60 * 8 + 12 = 492 
            ; with 24MHz crystal this amounts to 
            ; 20.5 microseconds

CF2:
djnz F2,CF3 ; decrement counter for freq. f2
cpl p1.6    ; if it reduces to 0 then complement the 
mov F2,#C2  ; logic on the corresponding pin and reload 
            ; the count value

CF3:
djnz F3,CF4
cpl p1.5
mov F3,#C3

CF4:
djnz F4,CF5
cpl p1.4
mov F4,#C4

CF5:
djnz F5,CF6
cpl p1.3
mov F5,#C5

CF6:
djnz F6,CF7
cpl p1.2
mov F6,#C6

CF7:
djnz F7,CF8
cpl p1.1
mov F7,#C7

CF8:
djnz F8,exit
cpl p1.0
mov F8,#C8 

exit:
reti        ; End of timer interrupt routine

main:
call init;

LOOP: JMP LOOP;


;------ Initialization Routine -----------
init:
        mov F1,#C1	;  Initialize count values
        mov F2,#C2
        mov F3,#C3
        mov F4,#C4
        mov F5,#C5
        mov F6,#C6
        mov F7,#C7
        mov F8,#C8

; The following code sets timer T0 to 25 microseconds
; and enables the timer intterupt
 
        mov TMOD, #02H   ; set timer mode to mode 2
        mov tl0,#0ceh    ; (8-bit auto-reload mode)
        mov th0,#0ceh    ; set reload value 
        setb ET0         ; enable timer interrupt
        setb EA          ; master interrupt enable
        setb TR0         ; start timer
ret

end