di2561.txt
;***************************************************************************************************
; LISTING 1 - ASSEMBLY CODE FOR SOUND GENERATION
;
; "Add harmony to your system," EDN, July 20, 2000, pg 150
; 
; http://www.ednmag.com/ednmag/reg/2000/072000/designideas.htm#15di6
;***************************************************************************************************

;***************************************
; Setup
;***************************************
  ldi  r_cnt1,first_note  ; load counter1 register with first_note value
  ldi  r_cnt2,second_note ; load counter1 register with second_note value
  ldi  r_vol,volume       ; load volume register with a value (0-80hex)
  ldi  r_lout,1           ; load output register with low value
  ldi  r_hout,2           ; load output register with high value

;***************************************
; Square Wave Generator
;***************************************
line1:
  dec  r_cnt1             ; decrement counter1 register
  breq line5              ; branch if result of decrement equals zero
  nop                     ; no operation (waste one instruction time)
  jmp  line7              ; jump to line7
line5:
  ldi  r_cnt1,first_note  ; load counter1 register with first_note value
  eor  r_vol1,r_vol       ; flip state of first_note intensity (0 or volume)

line7:
  dec  r_cnt2             ; decrement counter2 register
  breq line11             ; branch to line 11 if result of decrement equals zero
  nop                     ; no operation (waste one instruction time)
  jmp  line13             ; jump to line13
line11:
  ldi  r_cnt2,second_note ; load counter1 register with second_note value
  eor  r_vol2,r_vol       ; flip state of second_note intensity (0 or volume)

;***************************************
; NCO Summing
;***************************************
line13:
  add  r_acc,r_vol1       ; add first_note intensity to accumulator register
  brcs  line20            ; branch if result of add overflows
  add  r_acc,r_vol2       ; add second_note intensity to accumulator register
  brcs  line22            ; branch if result of add overflows
  nop                     ; no operation (waste one instruction time)
  out  portx,r_lout       ; output a low level to enunciator
  jmp  line1              ; jump to line1
line20:
  nop                     ; no operation (waste one instruction time)
  adc  r_acc,r_vol2       ; add second_note int. and overflow bit to accumulator
line22:
  out  portx,r_hout       ; output a high level to enunciator
  jmp  line1              ; jump to line1