;****************** Constant Definitions ******************
symbol	control	= outpinsA		;control lines to the AY chip
symbol	BC1		= A.0			;A.0 is BC1 on the AY chip
symbol	BDIR		= A.1			;A.1 is BDIR on the AY chip
symbol	bus		= outpinsB		;data/address bus to AY chip

symbol	AYpulse	= 4			;2.5 uSec for pulsout

;AY-8910 control codes and register addresses
symbol	inactive	= %00000000	
symbol	latchAddr	= %00000011		
symbol	Reg7		= %00000111	;Register 7 - data direction
symbol	Reg14		= %00001110	;Register 14 - Port IOB
symbol	IOAOut	= %11110111	;enable IOA and noise on A

;****************** Variable Definitions ******************

symbol  	value		= b0  ;register number (0-15)
symbol	register	= b1	;value to store in register
symbol	i		= b2	;index

;********************** Main Program **********************
pgm:
setfreq em64			;64 MHz external resonator
  dirsA = %00001111		;A.0-A.3 are outputs
  dirsB = %11111111		;B.0-B.7 are outputs
  dirsC 	= %00000010		;C.1 is AY clock
  
pwmout C.1, 7, 16			;start AY clock at 2 MHz
gosub init				;set up the AY chip 
 
  register = 8			;make Noise fixed on
  value = %00001111
  gosub store
  
main:
  
for i = 31 to 0 step -1
  
  register = 6			;Noise Frequency
  value = i
  gosub store
  
pause 800
  
next i 
  
					;endless loop to play A 440
goto main   
end	
		
;************* Initialize AY on power-up *************
init:
  control = inactive			;clear BC1, BDIR

  ;set up the AY Port IOA for output
  control = latchAddr			;latch address to register 7
  bus = Reg7								
  control = inactive
  bus = IOAOut				;enable IOA and Tone A
  pulsout BDIR,AYpulse		
  
  control = latchAddr			;latch address to register 15
  bus = Reg14								
  control = inactive
  bus = $FF					;turn all gates off
  pulsout BDIR,AYPulse			
   return

;*************** Send Value to AY Register **************
;Pass in register and value at entry

store:
  control = inactive			;clear BC1, BDIR
  control = latchAddr			;latch address of register
  bus = register								
  control = inactive
  bus = value				;get value and
  pulsout BDIR,AYPulse			;store it
return