;
; hello.ftdi.echo.44.asm
;
; 115200 baud FTDI character echo
;
; Neil Gershenfeld
; 10/17/10
;
; (c) Massachusetts Institute of Technology 2010
; Permission granted for experimental and personal use;
; license for commercial sale available from MIT.
;

.include "tn44def.inc"

.def bitcnt = R16; bit counter
.def temp = R17; temporary storage
.def temp1 = R18; temporary storage
.def ah = r19; adch
.def al = r20; adcl

.org 0
rjmp reset

;
; main
;
reset:
   ;
   ; set fuse low byte to 0x7E for 20 MHz resonator
   ;
   ; set clock divider to /1
   ;
   ldi temp, (1 << CLKPCE)
   ldi temp1, (0 << CLKPS3) | (0 << CLKPS2) | (0 << CLKPS1) | (0 << CLKPS0)
   out CLKPR, temp
   out CLKPR, temp1
   ;
   ; set stack pointer to top of RAM
   ;
   ldi temp, high(RAMEND)
   out SPH, temp
   ldi temp, low(RAMEND)
   out SPL, temp
   ;
   ; start main loop
   ;
	sbi PORTA, 2	; enable pullup on port A, pin 2
	sbi DDRA, 7	; port A, pin 7 output
	ldi temp, 0x00
	out prr, temp

	ldi temp, 0x02
	out admux, temp
	ldi temp, 0b11100111
	out adcsra, temp

	loop:

	in al, adcl			; adc low byte
	in ah, adch
	
	; shift ADC output right 2 bits (1024 to 256, divide by 4)
	clc
	ror ah
	ror al
	clc
	ror ah
	ror al
	subi al, 0x02
	pwm:
		mov bitcnt, al	; positive duty cycle
		mov temp, al
		com temp	; negative duty cycle
		sbi porta, 7
		pause: dec bitcnt
			brne pause
		cbi porta, 7
		pause2: dec temp
			brne pause2
	      rjmp loop