.file "tailA.asm"
;;
;; ATmega8 taillight circuit
;; 
;; Last edited:	12-7-2004
;; Base on 'tailA.c'
;; Purpose:	An assembly to strobe the LEDsvia Port D.
;;		  The strobe rate is to be set by adjusting the voltage  
;;		  drop over a potentiometer that is sampled by an ADC
;;
;; A compared C code is followed by "--> " and before the assembly

;; List of all registers are defined in this code
;;------------------------------------------------;;
;; R18 --- 0xFF ---> DDRD
;; R19 --- ADCH & OCR1AL
;; R20 --- ADCH & OCR1AH
;; R21 --- TIFR
;; R22 --- ADMUX
;; R23 --- ADCSRA
;; R25 --- Pattern
;; R26 --- 0x00 ---> TCCR1A
;; R27 --- TCCR1B
;;------------------------------------------------;;

	;; Access the register and register bit definitions
	.include "avr-mega8.asm"

	;; The variable 'pattern' is kept in register 25
	;; Bit set means LED on (complemented before output)
	;; --> unsigned char pattern = 0x00;
	LDI 	R25,0x00	 	;;Load immediate

	;; Set up ADMUX
	;; --> ADMUX = (1 << REFS0) | (1 << ADLAR);
	LDI 	R22,0x60	 	;;Load immediate
	OUT 	ADMUX,R22 	 	;;Out to I/O location

	;; Set up ADCSRA
	;; --> ADCSRA = (1 << ADEN) | 0x07;
	LDI 	R23,0x87	 	;;Load immediate
	OUT 	ADCSRA,R23 	 	;;Out to I/O location

	;; Set up timer 1A control registers
	;; --> TCCR1A = 0;
	LDI 	R26,0x00	 	;;Load immediate
	OUT 	TCCR1A,R26 	 	;;Out to I/O location

	;; Set up timer 1C : divide by 256 prescaler using OCR1A
	;; Frequency of operation is about 61 Hz
	;; --> TCCR1B = 0x0B;
	LDI 	R27, 0x0C	 	;;Load immediate
	OUT 	TCCR1B,R27 	 	;;Out to I/O location

	;; Set up Port D for OUTput
	;; --> DDRD = 0xFF;
	SER 	R18	  	 		;;Set all Bits in Register
	OUT 	DDRD, R18 	 	;;Out to I/O location

loop:
	;; Update LEDs
	;; --> PORTD = pattern;
	COM     R25				;;One's complement
	OUT 	PORTD,R25 	 	;;Out to I/O location
	
	;; Set up ADCSRA start a conversion
	;; --> ADCSRA |= (1 << ADSC);
	SBI 	ADCSRA,6	 	;;Set Bit in I/O Register

	;; Update OCR1A from ADC
	;; OCR1AH  is  0x2B
	;; --> OCR1AH = (ADCH >> 4); 
	IN  	R20,ADCH 	 	;;In from I/O location
	SWAP	R20				;;Swap Nibbles
	ANDI 	R20,0x0F 	 	;;Logical AND with Immediate
	OUT     0x2B,R20 	 	;;Out to I/O location
	;; OCR1AA  is  0x2A
	;; --> OCR1AL = (0x01 | ADCH) << 4; 
	IN  	R19,ADCH 	 	;;In from I/O location
	ORI 	R19,0x0F 	 	;;Logical OR with Immediate
	SWAP	R19				;;Swap Nibbles
	ANDI    R19,0xF0 	 	;;Logical AND with Immediate
	OUT     0x2A,R19 	 	;;Out to I/O location

checktime:
	;; Test OCF1A flag in TIFR
	;; --> while (!(TIFR & (1 << OCF1A)));
	IN  	R21,TIFR	 	;;In from I/O location
	ANDI	R21,0x10	 	;;Logical AND with Immediate
	BREQ	checktime	 	;;Branch if Equal

	;; OCF1A flag bit is set, so reset it (write 1 to it)
	;; --> TIFR |= (1 << OCF1A);
	ORI 	R21, 0x10 	 	;;Logical OR with Immediate
	OUT 	TIFR, R21 	 	;;Out to I/O location
	;; Continue forever
	RJMP	loop		 	;;Relative jump