; ConvertFtoC

; Version 1.0
; Date: November 11, 2014
; Written By : Yoseph Yegezu


.INCLUDE <m328pdef.inc>

.DSEG
		C: .BYTE 1
		F: .BYTE 1
			
.CSEG


.DEF Denominator=R19
.DEF Quotient=R22

.ORG 0x0000


// Input a Fahrenheit value into r17
TestConvertFtoC:
	ldi r17,124
	sts F, R17
// Call subroutine ConvertFtoC
	rcall ConvertFtoC
	rjmp TestConvertFtoC

/****************************
 * Subroutine converts a temperature reading in Fahrenheit (variable F) to Celsius (variable C).
 * (F  -  32)  x  5/9 = C
 * Range of F (32 to 255) therefore Cmax is 123.8
 ****************************/
    
ConvertFtoC:
	//Load the F value into r18
	lds r18, F
	//Input the constant value 18 in reg. 16
	ldi r16, 32
	//This part calculates (°F-32)
	sub r18, r16
	ldi r20, 5
	//This part calculates (°F-32)*5
	mul r18, r20
	//Move the products into r25H and r24L
	movw r25:r24, r1:r0
	//Input the denominator into r19
	ldi Denominator, 9
	//Call the 16 bit by 8 bit division
    rcall Div16_8
    sts  C, r22
	ret	

/************************************
 * Quotient = Numerator/Denominator *
 * r23:r22  = r25:r24 / r19         *
 ************************************/
Div16_8: 
    clr  r2
	clr  r23
	clr  Quotient          // r22

// Quotient is going to increment by 1 everytime L1 loops
// Loop  L1 stops when the numerator-denominator = less than 10(the demoninator)
// This part calculates (°F-32)*5/9
L1:
	inc Quotient
	adc r23,r2

	sub  r24,Denominator  // r19
	sbc  r25,r2

	brcc L1
// Since the quotient is incremented by 1 when the loop began, after the loop quotient is dec
	dec  Quotient
	sbc  r23,r2
// Notice L1 is going to branch off when the numerator is no lnger divisiable by the denominator. 
// Which means L1 is branching off when r24-denominator results in a negative value.
// Therefore, the denominator is going to be added to the r24 after the loop.
	add  r24,Denominator
	adc  r25,r2

	ret