; ConvertCtoF

; 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 celsius value into r17
TestConvertCtoF:
	ldi r17,74
	sts C, R17
	//call subroutine ConvertCtoF
	rcall ConvertCtoF
	rjmp TestConvertCtoF

/****************************
 * subroutine converts a temperature reading in Celsius (variable C) to Fahrenheit (variable F).
 * F=(C × 9/5) + 32 ==(C × 18/10) + 32
 * Range for C input is from (0 to 124), since F max is 255
 ****************************/
    
ConvertCtoF:
	//Load the C value into r18
	lds r18, C
	//Input the constant value 18 in reg. 16
	ldi r16,18
	//This part calculates (C*18)
	mul r18, r16
	//Move the products into r25H and r24L
	movw r25:r24, r1:r0
	//Input the denominator into r19
	ldi Denominator, 10  //r19,10
	//Call the 16 bit by 8 bit division
    rcall Div16_8
	// add 32 to the quotient (18*C)/10 + 32
    ldi  r26,32
	add  Quotient,r26   //r22,r26
	adc  r23,r2

    //Store the answer into F
	sts  F, r22
	ret	

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

// quotient is going to increment by 1 every time L1 loops
// loop  L1 stops when the numerator-denominator is less than the demoninator(10)
//(18*C)/10
L1:
	inc Quotient
	adc r23,r2

	sub  r24,Denominator  // r24,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