From: Scott Dattalo, notes
Comments added by Alex Forencich
;******************************** ;binary_to_bcd - 8-bits ; ;Input ; bin - 8-bit binary number ; A1*16+A0 ;Outputs ; hundreds - the hundreds digit of the BCD conversion ; tens_and_ones - the tens and ones digits of the BCD conversion binary_to_bcd: CLRF hundreds SWAPF bin, W ; swap the nibbles ADDWF bin, W ; so we can add the upper to the lower ANDLW B'00001111' ; lose the upper nibble (W is in BCD from now on) SKPNDC ; if we carried a one (upper + lower > 16) ADDLW 0x16 ; add 16 (the place value) (1s + 16 * 10s) SKPNDC ; did that cause a carry from the 1's place? ADDLW 0x06 ; if so, add the missing 6 (carry is only worth 10) ADDLW 0x06 ; fix max digit value by adding 6 SKPDC ; if was greater than 9, DC will be set ADDLW -0x06 ; if if it wasn't, get rid of that extra 6 BTFSC bin,4 ; 16's place ADDLW 0x16 - 1 + 0x6 ; add 16 - 1 and check for digit carry SKPDC ADDLW -0x06 ; if nothing carried, get rid of that 6 BTFSC bin, 5 ; 32nd's place ADDLW 0x30 ; add 32 - 2 BTFSC bin, 6 ; 64th's place ADDLW 0x60 ; add 64 - 4 BTFSC bin, 7 ; 128th's place ADDLW 0x20 ; add 128 - 8 % 100 ADDLW 0x60 ; has the 10's place overflowed? RLF hundreds, F ; pop carry in hundreds' LSB BTFSS hundreds, 0 ; if it hasn't ADDLW -0x60 ; get rid of that extra 60 MOVWF tens_and_ones ; save result BTFSC bin,7 ; remeber adding 28 - 8 for 128? INCF hundreds, F ; add the missing 100 if bit 7 is set RETURN ; all done!
Note: SKPDC is equivalent to BTFSS STATUS, DC and SKPNDC is equivalent to BTFSC STATUS, DC
Same code optimized for the PIC18F instruction set:
binary_to_bcd: clrf hundreds swapf bin, W, A ; swap the nibbles addwf bin, W, A ; so we can add the upper to the lower andlw B'00001111' ; and lose the upper nibble (W is in BCD from now on) btfsc STATUS, DC, A ; if we carried a one (upper + lower > 16) addlw 0x16 ; add 16 (the place value) (1s + 16 * 10s) daw ; check for digit overflows btfsc bin, 4, A ; 16's place addlw 0x16 - 1 ; add 16 - 1 btfsc bin, 5, A ; 32nd's place addlw 0x30 ; add 32 - 2 btfsc bin, 6, A ; 64th's place addlw 0x60 ; add 64 - 4 btfsc bin, 7, A ; 128th's place addlw 0x20 ; add 128 - 8 % 100 daw ; check for digit overflows rlcf hundreds, F ; pop carry in hundreds' LSB movwf tens_and_ones btfsc bin,7, A ; remember adding 28 - 8 for 128? incf hundreds, F ; add the missing 100 if bit 7 is set return
Don't need the hundreds? Get rid of everything after the second daw up to the return. The result will end up in W in that case, just add a movwf if the result is needed elsewhere. Want to pass in the byte in W? Just add a movwf bin at the beginning.
I am a Newbie and this is the exact information i was looking for, it works great. The only thing is i don't know how this code actually works, maybe i try to figure it out when i learn more about asm.+
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