PIC Microcontoller Bit Math Method

Converting a 3-bit number to a 8-bit mask

From: Mike Keitz. There is a trick method that works for 4 bits. It replaces a 2-bit number in a variable with a 4-bit mask:

        incf    BitP, W  ;W = 0001 0010 0011 0100 
        btfsc   BitP, 1  ;If 0 or 1, result is almost correct now. 
        iorwf   BitP, F  ;BitP = [0000] [0001] 0011 0111 
        incf    BitP, F  ;BitP = 0001 0010 0100 1000

It could be the core of an 8-bit routine thus:

; Convert 3-bit number (0-7) in INDEX to a 8-bit mask (00000001 ... 
; 10000000) in BitP. 
        movfw   INDEX 
        andlw   b'00000011'     ;Start with half of the mask. 
        movwf   BitP 
        incf    BitP, W         ;The 4-bit converter 
        btfsc   BitP, 1 
        iorwf   BitP, F 
        incf    BitP, F 
        btfsc   INDEX, 2        ;Is it high 4 bits? 
        swapf   BitP, F

Code:

mark_jeronimus-gmail- shares this code:

Still, a simple table lookup or 8-bits is less instruction cycles and doesn't need a temp register.

And it can do a little math too (set bit 7-WREG instead of bit WREG)

GET_BIT
	ADDWF	PCL,F
	RETLW	B'00000001'
	RETLW	B'00000010'
	RETLW	B'00000100'
	RETLW	B'00001000'
	RETLW	B'00010000'
	RETLW	B'00100000'
	RETLW	B'01000000'
	RETLW	B'10000000'

GET_BIT2
	ADDWF	PCL,F
	RETLW	B'10000000'
	RETLW	B'01000000'
	RETLW	B'00100000'
	RETLW	B'00010000'
	RETLW	B'00001000'
	RETLW	B'00000100'
	RETLW	B'00000010'
	RETLW	B'00000001'

file: /techref/microchip/math/bit/mask.htm, 2KB, , updated: 2008/10/8 04:23, local time: 2008/11/21 23:05,

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