PIC Microcontroller Bit Math Method

Detecting bit changes

by Scott Dattalo

Here's a version that saves the changes and then checks if wither input went high (but doesn't check high to lows):

 movf PORTB,w ;get current state
 xorwf last_in,w ;compare to previous
 xorwf last_in,f ;save changes
 andwf last_in,w ;did either go high?
 andlw (1<<4)||(1<<5) ;only want bits 4 and 5
 skpnz
 goto nolowtohighs
; either bit4 or bit5 went from 0 to 1
...
nolowtohighs

Here's one that does a check and then saves the changes (and will allow you to test high to lows if you want)

 movf PORTB,w ;get current state
 movwf temp
 xorwf last_in,w ;compare to previous
 andwf temp,w ;did either go high?
 andlw (1<<4)||(1<<5) ;only want bits 4 and 5
 skpnz
 goto save changes
; low to high change - do whatever
 btfsc temp,4
 do whatever for bit 4 going high
 btfsc temp,5
 do whatever for bit 5 going high
save_changes:
 movf temp,w
 xorwf last_in,w
 xorwf last_in,f

Also:

• PIC Microcontoller Methods Detecting a change in a register

See also:

• PICList Thread Bit high code challenge

Andrew D. Vassallo says:

Bob Ammerman says:

 ; compare register "last_inputs" to
 current PORTB reading ;
 ; direct computation of ~last_inputs & new_value
 comf last_inputs,W
 andwf PORTB,W
 andlw (1<<4)|(1<<5) skpz goto got_a_0_to_1 

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