PIC Microcontoller Range Check Math Methods

Test if a value is in a certain range

W = value

addlw 255 - Hi
addlw (Hi - Lo) + 1

Carry is set if W is in range Lo - Hi

It works for all values of Hi and Lo although it is not optimal for 255 and 0. It even works for ranges that wrap around 255 back to 0. For instance, if you set Lo to 240 and Hi to 15, it will match 0..15 and also 240..255. And it works for signed numbers. You can set Lo to -3 and Hi to 5 to match -3..+5.

The only requirement is that your assembler must truncate literals to eight bits. If not, you can still use it by masking the literals yourself:

        addlw (255 - Hi) & 0ffh
        addlw ((Hi - Lo) + 1) & 0ffh

For example:

Lower = 50
Upper = 60

addlw 255 - Hi equates to addlw 195
addlw (Hi - Lo) + 1 equates to addlw 11

W = 50          ; lower limit

addlw 195       ; w = 245
addlw 11        ; w = 0, C = 1, result = OK

W = 55          ; in range

addlw 195       ; w = 250
addlw 11        ; w = 5, C = 1, result = OK

W = 60          ; upper limit

addlw 195       ; w = 255
addlw 11        ; w = 11, C = 1, result = OK

W = 49          ; under minimum

addlw 195       ; w = 244
addlw 11        ; w = 255, C = 0, result = Not OK

W = 61          ; over maximum

addlw 195       ; w = 0
addlw 11        ; w = 11, C = 0, result = Not OK

Ondrej Zoubek Says: " please note, that 12-bit core (PIC16C5x, 12C5xx) has no addlw instruction " +

Code:

• Dear Piclist,
  
  re. Range check when limits are in RAM
  I have an app. where speed of execution is paramount while looking for the range but the range limits can be calculated at leisure in which case...
  ...the sums 255-hi and hi-lo+1 can be evaluated and stored in 2 RAM locns.
  The ADDLW are replaced with ADDWF and I have a 2 instr. range test. It still fails if hi is 255 and lo is 0 which is a shame. It would be a simple way of including an always true condition.
  
  regards,
  Colin Smith
  colin@auc.co.uk
  

  +
• TakeThisOuTharold at spamdovesystems.com shares this code:

  The above window detector is nice and simple for those
  applications where the window limits are constants. Here's
  a macro that does window detecting when the limits are in RAM.
  
  WindowTest macro value, min, max
  ; Call as WindowTest value, min, max. Carry is set if value is within min-max (wtpass),
  ; and cleared if not (wtfail). 
  ; The key to this is the subwf instruction. With subwf, the destination=f-w and
  ; carry =1 if destination=>0 (carry=1 is positive result). So, 
  ;	carry=1 if f=>w
  ;	carry=0 if f<w or w>f
    local wtexit		; local labels in macro
    							; For min, we want value=>min, so we'll put value in f and min in w
   	movfw	min				; Get minimum
  	subwf	value,w			; w=f-w , w=value-min. Result positive (carry set) if value=>min
  	bnc		wtexit			; If no carry, we're below min. Exit with fail (carry clear)
  							; For max, we want max=>value, so we'll put max in f, value in w
  	movfw	value			; Get current value in w
  	subwf	max,w			; w=f-w = max-value. Result positive (carry set) if max=>value. Exit with carry set
  wtexit	
  	endm
  
  

  +

See also:

• The above trick is basically the same as the one described in "Checking if the point belongs to an interval" at the strchr wiki:
  http://www.strchr.com/checking_if_point_belongs_to_interval
• /techref/microchip/swstmt.htm PIC Microcontoller Program Flow Context Switch Methods+

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