Hi,
recently I had to incorporate an RTC into an design, as most RTC =
provides the data
in packed BCD I also had to dwell into BCD conversion, addition etc.
Thre were a few snippets on the piclist site but there were one thing =
that I needed=20
that I could not find.
Namely an BCD test and clamp function, as my design has provisions for a =
user to input arbitray data input into the RTC I wanted to perform an =
BCD sanity check before the actual
write into the RTC.=20

The requirments:

Input: 0x00 - 0xFF
BCD Min limit: 0x00-0x98
BCD Max Limit: 0x01-0x99 (note: BCD Max > BCD Min )

Formula: If(Input!BCD)OutPut =3D MaxBCD;
           else if(Input>MaxBCD) Output =3DMaxBCD;
           else if(Input<MinBCD) Output =3DMinBCD;
           else OutPut =3D Input

As I'm using an 18F452 in this project, I allow use of the extended =
instruction of this=20
family.


This is my first take:
;*******************************************************************
; CHECK_W_BCD_DIGIT_CLAMP - Checks if the value in w is bcd and that is
; if within the range MinBCD-MaxBCD, if not bcd or outside range the =
value
; will be clamped to either MaxBCD or MinBCD.
; Input:=20
; W - The value to test/clamp
; MinBCD - Constant BCD,minimum accepted value, range 0x00-0x98 (must be =
less than MaxBCD)
; MinBCD - Constant BCD,maximum accepted value, range 0x01-0x99 (must be =
higher than MinBCD)
; Temp - An temp file register in *current* bank or access ram
; KnownZero - An file register in *current* bank or access bank,  that =
is zero =3D0x00
;=20
; Output:
; W - The clamped value of input (or Input if input is within range and =
BCD)=20
;=20
CHECK_W_BCD_DIGIT_CLAMP MACRO   MinBCD,MaxBCD,Temp,KnownZero
        MOVWF   Temp            ; save input
        MOVFF   KnownZero,STATUS ; clear status reg (C and DC required for DAW)
        DAW                     ; decimal adjust w, -> BCD
        XORWF   Temp,F  ; check if w has changed=20
        BTFSS   STATUS,Z
        MOVLW   MaxBCD  ; w has changed, ie input was not BCD, set to maxvalue
=09
        ; check if below max
        ADDLW   LOW(0xFF-MaxBCD)
        BTFSC   STATUS,C
        MOVLW   0xFF            ; overflow set to max
=09
        ; now check for minium value
        ADDLW   LOW((MaxBCD-MinBCD)+0x01)
        BTFSS   STATUS,C
        MOVLW   MinBCD-0x01     ; value to low, set to min
        ADDLW   0x01            ; finally fix-up W
        ENDM


Usage:
        MOVLW   0x1A    ; put in a *non* bcd number
        CHECK_W_BCD_DIGIT_CLAMP 0x01,0x12,Temp,KnownZero
        ; W =3D 0x12 here

        MOVLW   0x00    ; put in a to low bcd number
        CHECK_W_BCD_DIGIT_CLAMP 0x01,0x12,Temp,KnownZero
        ; W =3D 0x01 here

        MOVLW   0x61    ; put in a to high bcd number
        CHECK_W_BCD_DIGIT_CLAMP 0x00,0x59,Temp,KnownZero
        ; W =3D 0x59 here



I think the above works, I haven't tested it yet, but I see no obvious =
errors.

Anyone have anything better ?


/Tony

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