I need some help here... I can't get this thing to work.

I just started coding a I2C slave. Below is a bit of code that I wrote for
detecting START & STOP conditions, as well as raising and falling CLK edges.

The concept is simple. For every new sample form the PORTB I roll the bits
of I2C_TRANS_REG (transition register) twice to the left and add SDA and CLK
at the two LSBs. Obviously SCL is connected to PORTB[0], and SDA to PORTB[1]

Then, after I mask-off the 4 MSBs I use the 4 LSBs to detect how the signals
transitioned.

However it doesn't work. The PIC runs at 10MHz and I am sure it is enough to
keep up with the slow I2C master signals. I have been looking again and
again in the code but can't see what is wrong. Can you spot anything?  Also,
I have commented out part of the code to simplify it as an attempt to debug
it.


I2C_BUS_SCAN
        rlf     I2C_TRANS_REG, f
        rlf     I2C_TRANS_REG, f
        movlw   b'00001100'
        andwf   I2C_TRANS_REG, f
        movf    I2C_PORT, w             ; Read I2C port...
      andlw   b'00000011'             ; and keep only CLK and DTA pins...
      iorwf   I2C_TRANS_REG, f        ; Then mix them with the old pin
status

        sublw   b'00001101'             ; Check if DTA transitioned form 1 to 0 while CLK=1
        btfsc   STATUS, Z
        goto    I2C_START_DETECT

      movf    I2C_TRANS_REG, w            ; Get a fresh copy of TRANS_REG
        sublw   b'00000111'             ; Check is DTA transitioned form 0 to 1 while CLK=1
        btfsc   STATUS, Z
        goto    I2C_STOP_DETECT

;     movf    I2C_TRANS_REG, w            ; Get a fresh copy of TRANS_REG
;       andlw   b'00000101'             ; Keep only the old and new CLK bits
;       sublw   b'00000100'             ; Check if CLK transitioned form 1 to 0
;       btfsc   STATUS, Z
;       goto    I2C_CLK_FALL

;     movf    I2C_TRANS_REG, w            ; Get a fresh copy of TRANS_REG
;       andlw   b'00000101'             ; Keep only the old and new CLK bits
;       sublw   b'00000001'             ; Check if CLK transitioned form 0 to 1
;       btfsc   STATUS, Z
;       goto    I2C_CLK_RAISE

        goto    I2C_BUS_SCAN

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