PHXSYS wrote:
>
> Hi
>
> I am fairly new to programming PICs and I have some code I am trying to
> understand. It uses a MOVLW where the literal is a label for a routine. Can
> you help me understand?
>
> movlw   Ckstrt          ; then make sure start bit persists
>
> Is this a method of addressing? Is the code run for that label and the final
> value stored in W?

No. The instruction above just moves the LSB of 'Ckstrt' in the W
register. If Ckstrt is a label the instruction simply loads the W
register with the low byte of the address that the label refers.

> I've never used this before. Hre's an example of a serial routine.
>
> Idle    btfss   SERIN           ; skip if serial input is space
>         goto    idl15
>         movlw   BITTIME/2       ; delay 1/2 bit time
>         movwf   STime
>         movlw   Ckstrt          ; then make sure start bit persists**** LOOK H
ERE
>         movwf   SState
>         goto    w10
> ;
> Ckstrt  decfsz  STime,F  ; verify start bit by sampling at center **** LOOK HE
RE
>         goto    w15
>         btfss   SERIN           ; skip if start bit was valid
>         goto    idl13
>         movlw   BITTIME         ; delay 1 bit time
>         movwf   STime
>         movlw   Getbit          ; then receive data
>         movwf   SState
>         movlw   8
>         movwf   SCount          ; for 8 bits
>         goto    w6

I would have look at the rest of the code but it seems to me that this
implements a (sort of) state machine in which the 'SState' holds the
address of the routine to be called when the next event occurs. I don't
know what the 'w10' and 'w6' routines do, but I suppose that they do a
jump based on the value in SState like in:

        movf        SState, W     ;Load next routine address
        movwf       PCL           ;Set program counter to the routine

Please note that since PCL (as W) is an 8 bit register the code must be
located in the same 256 word segment. If this is not the segment
starting from address 0 you need to set the PCLATH register accordingly.

Hope this helps

Ciao
  Marco