Graham, Peter wrote:
>
> Hi all,
>
> This is my first time using the PIC controllers although I have had a
> reasonable amount of experience using the HC11.
>
> I have a question about indirect addressing data in the program area.
>
> I have an external compass unit which has a lot of commands that I want
> to access with  the 16C63. I want to store these commands in the program
> memory and then indirectly address these commands to be able to transmit
> them to the compass unit.
>
> I have found app notes regarding this for higher level controllers but
> not the 16C63. Is this possible or do I haveto load the commands into
> the data area to indirectly address them with this controller.
>
> Peter G.
>
>     ---------------------------------------------------------------
>
>                 Part 1.2       Type: application/ms-tnef
>                            Encoding: base64

There is no direct memory access on the 16C series, you have to set up a
jump table (and watch out for page boundaries)

It goes something like this (from memory, no pun intended)

main_prog       movlw offset
                call  table_rd
                ..


table_rd        addew   PCL,f
                retlw   'm'
                retlw   'e'
                retlw   's'
                retlw   's'
                retlw   's'
                retlw   'g'
                retlw   'e'
                retlw   0x0d

The problem arises if PCL+offset is >255 because you then have to add 1
to PCH too (page boundary). You may also need to implement some bounds
checking, as this can in theory jump to anywhere on the same page.

Consequently I usually put my tables on page 0 directly after the
reset/interrupt vectors. That way I don't have to allocate a whole page
for them, and I can see when they approach the page boundary.

Hope this helps,

Keith