Port Data Direction Register

sxman

Good morning-

Could any of you learned people out there tell me if it is possible to change the DDR for a specific pin?

ie I'd like to be able to (say) change the direction of rc.7 without having to write a complete new byte to the !rc register

eg setb !rc.7 (which I know doesn't work)

rather than

mov !rc, temp ( where temp.7 contains the value I want to write to !rc.7)

Thanks

4x5n

Have you tried putting the proper mask int 'w' and than using xor to toggle the bit based on the mask.

sxman

Thanks for the suggestion-
I assume you mean something like this:

mov w,!rc
xor w,#%10000000
mov !rc,w

Unfortunately, the mov w,!rc instruction is thrown out by the assembler.

4x5n

sxman wrote: »

Thanks for the suggestion-
I assume you mean something like this:

mov w,!rc
xor w,#000000
mov !rc,w

Unfortunately, the mov w,!rc instruction is thrown out by the assembler.

I was actually thinking of something like:

mv w, #%10000000
xor !rb, w

Unfortunately that isn't allowed by the assembler either.

I did a bit of checking around with google and it doesn't look like it's possible to do anything but write to the register with the "mov" command. You may need to use a register to save a copy of the port dir register make your changes to it and than move that into the !rx register.

sxman

Thanks for coming back again.
Yup, that was my conclusion earlier today- and it works OK. But (strange how there's always a but) I was trying to avoid using a dedicated register for this action as, due to the position of the routine using this command, it would really need to be a page $00 register and I've used them all up. Anyway, thanks again for your suggestions.

4x5n

I've always hated ports that I could only write to. So much easier to be able read back the status than have to keep track of it.

wass

Yeah, this is a week point with the SX. In fact SX/B reserves the last few bytes of RAM to keep copies of the TRIS registers for this purpose.