Can anyone help me with a vaguely PIC related problem...?

I have a project in which I need to control the brightness of a 12V lamp
from a PIC.  The lamp has to go from maximum brightness to the off state.  I
am planning on implementing this using varying mark-space ratios so to
minimise power loss.

Can anyone advise me on a cct. for doing this?  I am close by using a E2POT
digital potentiometer from Xicor and a CA555 timer, but this will only give
me a mark:space ratio of 1:1 at one extreme.  i.e. I can only get it half
dimmed, or half bright depending on the polarity of the output. However,
this cct. does have the advantage of only using two pins on the PIC.

If anyone could advise of a timer circuit with which I can use the E2POT and
still obtain a full range of mark:space ratios (with minimal component
count) I would be very grateful.  One additional point is that mains supply
is not available, so this cannot be used as a frequency generator.

I have a feeling the answer is fairly simple, but I just can't see the wood
for the trees! :-)

Thanks a lot!
Chris Windram