Floorplanning funnies

Ok, another problem, but this time it's a little less likely to be staring me in the face, I think :-) Any help/information/pointers to where I could figure it out from very gratefully received :-)

I have a placed, routed, and statically-timed design, and I wanted to see if I could floorplan it into a slightly faster one - the tool's effort being pretty dire as usual.

When you fire up the floorplanner in Webpack, you get the previous attempt (by the tools) in a read-only window, and your own blank-slate in an editable window. I started off by pulling a carry-chain into the editor window, finding luts that are associated and putting them into the same CLBs. This worked fine for the first carry chain, but the second one didn't (!)

The tool's version has a brown-coloured chain extending for 31CY's, with LUTs packed in up and down the length of the chain. The stem of the chain name is 'c_core_Maddsub_addsub_inst_cy_XXX [CY] ...' and the stem of the LUTs is 'c_core_Maddsub_addsub_inst_lut3_XXXX [FG] ...' (where XXX varies depending on the element). It's pretty obvious they go together, and the tool manages to place them together. It won't let me do that though ? Whenever I try to drop a LUT that I can *see* next to the chain in the tool's window, it turns the mouse into a 'no-entry' cursor (circle with diagonal slash across) and won't let me place it there. I can put it anywhere else. Even more peculiarly, if I put it down on the grid elsewhere, then pick it up again, I *can* put it next to the chain...

The fact that it's got [FG] after its name presumably means it's using both F and G function generators, but if I'm reading the datasheet correctly, that's not a problem:

p12 of the 2nd spartan-3 datasheet: Main Logic Paths:

4 lines F1-F4 (or G1-G4 on the upper path) enter the slice ... The function generators data output D offers 5 possible paths: ...
  1. With the carry chain, serve as an input to the XORF (or XORG) xor gate that performs arithmetic operations, producing a result on X (or Y)

... which is what I assume it's doing, so what am I missing ?

In case it's relevant, the carry chain I could place has only 2 inputs (I0,I1) whereas this one has 4 (I0,I1,I2,I3). A 3-input chain can be easily placed just like the tool one as well...

Simon

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Simon
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