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diff --git a/docs/bel_and_site_design.md b/docs/bel_and_site_design.md index 8f4519b..d7db3fb 100644 --- a/docs/bel_and_site_design.md +++ b/docs/bel_and_site_design.md @@ -213,3 +213,46 @@ two output BEL pins driving the net connected to the cell port. Having multiple BEL pins driving one net is not legal, except for the global logic 0 and 1. + +### Quicklogic EOS S3 logic cell + +The Quicklogic EOS S3 logic cell has an interesting LUT design because there +is not LUT element specifically. Instead, the fabric exposes a 8x3 mux, with +inverters at each of the mux inputs, see figure below: + + + +The way to approach this fabric is to first draw BEL boundaries around the 4x2 +mux and 8x3 mux present in the fabric: + + + + +The cell library should have +3 MUX cell types: + - 4-input 1-output 2-select MUX4x2 (maps to MUX4x2 BEL and MUX8x3 BEL) + - 8-input 1-output 3-select MUX8x3 (maps to MUX8x3 BEL) + - A macro cell that is 2x (4-input 1-output 2-select MUX4x2) 2xMUX4x2 (maps to MUX4x2 *and* MUX8x3 BEL) + +A fourth most general cell type is possible, which is to add a cell that also +has a cell port that maps to `TBS`, instead of tying `TBS` high as the +2xMUX4x2 cell would do. It is unclear how useful such a cell would be. +However given the BEL boundaries, adding such a cell would be easy after the +fact. + +In all of the cells above, all inputs to the muxes have statically +configured inverters. + +So the question becomes, how to model LUT cells in this fabric? The LUT cells +should be the regular LUT1, LUT2 and LUT3 cells. The LUT1 and LUT2 can map to +either the MUX4x2 or MUX8x3 BEL. The LUT3 can map to only the MUX8x3 BEL. +The question is only what is the cell port to BEL pin map? + +The solution is when mapping a LUT cell, to tie all of the MUX BEL pins to VCC +(or GND, whatever the default is) before the inverter. The place and route +tool can treat the BEL as a regular LUT, and only the bitstream generation +step will need to be aware that the inversion control is being used to +encode the LUT equation. + +This configuration allows most (if not all) of the logic to be available +to the place and route tool, without exposing unneeded complexity. |