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Add option to bit2fasm to skip emitting empty CLE sites #24

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litghost opened this issue Jul 23, 2020 · 3 comments
Open

Add option to bit2fasm to skip emitting empty CLE sites #24

litghost opened this issue Jul 23, 2020 · 3 comments
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enhancement New feature or request

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@litghost
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daveshah: UltraScale+ devices have all unused LUTs set to output 1 by default, to save power
daveshah: this results in loads of extra set bits in the FASM
daveshah: it might be nice to have a bit2fasm option that skips these
@litghost litghost added the enhancement New feature or request label Jul 23, 2020
@litghost
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I would propose we continue to emit the bits for unused LUT's within a used CLE, but totally empty CLE's we can safely drop. There would need to be a corresponding change to fasm2bit to emit unused CLE's with the default LUT configuration.

@daveshah1
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SGTM - so long as explicitly setting LUT init to 0, even if the site is otherwise empty, doesn't still set the default 1s as this may be a genuine situation for pseudo-ground drivers - UltraScale+ doesn't have xc7-style ground nodes.

@litghost
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litghost commented Jul 23, 2020 •
edited

SGTM - so long as explicitly setting LUT init to 0, even if the site is otherwise empty, doesn't still set the default 1s as this may be a genuine situation for pseudo-ground drivers - UltraScale+ doesn't have xc7-style ground nodes.

The logic would be "if there is a FASM feature using the CLE, do not emit defaults", so setting the ALUT.INIT = 64'b0 would work as expected.

tmichalak pushed a commit to tmichalak/prjuray that referenced this issue Aug 25, 2020
@litghost
Merge pull request f4pga#24 from litghost/docs_zynq_jtag
b4bae37 
Initial Zynq US+ JTAG documentation.
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@litghost @daveshah1