Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Extra/unused LUTs in output design #48

Closed
mbpeterson70 opened this issue Feb 20, 2021 · 1 comment
Closed

Extra/unused LUTs in output design #48

mbpeterson70 opened this issue Feb 20, 2021 · 1 comment

Comments

@mbpeterson70
Copy link

Currently, if any LUT is used in a tile, Fasm2bels creates primitives/cells for every LUT in that tile, even for LUTs that were not used in the original design. When this is the case, a LUT that was originally unused is listed in Fasm2bels' output Verilog file as having its inputs as all "1'b1" and its its O5 and O6 outputs effectively do not drive anything, as seen in the examples below:

// Unused LUT
  (* KEEP, DONT_TOUCH, BEL = "B6LUT" *)
  LUT6_2 #(
    .INIT(64'h0000000000000000)
  ) CLBLL_L_X2Y121_SLICE_X0Y121_BLUT (
.I0(1'b1),
.I1(1'b1),
.I2(1'b1),
.I3(1'b1),
.I4(1'b1),
.I5(1'b1),
.O5(CLBLL_L_X2Y121_SLICE_X0Y121_BO5),
.O6(CLBLL_L_X2Y121_SLICE_X0Y121_BO6)
  );

// Used LUT
  (* KEEP, DONT_TOUCH, BEL = "A6LUT" *)
  LUT6_2 #(
    .INIT(64'hff55ff55f5f5f5f5)
  ) CLBLL_L_X2Y121_SLICE_X0Y121_ALUT (
.I0(LIOB33_X0Y1_IOB_X0Y1_I),
.I1(1'b1),
.I2(CLBLM_R_X3Y130_SLICE_X2Y130_AQ),
.I3(CLBLM_R_X5Y126_SLICE_X7Y126_BQ),
.I4(1'b1),
.I5(1'b1),
.O5(CLBLL_L_X2Y121_SLICE_X0Y121_AO5),
.O6(CLBLL_L_X2Y121_SLICE_X0Y121_AO6)
  );

image
Original design

image
Fasm2bels output design

Is there a reason why these unused LUTs are included in Fasm2bels' output? It seems like Fasm2bels could create a cleaner Verilog file if it only included used LUTs.

fasm2bels_output.v.zip
@litghost
Copy link
Contributor

Simply because they are equivalent and cleaning up the output isn't the first priority. You are welcome to submit a patch to clean up the output.

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Assignees
No one assigned
Labels
None yet
Projects
None yet
Milestone
No milestone
Development

No branches or pull requests
2 participants
@litghost @mbpeterson70