The-OpenROAD-Project
padding on DECAPs creates overlap error with tapcells
Summary
While running the temperature sensor design, I got a new error related to how padding is used. Padding seems to be applied to sky130_fd_sc_hd__decap_4 cell and the tool thinks there is an overlap with the tapcells.
Observed behavior: Design fails at detail placement:
[INFO DPL-0020] Mirrored 63 instances
[INFO DPL-0021] HPWL before 5213.7 u
[INFO DPL-0022] HPWL after 5133.0 u
[INFO DPL-0023] HPWL delta -1.5 %
[WARNING DPL-0005] Overlap check failed (7).
TAP_119 overlaps PHY_13
TAP_123 overlaps PHY_17
TAP_128 overlaps PHY_25
TAP_133 overlaps PHY_33
TAP_138 overlaps PHY_41
TAP_143 overlaps PHY_49
TAP_146 overlaps PHY_53
[ERROR DPL-0033] detailed placement checks failed.
Error: detail_place.tcl, 121 DPL-0033
0:00.48elapsed 99%CPU 84476memKB
make[1]: *** [results/sky130hd/tempsense/3_4_place_dp.def] Error 1
make[1]: Leaving directory `/shared/OpenFASOC/generators/temp-sense-gen/flow’
Expected Behavior
Design should not fail at detail placement. My work around is to remove padding for the decap cells as follows:
set_placement_padding -left 0 -right 0 -masters “sky130_fd_sc_hd__decap_4” in tapcell.tcl
set_placement_padding -left 0 -right 0 -masters “sky130_fd_sc_hd__decap_4” in detail_place.tcl
Test case is provided here floorplan_tempsense_sky130hd__2021-12-17_03-43.tar.gz
DPL checks based on the LEF MACRO class. For sky130_fd_sc_hd__decap_4, the class is CORE which is probably not correct. It should be CORE SPACER.
Yes, I think that should fix it if we updated the LEF. Thanks @rovinski. I will need to try it. I think @maliberty mentioned he could handle this case (CORE cells versus TAPCELL + padding)
@rovinski why should decap be a SPACER? It isn't a fill cell. sky130_fd_sc_hd__fill_* are marked as SPACER. @msaligane is there a reason to use decap rather than fill?
@maliberty It would be nice if there was a LEF class for decap but there isn't. Decap cells don't have any pins except power/ground, so they fit the same profile as filler cells. Most (all?) commercial PDKs I surveyed mark their decaps cells as such (CORE SPACER).
