| # flipflop + inverter + rebuffer + output buffer | |
| Tool: Clock Tree Synthesis | |
| Subcategory: Unexpected optimization | |
| ## Conversation | |
| ### oharboe | |
| Is the "rebuffer" buffer superfluous here? | |
| The "rebuffer" buffer does not exist in placement, only after CTS. | |
| `make DESIGN_CONFIG=designs/asap7/mock-array/Element/config.mk gui_final`: | |
|  | |
| `make DESIGN_CONFIG=designs/asap7/mock-array/Element/config.mk gui_place`: | |
|  | |
| ### maliberty | |
| It is there for timing optimization as the path has negative slack which is addressed post-CTS. Is there a reason to think it superfluous? | |
| ### oharboe | |
| No extra buffers after removing `set_load -pin_load 10 [all_outputs]`: | |
| Untar [cts-nobuffer.tar.gz](https://github.com/The-OpenROAD-Project/OpenROAD/files/14554686/cts-nobuffer.tar.gz) | |
| ``` | |
| $ ./run-me-mock-array_Element-asap7-base.sh | |
| OpenROAD v2.0-12488-g85f541bb6 | |
| openroad> report_checks -to io_lsbOuts_7 | |
| Startpoint: _755_ (rising edge-triggered flip-flop clocked by clock) | |
| Endpoint: io_lsbOuts_7 (output port clocked by clock_vir) | |
| Path Group: clock_vir | |
| Path Type: max | |
| Delay Time Description | |
| --------------------------------------------------------- | |
| 0.00 0.00 clock clock (rise edge) | |
| 74.98 74.98 clock network delay (propagated) | |
| 0.00 74.98 ^ _755_/CLK (DFFHQNx2_ASAP7_75t_R) | |
| 45.97 120.95 v _755_/QN (DFFHQNx2_ASAP7_75t_R) | |
| 13.06 134.01 ^ _435_/Y (INVx4_ASAP7_75t_R) | |
| 18.04 152.05 ^ output265/Y (BUFx2_ASAP7_75t_R) | |
| 0.01 152.06 ^ io_lsbOuts_7 (out) | |
| 152.06 data arrival time | |
| 300.00 300.00 clock clock_vir (rise edge) | |
| 70.00 370.00 clock network delay (ideal) | |
| -20.00 350.00 clock uncertainty | |
| 0.00 350.00 clock reconvergence pessimism | |
| -240.00 110.00 output external delay | |
| 110.00 data required time | |
| --------------------------------------------------------- | |
| 110.00 data required time | |
| -152.06 data arrival time | |
| --------------------------------------------------------- | |
| -42.06 slack (VIOLATED) | |
| ``` | |
| ### maliberty | |
| > How do I stop "don't touch" from removing the output buffer so I get the comparison I'm after? | |
| Set don't touch after the output buffer is created. | |
| > If I delete set_load -pin_load 10 [all_outputs], the extra buffer goes away. | |
| Probably the timing improves and the net no longer needs repair. Similarly with the set_driving_cell changing the timing. | |