o Fixing IR drop involves 1) strengthening the Power Distribution Network (PDN) to reduce its resistance or 2) reducing the current drawn by the logic.
- During Placement/Floorplan (Preventative)
- Robust PDN Design: Plan a dense power grid using wide straps/rings on low-resistance metal layers with ample vias
- Macro Placement: Place high-power macros near power sources or ensure they have strong connections to the power grid.
- Cell Placement: Avoid clustering high-power or high-switching activity cells in one area. Use density controls.
- During Post-Route Optimization / ECO Stage:
- Strengthen PDN:
- Add/Widen Power Straps: Introduce more power/ground stripes in areas with high voltage drop or increase the width of existing straps.
- Add Power Vias: Increase the number of vias connecting different layers of the power grid, at connections to cell rails, to reduce vertical resistance.
- Add Decap Cells (Primarily for Dynamic IR):
- Reduce Current Draw:
- Cell Downsizing: Replace high-power cells in the violating region with smaller drive-strength equivalents, if no timing violations.
- VT Swapping (Higher Vt): Swap cells to higher-Vt to reduce leakage current (helps static IR) and slightly reduce peak dynamic current (helps dynamic IR), If no timing issues.
- Spread High-Activity Cells: If dynamic IR drop is caused by simultaneously switching cells clustered together, try spreading these cells apart during ECO placement