o Handling Static IR Drop: Static IR drop is primarily due to the resistance of the power grid (Vdrop=Iavg×Rgrid). If you cannot improve Rgrid (by widening straps/adding vias):
- Reduce Average Current (Iavg):
Downsize Non-Critical Cells: Replace cells contributing significantly to static leakage in the affected region with smaller drive-strength variants (if timing permits).
VT Swapping (High VT): Swap cells to higher Vt variants (LVT -> SVT -> HVT) in the affected area. Higher Vt cells have significantly lower leakage current. This requires available timing slack.
Power Gating (Architectural): If the block can be periodically shut down using power gating, its contribution to average static IR drop over time is reduced (This is a design architecture change, not a simple fix).
- Optimize Power Grid Path: Ensure the existing power grid connections are optimal. Sometimes, reconnecting a cell group to a different, less resistive part of the existing grid (if possible without causing other issues) might help slightly.
- Re-evaluate Floorplan: Move the high-leakage block to an area with a stronger existing power grid, if possible.
Handling Dynamic IR Drop (when standard fixes are limited): Dynamic IR drop is due to peak current demands during switching (Vdroop≈LdtdI+IpeakR). If you cannot add decaps (the primary fix) or strengthen the grid:
- Reduce Peak Current (Ipeak / dI/dt):
Activity Spreading (Timing Adjustment): If multiple cells switch simultaneously causing a large current spike, try adjusting timing slightly (e.g., using useful skew, adding small delays) on non-critical paths to stagger the switching times, thus reducing the peak current draw at any single moment. This is complex and has timing implications.
Downsize Switching Cells: Reduce the size/drive strength of the heavily switching cells, if timing slack allows. Smaller cells draw less peak current.
VT Swapping (High VT): Using higher Vt cells can slightly reduce peak current (due to slower switching), but the main impact is slower performance.
Reduce Clock Frequency: Lowering the clock frequency reduces the dI/dt component and overall activity (architectural change).
Clock Gating: Ensure clock gating is aggressively applied to prevent unnecessary switching in the affected region.
Spreading cells
- Optimize Local Power Delivery: Ensure the existing connections from the local power rails/straps to the high-switching cells are as low-resistance as possible (e.g., maximizing local vias if that specific fix is allowed).