Why applying a hold ECO causes legalization errors?

o   A "cell cannot be legalized" error during an ECO (especially a hold ECO, which often involves adding buffers) means the PnR tool cannot find a valid, DRC-clean placement location for the newly added or modified cells that aligns with the site rows and doesn't overlap with other cells.

o    High Cell Density/Congestion: There's simply no physical space (empty sites) to place the new buffer without causing overlaps.

o    Filler cells not removed from the design.

o    Frozen Placement: If too many surrounding cells are marked as fixed or have dont_touch attributes, the legalizer has very little flexibility to move existing cells to make space for the ECO cell.

o    Large ECO Cell Size: If the ECO involves adding a large buffer or ECO is adding so many buffers in certain area causing placement issues.

o    Row Utilization Limits: Some tools might have constraints on maximum row utilization that prevent adding more cells even if a few empty sites seem available.

o    Multi-Height Cells: If the ECO cell is a multi-height cell and the required space pattern (e.g., two adjacent free rows) isn't available.

o    Power Domain Constraints: If the ECO cell needs to be in a specific power domain, and that domain's physical voltage area is full.

o   What Can Be Done?

o    Check for Filler or placement blockage issue, this is easy to resolve. By deleting.

o    Unfix Nearby Cells Temporarily: or manual movement to fix placement issue.

o    Use Spare Cells (If Available and Pre-placed): If spare cells using too much space, can we reduce it if allowed?

o    Increase cell shift distance limit so tool can move cell to further distance and resolve legalisation.

o    Check ECO quality if ECO is simply too large.