o Target Utilization at Start: The initial target core utilization for PnR typically ranges from 50% to 70%.
Lower utilization (e.g., 50-60%) provides more whitespace, making routing easier, reducing congestion, potentially improving timing (less detour), and offering more flexibility for CTS and ECOs. This is often preferred for high-performance designs or designs with known congestion challenges.
Higher utilization (e.g., 65-70%, sometimes even higher for specific blocks) aims to minimize die area (cost). However, it increases the risk of congestion, may make timing closure harder, and leaves less room for post-route optimizations and ECOs.
70% vs 50%: The choice depends heavily on design goals and characteristics:
Choose 50% if:
High performance/frequency is the primary goal.
The design has complex routing or known congestion hotspots.
Significant CTS overhead or late-stage ECOs are anticipated.
Power density / thermal concerns require spreading cells out.
Using an advanced node with restrictive routing rules.
Choose 70% if:
Area/cost minimization is the primary goal.
The design is relatively simple with moderate performance targets.
Congestion is not expected to be a major issue.
Minimal ECOs are expected.