How do you constrain which metal layers a clock tree is allowed to route on, and can you be more specific than "the whole tree"?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
set_clock_routing_rules -min_routing_layer M4 -max_routing_layer M7 applies to all clock trees by default, but you can scope it to specific clocks (-clocks), or to specific net types within the tree using -net_type root|sink|internal -- root nets run from the clock root to the first branch, internal nets run from there toward sinks, and sink nets connect directly to leaf sinks.
Technical Explanation
- set_clock_routing_rules -min_routing_layer M4 -max_routing_layer M7 applies to all clock trees by default -- a global layer constraint unless scoped down.
- -clocks scopes the constraint to specific clocks rather than the whole design.
- -net_type root|sink|internal scopes to a specific portion of the tree structurally: root nets (clock root to first branch), sink nets (connecting directly to leaf sinks), and internal nets (everything in between).
- You can also assign an NDR to clock nets directly via -rules <rule_name> instead of, or alongside, layer bounds.
Common Mistake
The Trap: Applying a single global layer constraint to the entire clock tree when the actual congestion or EM concern only applies to root nets or sink nets specifically -- over-constraining the rest of the tree unnecessarily.
Follow-up Question & Model Response
"Why would root nets specifically be the most likely candidates for a tighter layer or NDR constraint?"
Candidate Model Response: Root nets carry the most fanout and the highest current demand of any segment in the tree, making them the most likely candidates for EM concerns or congestion issues that a tighter layer or NDR constraint would specifically address.
Practical Example
Debug Scenario: EM concerns on the highest-current segment of a clock tree (the root net, carrying the most fanout) are addressed with set_clock_routing_rules -net_type root -min_routing_layer M6, leaving sink and internal nets unconstrained.
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