What must be true before you start routing?

Short Answer

The block has to be routable and roughly timing-clean before the router touches it. That means power and ground already routed, clocks built and propagated, estimated congestion acceptable, estimated timing close to zero slack, no max capacitance or transition violations, and a clean check_routability. Zroute also needs every design rule defined in the technology file, because that is the only place it reads rules from.

Technical Explanation

• Zroute takes all of its design rule information from the technology file. If a spacing or via rule is missing there, the router will not honour it, and the error only appears later in signoff DRC.
• Power and ground should already be routed, normally during design planning, before placement. Signal routing has to work around the PG straps and rails, so routing signals first and adding PG afterwards does not work.
• CTS and its optimization should be complete, with clocks propagated. If the clock tree changes after signal routing, the router has to rip up and redo wires around it.
• Estimated congestion must be acceptable. Global routing overflow is the router telling you there are not enough tracks. A hot spot at this stage will turn into a cluster of detail routing DRCs that refuse to converge.
• Estimated timing should sit near 0 ns slack, and there should be no max capacitance or max transition violations. Routing adds delay; it does not remove it. A path that is already -50 ps before routing will be worse after.
• check_routability should come back clean. It finds blocked standard cell and macro pins, pins outside the block boundary, off-grid pins and bad via definitions, all of which the router cannot fix.

What To Check

• check_routability output: zero blocked ports, zero out-of-boundary pins, zero min-grid violations.
• report_congestion: overflow total and the number of overflowing global routing cells per layer.
• report_timing worst setup path on estimated parasitics, and its slack.
• report_constraints -all_violators: any max transition or max capacitance violations.
• check_legality: every cell on a legal site with no overlaps.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): check_routability clean, overflow 0 on every layer, worst setup slack at or above -5 ps, zero transition and capacitance violations.
• Suspicious (illustrative): Fewer than 0.5% of global routing cells overflowing, or a few tens of picoseconds of negative slack that you can explain.
• Hard stop: Any blocked pin reported by check_routability, PG not yet routed, or clocks still ideal. Stop and fix these before routing.

What The Interviewer Is Testing

• Whether you know that the tech file is the router's only rule source.
• Do you treat congestion and pin access as entry gates rather than routing problems?
• A strong answer shows you can name the checking command for each prerequisite instead of listing the prerequisites from memory.

Follow-up Question & Model Response

"What does check_routability catch that report_congestion cannot?"

Candidate Model Response: report_congestion measures demand against supply in each global routing cell, so it tells you about crowding. check_routability looks at individual pins and definitions. It finds a pin that has no legal way out because a blockage or PG shape covers it, a pin outside the block boundary, and pins off the minimum grid. A block can have zero overflow and still have one blocked macro pin that will become an open net. You need both checks.