What changes when routing across voltage areas?

Short Answer

Signals crossing between voltage areas must go through the right isolation cells or level shifters, and the router must not create physical paths that break the power intent. Voltage area rules control whether nets may pass through an area and whether buffers can be added there. Cells with secondary power pins, such as always-on buffers, also need those pins routed to the correct supply.

Technical Explanation

• A voltage area is a region of the layout tied to a power domain. Cells inside it run from that domain's supplies.
• A signal crossing between domains at different voltages needs a level shifter, and one leaving a domain that can be switched off needs isolation. These cells come from the power intent, and routing must connect them as intended, not bypass them.
• By default, physical feedthrough nets are allowed through voltage areas. create_voltage_area_rule with -allow_pass_through false stops that, so nets detour around the area instead.
• Voltage area rules also control buffering: whether new cells, buffers and physical or logical feedthroughs are allowed. This matters when routing and optimization add buffers on long nets.
• Cells like always-on buffers have secondary power and ground pins. They need is_secondary_pg and port_type attributes in the frame view, and they are routed with route_group on the supply net, for example the always-on supply.
• check_routes reports voltage area violations, and check_mv_design checks the multivoltage design for power intent problems.
• Always-on nets passing through a switchable area should be buffered only with always-on cells powered from the always-on supply.

What To Check

• check_mv_design result after routing and any ECO.
• Voltage area violations from check_routes.
• That secondary PG pins are all connected to the intended supply.
• Nets passing through voltage areas where pass-through should not be allowed.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): check_mv_design clean, no voltage area violations, and every secondary PG pin connected.
• Suspicious (illustrative): A few nets crossing a voltage area they should avoid, found before signoff and rerouted.
• Hard stop: A domain-crossing signal routed without its level shifter or isolation, or an always-on buffer whose secondary pin is unconnected.

What The Interviewer Is Testing

• Whether you know voltage area rules and what they control.
• Do you know how secondary PG pins are routed?
• A strong answer shows you check with multivoltage checks, not just DRC.

Follow-up Question & Model Response

"Why would you forbid nets from passing through a voltage area at all?"

Candidate Model Response: A net passing through an area that can be switched off is at risk if anything along it needs buffering, because a buffer placed there would be powered by that area. There can also be noise or layout concerns in sensitive domains. Forbidding pass-through makes nets detour around, which costs wire length, but guarantees nothing in that domain carries unrelated signals.