ExpertQuestion 32 of 50Source: Synopsys IC Compiler II Multivoltage User Guide: Feedthrough Buffering With Missing Voltage Areas, Assigning Related Supplies for Feedthrough Ports, The Incomplete UPF Flow

A feedthrough crosses a domain with no voltage area defined yet. What breaks, and how do you handle it?

From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide

By Tabish Iqbal · 9+ Years VLSI Physical Design Experience

Short Answer

With a voltage area missing, ICC2 puts that domain in DEFAULT_VA, and it only buffers a feedthrough through the domain if the domain primary supply matches the top domain primary. Everything you implement there is provisional: buffers may sit on a supply that later turns out to be switched, and checks that need the real VA cannot run properly. Keep the net known, assign feedthrough supplies explicitly, and re-check once the voltage area exists.

Floorplan with net dsp2cpu_req from PD_DSP to PD_CPU crossing PD3, first shown with no voltage area (DEFAULT_VA, buffered because PD3 shares the top VDD1p0 primary), then after PD3 gets its own switchable VA_PD3 on VDD1p0_SW (hatched, red buffers), and a note on assign_feedthrough_supply (ICC2). — Technical Reference DiagramA feedthrough crosses a domain with no voltage area defined yet. What breaks, and how do you handle it?

Technical Explanation

Default: mv.upf.enable_missing_voltage_area (ICC2) is true, so missing VAs are allowed and top-level domains without one go to DEFAULT_VA.
Global policy trap: when Early Data Check runs with a global policy, ICC2 ignores mv.upf.enable_missing_voltage_area (ICC2), so the option no longer controls this behavior.
Buffering rule: a buffer may go in a domain with no VA only if that domain primary is equivalent to the topmost primary.
What breaks: if a later UPF makes that domain switchable, buffers already placed in its region sit on a rail that turns off.
Checks: check_mv_design (ICC2) still warns about missing voltage areas, so the warning list becomes your to-do list, not noise.
Supply assignment: assign_feedthrough_supply (ICC2) picks related supplies for feedthrough ports to reduce dual-rail cells and avoid new violations.
Feedback: generate_mv_feedback (ICC2) suggests isolation and level-shifter changes for feedthrough violations; you apply the fixes yourself.

# [ICC2]  icc2_shell
get_app_option_value -name mv.upf.enable_missing_voltage_area
check_mv_design
assign_feedthrough_supply -net {dsp2cpu_req} -verbose
generate_mv_feedback -net {dsp2cpu_req}
report_early_data_checks

What To Check

Every domain still on DEFAULT_VA is listed and owned.
Buffers on feedthrough nets are rechecked after the real VA is created.
Feedthrough ports have assigned supplies, not guesses.
Missing-VA warnings shrink over time, not grow.

Command Checks & Actions

ICC2 (icc2_shell)get_app_option_value -name mv.upf.enable_missing_voltage_area

Confirm missing voltage areas are allowed

ICC2 (icc2_shell)check_mv_design

List missing voltage area warnings and crossing violations

ICC2 (icc2_shell)assign_feedthrough_supply -net {dsp2cpu_req} -verbose

Assign related supplies to the feedthrough ports on the net

ICC2 (icc2_shell)generate_mv_feedback -net {dsp2cpu_req}

Get suggested strategy changes for the feedthrough

ICC2 (icc2_shell)report_early_data_checks

See which checks were tolerated or repaired, such as mv.va.missing_voltage_area

Healthy, Suspicious & Hard-stop Results

Healthy (illustrative): Only PD3, which shares the top primary, remains on DEFAULT_VA, and assign_feedthrough_supply succeeds on all nets.
Suspicious (illustrative): The feedthrough is buffered, but the MV-1057 count shows one net could not be assigned.
Hard stop: Buffers sit in a region later made a switchable VA, or assign_feedthrough_supply reports MV-1061 on the net.

Common Mistake

The Trap: Treating the clean buffering result as final because no error appeared.

When the VA arrives and the domain turns out switchable, those buffers die at shutdown and the feedthrough net goes X.
Because the net passed every earlier check, nobody looks at it again unless the missing-VA warnings are tracked to closure.

What The Interviewer Is Testing

Do you know how ICC2 treats a domain without a VA?
Would you plan a re-check once the VA lands?

Follow-up Question & Model Response

"What changes if you disable missing voltage area support?"

Candidate Model Response: Domains without a VA are no longer assigned to DEFAULT_VA in either the top or the lower-level block. Implementation commands that need voltage areas then stop with an MV-51 error. That is stricter and safer late in the flow, because nothing gets built on a guess. Early on it blocks exploration, so most teams leave it on and track the warnings.

Practical Example

Design Scenario: (illustrative) The net dsp2cpu_req runs from PD_DSP to PD_CPU across the region of PD3, which has no VA yet and shares the top primary VDD1p0. ICC2 inserts two buffers there on DEFAULT_VA. Two weeks later a UPF update moves PD3 onto a switched primary, VDD1p0_SW, and gives it its own VA. The re-run of check_mv_design flags both buffers; the team reruns assign_feedthrough_supply and fix_mv_design, which converts them to dual-rail with backup pins on VDD1p0. The missing-VA warning list is now empty, so the feedthrough plan is final.

Low-Power & UPF Handbook