How does ICC2 legalize always-on buffers that already exist in the RTL netlist?

Short Answer

Buffers and inverters written in the RTL are hard terminals with a fixed supply, so on an always-on net inside a shutdown domain they create violations the tool cannot fix by adding cells around them. AO legalization lets ICC2 change their supply: it converts single-rail ones to dual-rail user library cells, or back, at the same time as level-shifter insertion. You enable it with mv.upf.ao_legalize_gtech_buf (ICC2) for GTECH cells, or run create_mv_cells -always_on (ICC2) to cover every buffer and inverter.

Technical Explanation

• The problem: an RTL buffer in PD_COP takes the switched rail, so an always-on control through it dies at shutdown, and the tool treats it as fixed.
• Option: mv.upf.ao_legalize_gtech_buf (ICC2) is false by default; set it true before compile_fusion (FC) or create_mv_cells (ICC2).
• In compile_fusion: legalization runs after isolation insertion, so it does not disturb source and sink isolation strategies, and alongside level-shifter insertion.
• Joint fix: the tool can add a level shifter, convert a buffer to dual-rail, or both, which solves voltage violations that were stuck before.
• User cells: it converts to dual-rail user library cells, not GTECH ones, so the secondary supply connection survives mapping.
• All buffers: create_mv_cells -always_on (ICC2) also covers non-GTECH buffers and inserts nothing else; add -level_shifter to combine both jobs.
• Priority: with -always_on -level_shifter, ICC2 minimizes the number of level shifters before the number of dual-rail buffers.

# [ICC2]  icc2_shell
set_app_options -as_user_default -name mv.upf.ao_legalize_gtech_buf -value true
create_mv_cells -always_on -level_shifter
check_mv_design
# [FC]  fc_shell
set_app_options -as_user_default -name mv.upf.ao_legalize_gtech_buf -value true
compile_fusion

What To Check

• MV-055 counts match the number of RTL buffers you expected to change.
• Converted dual-rail buffers have their backup pin on the always-on rail.
• No voltage or isolation violation remains on the always-on nets.
• The library has usable dual-rail buffers and inverters.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): MV-055 reports 6 GTECH buffers converted from single-rail to dual-rail, and check_mv_design is clean.
• Suspicious (illustrative): MV-055 also reports conversions back to single-rail on nets you thought were always-on.
• Hard stop: No MV-055 message appears and check_mv_design still flags the same always-on nets, so legalization never ran.

What The Interviewer Is Testing

• Do you know why RTL buffers block MV fixing?
• Can you say what the option covers and what -always_on adds?

Follow-up Question & Model Response

"What if the library has no dual-rail buffers at all?"

Candidate Model Response: Then conversion is impossible, and the fallback is to move the buffer. fix_mv_design -buffer (ICC2) with -move_within_x / -move_within_y (ICC2) can move a single-rail buffer into a nearby voltage area where its needed supply is the primary. That needs feedthroughs allowed in the target VA and a legalize_placement (ICC2) afterwards. If neither works, change the RTL to route the net around the shutdown domain.