Why do tie cells cause multivoltage violations, and how do you fix them?

Short Answer

A tie cell drives a constant, but that constant is only valid while the tie cell supply is on and at the right voltage. When you add tie cells by hand, ICC2 uses single-rail ones, which take the domain primary supply; if the load wants a different supply, check_mv_design (ICC2) reports a voltage or isolation violation. fix_mv_design -tie_cell (ICC2) repairs them by switching between single-rail and dual-rail tie cells or reconnecting the backup PG pin.

Technical Explanation

• Mechanism: the tie output related supply is the tie cell supply; the load checks it like any other driver.
• Isolation case: tie on VDD1 feeding a load on VDD2, where VDD1 is less always-on than VDD2, gives an isolation violation.
• Voltage case: if VDD1 and VDD2 differ in voltage in the power state table, it is a voltage violation.
• Fix directions: fix_mv_design -tie_cell (ICC2) converts single-rail to dual-rail with the backup on the load supply, dual-rail back to single-rail, or reconnects a wrong backup pin.
• Constraints it honours: it will not use or modify dont_touch cells, keeps library cell purposes, stays within library subsets and site definitions, and respects bias rules.
• Literal supply: it keeps any supply set with the -literal_supply option of set_port_attributes (UPF) when it converts a tie cell.
• AO backplane ties: with mv.cells.ao_cells_without_primary_pg_pin (ICC2) set before commit_upf (ICC2), tie insertion uses always-on backplane tie cells.

# [ICC2]  icc2_shell
check_mv_design
fix_mv_design -tie_cell -report_only -verbose
fix_mv_design -tie_cell -verbose
report_mv_lib_cells -standard -verbose
check_mv_design

What To Check

• Every tie cell output supply is at least as always-on as its loads.
• Tie and load supplies are at the same voltage in every power state.
• Dual-rail tie cells have their backup pin on the intended supply.
• No tie cell is dont_touch where a fix is needed.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): The fix_mv_design report shows MV Illegal Tie Lib Cell at 0 after the run, and check_mv_design is clean.
• Suspicious (illustrative): Violations are fixed, but several tie cells became dual-rail in a VA with no secondary strap.
• Hard stop: A dont_touch tie cell still shows an isolation violation, or, with backplane support on, check_mv_design reports MV-546 for a regular dual-rail tie.

What The Interviewer Is Testing

• Do you see a tie cell as a driver with a supply?
• Can you name the isolation and voltage cases?

Follow-up Question & Model Response

"What is an always-on backplane tie cell, and when would you use one?"

Candidate Model Response: It is a tie cell with an always-on attribute and a backup power pin but no primary power pin. You enable support with mv.cells.ao_cells_without_primary_pg_pin (ICC2) before commit_upf (ICC2), and then tie insertion uses it where a dual-rail tie is needed. fix_mv_design -tie_cell (ICC2) can swap pre-instantiated regular dual-rail ties for it. You must connect the secondary supply to its PG pin explicitly.