What library cell attributes does a low-power cell need to have (e.g. is_isolation_cell, is_level_shifter, switch_cell_type)?

Short Answer

The tool only uses a cell for isolation, level shifting, switching, retention or always-on buffering if its Liberty model says so. Each type has a cell flag, such as is_isolation_cell or is_level_shifter, plus pin attributes that mark enable pins, PG pin types and which supply each signal pin belongs to. You check what the tool actually read with report_mv_lib_cells (ICC2).

Technical Explanation

• Isolation: is_isolation_cell true, and isolation_cell_enable_pin on the enable pin.
• Level shifter: is_level_shifter true; an enable level shifter also needs level_shifter_enable_pin.
• Power switch: switch_cell_type coarse_grain or fine_grain tells tools what kind of switch it is.
• Retention: the retention_cell attribute defines the retention style the tool matches against map_retention_cell.
• Always-on: always_on true on buffers and inverters, so ICC2 can use them in always-on optimization.
• PG pins: each pg_pin carries a pg_type such as primary_power or backup_power; signal pins carry related_power_pin and related_ground_pin.
• Without these, the cell looks like ordinary logic; ICC2 falls back to other cells or leaves GTECH cells.

# [ICC2]  icc2_shell
report_mv_lib_cells -verbose
check_mv_design

What To Check

• Every isolation and level-shifter cell you plan to use shows its flag and enable pin.
• PG pin types match the cell role, such as backup_power on an always-on or dual-rail pin.
• Signal pins have related_power_pin and related_ground_pin.
• Macro pins with no related supply are either feedthrough or truly unconnected.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): report_mv_lib_cells lists ISO_AND_X1 as an isolation cell with EN as enable, and HEADER_X4 as a switch cell.
• Suspicious (illustrative): A cell you expected is missing from the report, often because an attribute or PG pin is absent.
• Hard stop: Inserted isolation cells stay GTECH after create_mv_cells, because no library cell qualified.

What The Interviewer Is Testing

• Do you know the tool selects power-management cells from Liberty attributes, not cell names?
• Can you name the attribute for each cell type?

Follow-up Question & Model Response

"What does power_down_function add for a low-power cell?"

Candidate Model Response: It states when an output is not driven because its supplies are off, for example Y becomes undefined when VDD is off. Tools use it to model the output as corrupted during shutdown instead of trusting the logic function. Without it, a cell that loses power looks like it still drives a valid value. You read it in the Liberty model; report_mv_lib_cells -verbose (ICC2) shows the supplies each pin depends on.