How do you force a specific library cell for an isolation or level-shifter strategy?

Short Answer

Use use_interface_cell (UPF) to tie an isolation strategy, a level-shifter strategy, or one of each to the library cells you list. It is the only mapping command that can map a combined cell such as an enable level shifter, and in ICC2 it wins over map_isolation_cell (UPF) and map_level_shifter_cell (UPF) on the same strategy. Then check with report_mv_lib_cells (ICC2) that the forced cell really has the attributes the strategy needs.

Technical Explanation

• Default mapping: with no mapping command, ICC2 picks any suitable isolation or level-shifter cell in the library for each strategy.
• Why force a cell: a combined enable level shifter for a crossing that needs both functions, a preferred drive strength, or a cell qualified for that voltage pair.
• Scope: use_interface_cell (UPF) takes at most one isolation and one level-shifter strategy, and can map combined cells the older map commands cannot.
• Priority: when both use_interface_cell (UPF) and an older map command target the same strategy, use_interface_cell (UPF) wins.
• Existing cells: instantiated isolation cells tied to the strategy are remapped unless dont_touch; level-shifter mapping restricts the choice but never forces insertion.
• Validity: each listed cell must model the strategy function in Liberty or a define command; if none does, it is an error unless -force_function is used.
• What breaks: if every listed cell is invalid, the default policy drops the strategy with UPF-108, and that crossing gets no cells.

# [UPF]  design.upf
set_isolation ISO_GPU -domain PD_GPU -applies_to outputs -isolation_supply SS_AON -clamp_value 0 -isolation_signal U_PMU/iso_gpu -isolation_sense high -location parent
set_level_shifter LS_GPU -domain PD_GPU -applies_to outputs -rule low_to_high -location parent
use_interface_cell ELS_GPU -strategy {ISO_GPU LS_GPU} -domain PD_GPU -lib_cells {ELS_LH_AND_X2}
# older isolation-only form, still accepted by ICC2; loses to use_interface_cell here
map_isolation_cell ISO_GPU -domain PD_GPU -lib_cells {ISO_AND_X1}
# [ICC2]  icc2_shell
report_mv_lib_cells -verbose
create_mv_cells
check_mv_design

What To Check

• report_mv_lib_cells -verbose (ICC2) shows the forced cell with the PG pins and pin functions the strategies need.
• The load log shows no UPF-108 for any mapped strategy.
• Inserted cells on PD_GPU outputs are the forced ELS, not a fallback pair.
• No dont_touch isolation cell still uses a cell you meant to replace.

Healthy, Suspicious & Hard-stop Results

• Healthy (illustrative): All 256 PD_GPU outputs get ELS_LH_AND_X2, one cell per crossing instead of an isolation cell plus a shifter.
• Suspicious (illustrative): Some outputs still get ISO_AND_X1 plus a separate shifter: the mapping did not reach every element, so check the element lists.
• Hard stop: UPF-108 reports the strategy dropped because the listed cell is invalid: those outputs have no isolation at all.

What The Interviewer Is Testing

• Do you know which mapping command wins when two target the same strategy?
• Can you explain why a combined enable level shifter needs use_interface_cell?

Follow-up Question & Model Response

"Why not just set dont_use on every other isolation cell instead of mapping?"

Candidate Model Response: A dont_use setting is global, so it removes those cells from every strategy and domain that could have used them. Mapping is per strategy, so PD_GPU gets the enable level shifter while other domains keep their normal choices. It also records the choice in the UPF, where a reviewer looks for it. Use dont_use for cells nobody should ever get, and mapping for a local preference.