BeginnerQuestion 40 of 50

Can you describe a basic low-power implementation mistake, like a missing isolation cell?

From PDVerse Low-Power Physical Design Mentor Guide, part of the pdVerse Mentor Guide

By Tabish Iqbal, who has 9+ years of VLSI Physical Design experience

Short Answer

A missing isolation cell leaves an output of a switchable domain driving always-on logic with nothing to hold it when the source powers down. The receiver sees a floating value, which causes crowbar current and random logic behaviour only when the domain actually turns off.

Schematic of switched PD_COP output irq_out driving an always-on PD_MYCHIP flop with no isolation cell, a red X on the net and a floating X value, next to the fixed version with an AND clamp driven by U_PC/ISE. — Technical Reference DiagramCan you describe a basic low-power implementation mistake, like a missing isolation cell?

Technical Reference DiagramCan you describe a basic low-power implementation mistake, like a missing isolation cell?

Technical Explanation

Symptom: power-aware simulation shows X on the receiver after shutdown, or silicon draws extra current and misbehaves in sleep.
Normal functional simulation and DRC/LVS pass, because they never model the source domain being off.
Detection: check_mv_design (ICC2) reports the missing cell; check_lp (VCLP) reports ISO_STRATEGY_MISSING at the UPF stage and ISO_INST_MISSING at the design stage.
Cause: usually no set_isolation (UPF) covers that port, or its filters exclude it.
Fix: write or widen the strategy with an always-on -isolation_supply, then rerun create_mv_cells (ICC2) and the checks.
Legacy form (still accepted by ICC2/PT): -isolation_power_net / -isolation_ground_net on set_isolation (UPF), plus set_isolation_control (UPF) for the signal.

# [UPF]  design.upf
set_isolation ISO_COP_OUT -domain PD_COP -applies_to outputs -isolation_supply SS_AON -clamp_value 0 -isolation_signal U_PC/ISE -isolation_sense high -location parent
# [ICC2]  icc2_shell
create_mv_cells
check_mv_design -isolation

What To Check

Every output of a switchable domain whose receiver can stay on is covered by a strategy.
The isolation supply is always-on.
The clamp value suits the receiver: 0 for active-high inputs.
The isolation signal comes from always-on logic.

Command Checks & Actions

UPF (design.upf)set_isolation ISO_COP_OUT -domain PD_COP -applies_to outputs

Cover every PD_COP output with isolation

ICC2 (icc2_shell)check_mv_design -isolation

Report crossings still missing isolation

ICC2 (icc2_shell)report_mv_path -cell U_COP/DATA_UPF_ISO

Show why a cell was or was not inserted

VC LP (vc_static_shell)check_lp -stage design

Static check of the netlist against the UPF

VC LP (vc_static_shell)report_violations -app LP

List isolation violations by tag

Healthy, Suspicious & Hard-stop Results

Healthy (illustrative): No isolation errors in check_mv_design and no ISO_* errors in check_lp.
Suspicious (illustrative): Isolation present but clamp or supply unchecked on a new port.
Hard stop: An output of PD_COP reaches PD_MYCHIP with no isolation cell.

Common Mistake

The Trap: Skipping isolation because the two domains run at the same voltage.

Isolation depends on whether the source can be off while the receiver is on, not on the voltage. Same-voltage domains still need it.

What The Interviewer Is Testing

Can you walk from symptom to detection to fix?
Do you know why normal simulation misses it?

Follow-up Question & Model Response

"Why must the isolation signal come from always-on logic?"

Candidate Model Response: The isolation cell has to hold its clamp for the whole time the source domain is off. If its enable came from the switched domain, the enable would float at the same moment the data does. The clamp would then release at random. So the power controller in the always-on domain drives it, through always-on buffers if it needs buffering.

Practical Example

Design Scenario: (illustrative) In MYCHIP, U_COP output irq_out feeds an always-on interrupt flop in PD_MYCHIP. Both run at 1.0 V, so the designer skipped it. When PD_COP turns off, irq_out floats and fires false interrupts. Adding ISO_COP_OUT with an AND clamp to 0 (ISO_AND_X1, an illustrative cell name), enabled by U_PC/ISE, fixes it.

Low-Power & UPF Handbook