What happens if an isolation cell has the wrong clamp value, and how would this surface during verification?
From PDVerse Low-Power Physical Design Mentor Guide · pdVerse Mentor Guide
Technical Explanation
set_isolation's -clamp_value option (0, 1, or latch) determines what value the isolated output is forced to when isolation is active: 0 uses an AND-gate-style clamp, 1 uses an OR-gate-style clamp, and latch holds whatever value the non-isolated port had at the moment isolation activated. If the clamp value specified doesn't match what the downstream logic actually expects when the domain is shut down, the isolated output will present the wrong constant (or a stale latched value) to always-on logic during shutdown — a functional correctness bug, not merely a mapping issue.
Architecture-level Reasoning
The clamp value is a purely logical/electrical choice made by the strategy author; the tool will happily map and insert whichever clamp type you asked for, so a wrong clamp value is invisible to feasibility mapping and only shows up as a functional or verification-level problem once the isolated signal's expected behavior is checked against what's actually driven.
Step-by-step Walkthrough
1) The isolation strategy is declared with a chosen clamp value, e.g. set_isolation ISO_OUTPUT -domain PD_SHUTDOWN isolation_power_net VDDG -isolation_ground_net VSS -clamp_value 0 -applies_to outputs. 2) If downstream always-on logic actually requires the isolated signal to clamp to 1 (or latch) during shutdown but the strategy specifies 0, the domain's outputs present a logic-0 instead of the required value once isolation engages. 3) A GUI can distinguish clamp variants visually — Clamp 0, Clamp 1, Clamp Latch, and No isolation are each a distinct symbol — so a visual/report inspection of the isolation cell's clamp value against the intended behavior is one direct way to catch the mismatch. 4) report_mv_path -pin/-net/-cell on the isolated signal reports its strategy/domain/clamp details for correct cells, letting you compare the reported clamp against the design's functional requirement.
Command
set_isolation ISO_OUTPUT -domain PD_SHUTDOWN -isolation_power_net
VDDG -isolation_ground_net VSS -clamp_value 0 -applies_to outputsSwitch-by-switch
-clamp_value 0: AND-gate-style clamp, forces isolated output to 0
when isolation is active. -clamp_value 1: OR-gate-style clamp,
forces isolated output to 1. -clamp_value latch: latches the nonisolated port's value at the moment isolation becomes active.Expected Result
The isolated output presents exactly the constant (or latched) value that downstream always-on logic expects during domain shutdown, matching the functional requirement the clamp was chosen for.
Possible Implementation Error
Specifying -clamp_value 0 when the downstream always-on consumer's safe/expected idle value is actually 1, so once the domain shuts down the consumer sees an incorrect logic-0 and behaves as if an active condition were asserted.
Likely Tool / Clp Warning
There is no specific verification error code for a wrong clamp value; the GUI's isolation-cell symbol variants (Clamp 0/Clamp 1/ Clamp Latch/No isolation) are the documented visual mechanism for inspecting what clamp value is actually applied.
Root Cause
-clamp_value is a purely author-specified logical choice with no automatic cross-check against what downstream logic functionally requires; a wrong value is syntactically valid and maps to a real library cell without complaint.
Debugging Sequence
1) Identify the functional requirement for the isolated signal's value during shutdown from the design intent. 2) Use report_mv_path on the relevant pin/net/cell to see the strategy's actual clamp value. 3) Inspect the GUI's isolation cell symbol (Clamp 0/1/Latch) for a visual cross-check. 4) Correct the clamp_value argument in set_isolation if it doesn't match the required behavior. A wrong clamp value is a functional-correctness defect that would only be caught by behavioral/formal verification against the intended shutdown behavior, not by structural UPF checks — it must be resolved before functional signoff.
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