What is an isolation control signal, and why must it stay "alive" even when a domain shuts down?
From PDVerse Low-Power Physical Design Mentor Guide, part of the pdVerse Mentor Guide
Short Answer
The isolation control signal is the enable that tells isolation cells when to clamp. It must stay driven while the protected domain is off, because the clamp has to hold for the entire shutdown. So it comes from always-on logic and is buffered only by always-on cells along its whole route.
Technical Explanation
- The isolation control signal (often ISE) switches isolation cells between pass-through and clamp.
- It is driven by the power controller, which lives in an always-on domain.
- If the net routes across a shutdown area, any buffer on it must be an always-on buffer with a backup supply.
- In IEEE 1801-2015 you name it on
set_isolation(UPF) with-isolation_signaland-isolation_sense. - Legacy form (still accepted by ICC2/PT):
set_isolation_control(UPF) with-isolation_signaland-isolation_sense. - If the signal dies with the domain, the clamp releases and the always-on side sees invalid values.
- It must also meet timing: the enable has to reach every isolation cell before the switch starts to open.
Common Mistake
The Trap: Generating ISE inside the domain it protects, or buffering it with ordinary cells on the switched rail.
- When the domain powers down, the enable floats, the clamp lets go, and the fault only appears in power-aware simulation or silicon.
- Static checks exist for this:
check_lp(VCLP) flags a control signal corrupted by an off rail on its way to the sink underCORR_CONTROL_STATE.
Follow-up Question & Model Response
"How does ICC2 make sure buffers on the isolation enable are always-on?"
Candidate Model Response: The ICC2 MV guide calls nets like this always-on paths, and isolation enable paths are one of its examples. When such a path crosses a shutdown voltage area and needs buffering, the tool uses always-on buffers and inverters. It performs this optimisation only when the library contains such cells, marked with the always_on attribute. Save and restore nets get the same treatment. So check the library first, then run the multivoltage checks to confirm every enable path.
Practical Example
Design Scenario: (illustrative) In MYCHIP, U_PC in PD_MYCHIP drives ISE to the isolation cells on PD_COP outputs. The net crosses 400 µm of the PD_COP area, so it needs two buffers there. Both are dual-rail always-on buffers taking power from VDD1p0 through secondary pins. The IEEE 1801-2015 strategy names U_PC/ISE as the isolation signal with sense high. ISE rises at least one clock before PSE drops, so every clamp is set before VDD1p0_SW starts to fall. If U_PC sat inside PD_COP instead, nothing could release isolation or turn the domain back on.
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