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 · pdVerse Mentor Guide
Definition
The isolation control signal is the logic net that tells an isolation cell whether to pass its data input through normally or clamp its output to the fixed isolation value; because isolation cells must clamp correctly at exactly the moment the source domain shuts off, this control signal must remain driven by an always-on source even though the domain it is protecting is powered down.
Mentor Explanation
This follows a firm design rule for power gating: control logic that generates shutdown control signals must itself always be 'on,' never switched off inadvertently. This is exactly why always-on cells exist — the automatic inferencing rule traces backward from the enable pins of isolation and level-shifter cells, tracking whichever nets and cells lie on that control path, and marks them always-on until reaching an always-on region. In implementation, 'set_isolation_control' defines the isolation control signal and its location, and that isolation signal must be a net, not a port or pin; the '-isolation_sense' option specifies whether high or low asserts isolation mode (default high).
Example
If the isolation-enable signal for a shutdown block were itself sourced from inside that same shutdown block, then the moment the block powers off, the control signal would also go dead — leaving the isolation cell unable to clamp its output correctly.
Why It Matters
This exposes a subtle but critical dependency: isolation only works if its own enable path is guaranteed to be alive, which is why always-on buffering of control signals is treated as mandatory infrastructure, not an afterthought.
Command
set_isolation_control ISO_OUTPUT -domain PD_SHUTDOWN -isolation_signal
ISOLATE_CTRL -isolation_sense low -location parentCommon Beginner Mistake
Routing an isolation control signal through ordinary (non-always-on) cells inside the shutdown domain — this creates a chicken-and-egg failure where the very signal meant to protect against a dead domain dies along with it.
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