An isolation cell is a special standard cell that clamps its output to a known safe value (logic 0 or logic 1) when its associated power domain is shut down — preventing the unpowered domain's floating, undefined outputs from corrupting the logic of neighboring powered-on domains.

Why Isolation Is Necessary

• When a power domain is gated off (power switches disconnected), the flip-flops and gates in that domain lose their supply voltage.
• The outputs of the powered-off domain become undefined — they may float to any voltage, oscillate, or settle at intermediate levels.
• These garbage values propagate to the powered-on logic connected to them, causing:
• Functional Errors: Downstream logic receives random inputs → incorrect computation.
• Short-Circuit Current: Intermediate voltage levels at receiver inputs cause both PMOS and NMOS to conduct → excessive current draw.
• Latch-Up Risk: Unexpected voltage levels can trigger parasitic SCR paths.
• Isolation cells clamp the output to a defined value before the domain powers down, and hold it there throughout the power-off period.

Isolation Cell Operation

• Normal Mode (ISO = 0): The isolation cell is transparent — it passes the input signal to the output like a buffer.
• Isolation Mode (ISO = 1): The output is forced to a fixed value (0 or 1) regardless of the input.
• Sequencing: The isolation signal must be asserted before the power switches turn off, and de-asserted after the power domain is fully powered up.

Isolation Cell Types

• Clamp-Low Isolation: Output forced to logic 0 during isolation. Uses AND-based logic: output = data AND (NOT ISO).
• Clamp-High Isolation: Output forced to logic 1. Uses OR-based logic: output = data OR ISO.
• Latch Isolation: The output latches the last valid value before power-down — preserves the most recent state instead of forcing 0 or 1.

Isolation Cell Power Supply

• The isolation cell must remain powered on while the source domain is off — it is connected to the always-on supply (or the receiving domain's supply).
• The input side connects to the powered-off domain.
• The output side connects to the powered-on domain.

Isolation in the Design Flow

• UPF/CPF: The power intent file specifies which domain boundaries need isolation, the isolation type (clamp-0 or clamp-1), and the isolation control signal.
• Automatic Insertion: Synthesis/P&R tools insert isolation cells at every output port of a power-gated domain.
• Placement: Typically placed at the boundary between power domains — on the "always-on" side.
• Verification: Power-aware verification tools check that:
• Every output of a power-gated domain has an isolation cell.
• The isolation control signal is asserted in the correct sequence relative to power switching.
• The clamped value is functionally correct for the receiving logic.

Isolation cells are essential safety infrastructure for power-gated designs — they prevent the chaos of floating signals from propagating across domain boundaries and corrupting the chip's functional behavior.