Fundamentals of Isolation Cells in Low Power VLSI Design

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Isolation Cells in Multi-Voltage VLSI Design

As modern VLSI designs focus heavily on reducing power consumption, multiple voltage and power domains are commonly used. In such designs, isolation cells play a critical role in ensuring reliable communication between power domains operating in different states.

Why Are Isolation Cells Needed?

Consider a design with two voltage domains:

  • VDDA: Always ON
  • VDDB: Can be turned ON or OFF to save power

When the VDDB domain is powered down, all logic gates within that domain may produce unknown or unpredictable outputs (X states).

If signals from the powered-down VDDB domain are connected directly to logic in the active VDDA domain, these unknown values can propagate into the always-on circuitry, causing:

  • Functional errors
  • Incorrect logic evaluations
  • Potential corruption of the active domain's operation

To prevent this issue, isolation cells are inserted on signals crossing from the OFF domain to the ON domain.

What Are Isolation Cells?

Isolation cells are special logic elements used to block invalid signals coming from a powered-down domain. Instead of allowing unknown values to propagate, they force the output to a predefined safe value.

Depending on the design requirement, an isolation cell can:

  • Clamp output to Logic 0 (Pull-Down)
  • Clamp output to Logic 1 (Pull-Up)
  • Retain the last valid value (Latch-Based Isolation)

This ensures that the active domain continues to operate correctly even when another domain is switched off.

Types of Isolation Cells

1. Clamp-to-Zero (Pull-Down) Isolation Cell

These cells force the output to Logic 0 whenever isolation is enabled.

AND Gate-Based Clamp-to-Zero

The output is controlled using an AND gate:

Function:

X = A × EN

Where:

  • A = Input signal
  • EN = Isolation enable signal

When isolation is active, the output is forced to 0, preventing unknown values from propagating.

NOR Gate-Based Clamp-to-Zero

A NOR-based isolation cell uses an inverted input and is particularly useful when the isolation cell is placed within the power-gated domain itself.


Advantages:

  • Requires only a single power supply.
  • Suitable for placement in the OFF domain.
  • Does not need an additional powered supply to force the output low.

2. Clamp-to-One (Pull-Up) Isolation Cell

These cells force the output to Logic 1 when isolation is enabled.

OR Gate-Based Clamp-to-One

Function:

X = A + EN̅

Where:

  • A = Input signal
  • EN̅ = Active-low isolation control

When isolation is asserted, the output is driven to 1, ensuring a known and safe logic state.

3. Latch-Based Isolation Cell

Latch-type isolation cells not only isolate signals but can also hold the last valid value before the source power domain is turned off.

Benefits:

  • Preserves the last known state.
  • Prevents unwanted logic transitions.
  • Useful in designs requiring state retention during power-down events.

Key Design Considerations

✅ NOR-based pull-down isolation cells require only a single power supply.

✅ Latch-based isolation cells typically require dual power supplies to retain data reliably.

✅ AND-based and OR-based isolation cells may use dual supplies and can be placed near the OFF domain while remaining powered by an always-on supply.

✅ Isolation cells help prevent unknown (X) values from entering active logic and are essential for robust low-power design.

Conclusion

Isolation cells are a fundamental component of modern low-power, multi-voltage VLSI systems. By blocking or controlling signals from powered-down domains, they protect active circuitry from unknown values and ensure reliable system operation while enabling significant power savings.

Happy Learning! 🚀

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