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日志 [2017年03月18日]Power Management Cells

已有 792 次阅读| 2017-3-18 12:58 |个人分类:PR|系统分类:芯片设计

Power Management Cells

Voltage differences and shutdown design partitions demand special cells to maintain the electrical integrity of the design. These power management cells are complex circuits and typically have multiple power pins. Liberty syntax is enhanced with additional concepts to model these cells correctly.

- Isolation Cells

When a design partition is shutdown, the outputs from the shutdown partition into the active partition need to be maintained at predictable signal levels. This signal isolation is achieved by using a special cell called an isolation cell. The isolation logic ensures that all inputs to the active partition are clamped to a fixed value.

- Level Shifters

In a multivoltage design, at the voltage interface, a signal connects to logic at different voltage levels. To transit the signals correctly, level shifter cells are used for all the nets at the voltage interface.

In multivoltage and shutdown designs, both level shifting and isolation are required. An enable level shifter cell serves both these requirements as it can function as an isolation cell or as a level shifter cell.

- Retention Registers

When a partition is shut down and later powered-up again, it is often necessary for the partition to resume operation based on its last good state. One commonly used design technique to implement this is to use special registers that can store the state during shutdown mode. These special registers are referred to as state-retention registers or simply retention registers.

- Power Gating Cells

When the design partitions are shut down, the standard cells are disconnected from their power or ground sources, thus reducing leakage current flow. This is fundamentally achieved by adding power gating cells to the power or ground network. These cells act as switches to connect or disconnect the power supplies.

- Always-On Cells

When dealing with shutdown partitions, there can be situations where certain cells in the shutdown partition need to stay active continuously. This type of logic is referred to as always-on logic. Some examples include retention registers, isolation cells and the logic in the retention control and isolation enable paths.

This type of logic is generally synthesized with special cells called always-on cells. The basic difference between an always-on cell and a regular cell is that the always- on cell has an extra power supply that serves as back-up during shut-down mode.