Logic used to isolate signals between two power domains where one is switched on and one is switched off. The most common usage of such cell is to isolate signals originating in a power domain that is being switched off, from the power domain that receives these signals and that remains switched on.
Isolation logic is typically used at the output of a powered-down block to prevent floating, unpowered signals (represented by unknown or X in simulation) from propagating from powered-down blocks.
The outputs of blocks being powered down need to be isolated before power can be switched off; and they need to remain isolated until after the block has been fully powered up. Isolation cells are placed between two power domains and are typically connected from domains powered off to domains that are still powered up.
In some cases, isolation cells may need to be placed at the block inputs to prevent connection to powered-down logic. If the driving domain can be OFF when the receiving domain is ON, the receiving domain needs to be protected by isolation. The isolation cells may be located in the driving domain, with special isolation cells, or they may be in the receiving domain.
ATPG tools need to understand low-power structures such as level shifters, isolation gates, and state retention power gates (SRPGs) instantiated during synthesis and target them for structural test.
Isolation cells are placed as close to the power shut-off domain as possible, but usually reside in the always-on domain.
Common problems that may occur while inserting isolation cells include placing the isolation cells in the wrong power domain or hooking up the isolation power supply to the switchable power supply instead of the always-on power supply. These are catastrophic issues!
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