Posted by Godwin Maben on January 28th, 2010
There were many questions on why output isolation is preferred over input isolation logic, sorry could not get time to respond to all the queries related to this. Here is my view point on this
Output signal isolation method is usually a preferred choice than the input isolation method as former leads to fewer isolation cells and gives better control over how the enable signal can be propagated to the isolation logic.
Input isolation will be a better choice if there are very few independent domains. In such situation isolation enable signal management is very straight-forward. Here number of power domain is not the main component, its the sequence in which they are powered down and up. In this case isolation cell is only required when the power island is active. During the power down period, the power island is no longer functional and no power is supplied to the island. Therefore, neither the floating inputs nor the input transient state matters to the power island, which is overall good from total power perspective.
For this method, probably regular standard cells (NAND and NOR) can be used as isolation cells rather than requiring a custom cell.