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[SOLVED] In backend , in which case, I can use the don't use cell ?

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May 27, 2022
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As I known, the vendor can define that whether the cell is dont use or not . But in backend flow , I found that the dont use cells are still used by EDA-tools . So how can I know in which case, I can use the don't use cell ?
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There is no rule. Vendors set default behavior for the cells in the library based on their own reasons, it is up to you to understand what they are and change the cell to use or no use.

Example 1: most cell library vendors mark physical cells as dont_use just so logic synthesis doesn't even try to instantiate them. Cells like fillers, decaps, and endcaps fall under this category.
Example 2: a cell library vendor marks a specific mux cell as dont_use because it has a lot of pins and causes pin access issues during routing. However, the cell is very very fast and in many cases you want to use it.
Example 3: power gating/power management cells are marked as dont_use. Examples are ISO cells that have the same behavior as a buffer. Yet, you don't want them to appear in the middle of your logic, you want to instantiate them by hand when doing your UPF definitions. Here the behavior being use/dontuse is irrelevant, they will be used.
Example 4: scan flops are sometimes marked as dont_use so synthesis only infers DFFs. Later, when doing the DFT steps, you tell the tools to replace all DFFs for SDFFs. The use/dont_use behavior is ignored at this point.
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