diffrence between spare cell and unconnected cell

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Spare cells input must be tied to HIGH or LOW. And it is used for ECO.
Unconnected cells shouldn't be their in your design
 

Spare cells could be added in the RTL or by the Backend tools.
Unconnected cell is not very usefull, because you did not control their input then the power consume by this cell.
 

To Perform ECO, spare cells were been used for the older process nodes wherein they did not have any ECO libraries. So for the latest technologies or process nodes you can find the ECO libraries in place and the concept of spare cells getting erased. For the ECO, you need to have the knowledge of the exact amount of the spare cells req'd, if somehow the ECO changes are vast, then due to spare cells scarcity, you might run into spare cells shortage and will would trigger the re-iteration of Base PG.
This is the reason why spare cells are replaced by ECO libraries in latest process nodes. I hope this would be useful.
 

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