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whenever it is required to perform some functional ECO, spare cells would be used .. these are extra cells, floating in an ASIC .. and if you want to include some more functionality, after chips is taped-out, you can use these cells to get avail of required functionality ..
The key in having spare cells in your design is that you only need to change the metal layers in order to rewire the logic and fix any bugs. This means you only need to pay for new metal masks, thus SAVING MONEY.
After a 1.0 tapeout, a 1.1 tapeout means you only changed some metal layers while 2.0 tapeout means you changed your base layers as well.
Re: what is Spare-cells ?where i can use those cells
By default the PnR tool, connect the inputs pins to a tie cell high or low.
In our flow there are two type of spare cells:
1- the spare cells added during the PnR flow.
2- the RTL code not needed but present in case, some bank of registers already mapped but without any functions which are already connected to the analog macro, in case the analog macro, need more cfg, and in the other way, some address to read back some nets from the analog, in case, we want check/debug others stuffs. And of course the level shifter to be able to access all differents analog power domain, and also all default reset values.