What are spare cells and why the heck do we need them?
Spare cells are basically elements embedded in the design which are not driving anything. The idea is that maybe they will enable an easy (metal) fix without the need of a full redesign.
Sometimes not everything works after tape-out, a counter might not be reseted correctly, a control signal needs to be additionally blocked when another signal is high etc. These kind of problems could be solved easily if “only I would have another AND gate here…”
Spare cells aim to give a chance of solving those kind of problems. Generally, the layout guys try to embed in the free spaces of the floor-plan some cells which are not driving anything. There is almost always free space around, and adding more cells doesn’t cost us in power (maybe in leakage in newer technologies), area (this space is anyhow there) or design time (the processes is 99% automatic).
Having spare cells might mean that we are able to fix a design for a few 10K dollars (sometimes less) rather than a few 100K.
So which spare cells should we use? It is always a good idea to have a few free memory elements, so I would recommend on a few flip-flops. Even a number as low as 100 FF in a 50K FF design is usually ok. Remember, you are not trying to build a new block, but rather to have a cheap possibility for a solution by rewiring some gates and FFs.
What gates should we through in? If you remember some basic boolean algebra, you know that NANDs and NORs can create any boolean function! This means that integrating only NANDs or NORs as spare cells would be sufficient. Usually, both NANDs and NORs are thrown in for more flexibility. 3 input, or even better 4 input NANDs and NORs should be used.
A small trick is tying the inputs of all NANDs to a logical “1” and all inputs of the NORs to a logical “0”. This way if you decide to use only 2 of the 4 inputs the other inputs do not affect the output (check it yourself), this in turn means less layout work when tying and untying the inputs of those spare cells.
The integration of spare cells is usually done after the synthesis step and in the verilog netlist basically looks like an instantiation of library cells. This should not done before, since the synthesis tool will just optimize all those cells away as they drive nothing. The layout guy has to somehow by feeling (or black magic) spread the spare cells around in an even way.
I believe that when an ECO (Engineering Change Order) is needed and a metal-fix is considered – this is where our real work as digital designers start. I consider ECOs, and in turn the use of spare cells to solve or patch a problem, as the epitome our usage of skills, experience, knowledge and creativity!
More on ECOs will be written in the future…