Spare Cells

We have discussed in our post titled 'Engineering Change Order' about the important to have a uniform distribution of spare cells in the design. Nowadays, there is a trend among the VLSI corporations to implement metal-only functional and timing ECOs due to their low-cost. Let us discuss about the spare cells in a bit more detail here.

Figure showing spare cells in the design

Spare cells are put onto the chip during implementation keeping into view the possibility of modifications that are planned to be carried out into the design without disturbing the layers of base. This is because carrying out design changes with minimal layer changes saves a lot of cost from fabrication point of view as each layer mask has a significant cost of its own. Let us start by defining what a spare cell is. A spare cell can be thought of as a redundant cell that is not used currently in the design. It may be in use later on, but currently, it is sitting without doing any job. A spare cell does not contribute to the functionality of the device. We can compare a spare cell with a spare wheel being carried in a motor car to be used in case one of the wheels gets punctured. In that case, the spare wheel will be replacing the main wheel. Similarly, a spare cell can be used to replace an existing cell if the situation demands (eg. to meet the timing). However, unlike spare wheels, spare cells may be added to the design even if they do not replace any existing cell according as the need arises.

Kinds of spare cells: There are many variants of spare cells in the design. Designs are full of spare inverters, buffers, nand, nor and specially designed configurable spare cells. However, based on the origin of spare cells, these can be divided into two broad categories:

• Those used deliberately as spare cells in the design: As discussed earlier, most of the designs today have spare cells sprinkled uniformly. These cells have inputs and outputs tied to either ‘0’ or ‘1’ so that they contribute minimum to static and dynamic power.
• Those converted into spare cells due to design changes: There may be a case that a cell that is being identified as a spare now was a main cell in the past. Due to some design changes, the cell might have been replaced by another cell. Also, some cells have floating outputs. These can be used as spare cells. We can also use the used buffers as spare cells if removing the buffer does not introduce any setup/hold violation in the design.

Advantages of using spare cells in the design: Introduction of spare cells into the design offers several advantages such as:

• Reusability: A design change can be carried out using metal layers only. So, the base layers can be re-used for fabrication of new chips.
• Cost reduction: Significant amount of money is saved both in terms of engineering and manufacture costs.
• Design flexibility: As there are spare cells, small changes can be taken into the design without much difficulty. Hence, the presence of spare cells provides flexibility to the design.
• Cycle time reduction: Nowadays, there is a trend to tape out base layers to the foundry for fabrication as masks are not prepared in parallel. In the meantime, the timing violations/design changes are being carried out in metal layers. Hence, there is cycle time reduction of one to two weeks.

Disadvantages of using spare cells: In addition to many advantages, usage of spare cells offers some disadvantages too. These are:

• Contribution to static power: Each spare cell has its static power dissipation. Hence, greater amount of spare cells contribute more to power. But, in general, this amount of power is insignificant in comparison to total power. Spare cells should be added keeping into consideration their contribution to power.
• Area: Spare cells occupy area on the chip. So, more spare cells mean more density of cells.

Thus, we have discussed about the spare cells here. Spare cells are used almost in every design in each device manufactured today. It is important to make an intelligent selection of spare cells to be sprinkled in the design. Many technical papers have been published stating its importance and on the structure of the spare cells that can be generalized to be used as any of the logic gate. In general, a collection of nand/nor/inverters/buffers is sprinkled more or less uniformly. The modules where more number of ECOs are expected, (like a new architecture being used for the first time) should be sprinkled with more spare cells. On the contrary, those having stable architectures are usually sprinkled with less number of spare cells as the probability of an ECO is very less in the vicinity of these modules/macros.

I hope you’ve found this post useful. Let me know what you think in the comments. I’d love to hear from you all.