Physical Design - Decap Cells

Throughout the physical cells serial article, we have seen Filler cells, Boundary cell and Tap cells. Today are going to discuss the last cell in this series named Decap Cells.

 

Decap Cells

• Decap cells also known as Decoupling capacitor cell. 
• These cells don’t have any logical functionality. 
• Decap cell is a capacitor which is added in the design between the power rails. 
• At the active edge of the clock, sequential cells are consumed power. 
• The voltage source is never ideal and hence, there is a huge current drawn from the power grid at the active edge of the clock. 
• These power rails are made of metal and metal has its own resistance and inductance. 
• This leads to the voltage drop across the power rails. 
• To avoid this voltage drop, we used Decap Cells which is placed throughout the power rails to provides extra charge to standard cells. 
• We have placed the Decap cells with precaution because they are very leaky.

*Figure: Decap Cells (Reference: paper)

In the new generation of EDA tool, there is a particular Decap cell insert flow which provides the optimum placement of Decap Cells after analysing power mesh/ring/grids and cell density.

 

At this note of discussion, I wrapped this topic here and we will meet soon with a new topic of Placement. Thank you. Have a nice day!!!!!