Standard Cell Library for ASIC Design

• Standard cells are designed based on power, area, and performance(PPA), which is used in digital cell libraries. Standard cells are the pre-defined, pre-characterized, and pre-verified cells, Standard cell architecture is defined based on cell height which is determined on the basis of a number of tracks, beta-ratio, pitch, and transistor widths.

How do you decide the height of Standard cells?

The track is generally used as a unit to define the height of the std cell. The track can be related to lanes.

Standard cell height = Pitch * (N-1) where N represents the number of tracks. 

What is Pitch?

The distance between two tracks is called a pitch.

Component of generic library

1. Basic gates (AND, OR, NAND, NOR, INV, EXOR, EXNOR)
2. MUX
3. AOI
4. OAI
5. Fillers, Tap cells, End Cap, Decap cells
6. Tie CellsBoolean function cells
7. Flip-Flops

Tap cells

• Tap cells are used to provide substrate connection.
• They are used to avoid latch-up violations.
• They connect n-well to VDD and p-sub to VSS.
• It is also used to fill the gap between two std cells.

Tie cell

• Tie cells are used to avoid direct gate connection to the power or ground network thereby protecting the cell from damage.
• In Design, some cell inputs may require a logic 0 or logic 1 value. Instead of connecting these to the VDD/VSS rails/rings, you connect them to special cells available in your library called tie cells library.
• There are the following two types of Tie cells
• Tie High cell:  Tie high cell, nmos acts as diode-connected and gives logic 0 to the gate of pmos, so we will get logic 1 as output
• Tie Low cell: Tie low cell, pmos act as diode-connected and give logic 1 to the gate of nmos, so we will get logic 0 as output.

Filler cells

• Filler cells are used to provide power/ground rail continuity, and n-well continuity so that DRC violations will not come
• They contain n-well and p substrate.
• Filler cells primarily are non-functional cells used

Decap cells 

• Decap cells also called Decoupling capacitor cells
• When there is a drop in power rail, these cells act as a battery and maintain the voltage across rails.
• These cells aid the IR drop issue and remove glitches in power.
• Decap cells are capacitors added in the design between power and ground rails.

End cap cells 

• End cap cells are added near the end of rows to terminate the rows properly.
• The n-wells of end cap cells are properly terminated within the cell.
• End cap cells are used for DRC purpose

What is a standard cell used for?

A standard cell is a collection of transistor and interconnect structures that performs a specific boolean logic function (such as AND, OR, XOR, XNOR, or inverters) or a storage function (flip-flop or latch).