Figure 3.19 shows the components of the standard cell from Figure 1.3. Each standard cell in a library is rectangular with the same height but different widths. The bounding box ( BB ) of a logic cell is the smallest rectangle that encloses all of the geometry of the cell. The cell BB is normally determined by the well layers. Cell connectors or terminals (the logical connectors ) must be placed on the cell abutment box ( AB ). The physical connector (the piece of metal to which we connect wires) must normally overlap the abutment box slightly, usually by at least 1 l , to assure connection without leaving a tiny space between the ends of two wires. The standard cells are constructed so they can all be placed next to each other horizontally with the cell ABs touching (we abut two cells).
A standard cell (a D flip-flop with clear) is shown in Figure 3.20 and illustrates the following features of standard-cell layout:
- Layout using 45° angles. This can save 10%–20% in area compared to a cell that uses only Manhattan or 90° geometry. Some ASIC vendors do not allow transistors with 45° angles; others do not allow 45° angles at all.
- Connectors are at the top and bottom of the cell on m2 on a routing grid equal to the vertical (m2) track spacing. This is a double-entry cell intended for a two-level metal process. A standard cell designed for a three-level metal process has connectors in the center of the cell.
- Transistor sizes vary to optimize the area and performance but maintain a fixed ratio to balance rise times and fall times.
- The cell height is 64 l (all cells in the library are the same height) with a horizontal (m1) track spacing of 8 l . This is close to the minimum height that can accommodate the most complex cells in a library.
- The power rails are placed at the top and bottom, maintaining a certain width inside the cell and abut with the power rails in adjacent cells.
- The well contacts (substrate connections) are placed inside the cell at regular intervals. Additional well contacts may be placed in spacers between cells.
- In this case both wells are drawn. Some libraries minimize the well or moat area to reduce leakage and parasitic capacitance.
- Most commercial standard cells use m1 for the power rails, m1 for internal connections, and avoid using m2 where possible except for cell connectors.
When a library developer creates a gate-array, standard-cell, or datapath library, there is a trade-off between using wide, high-drive transistors that result in large cells with high-speed performance and using smaller transistors that result in smaller cells that consume less power. A performance-optimized library with large cells might be used for ASICs in a high-performance workstation, for example. An area-optimized library might be used in an ASIC for a battery-powered portable computer.