Standard Cell - Construction of A Standard Cell

Construction of A Standard Cell

A standard cell is a group of transistor and interconnect structures that provides a boolean logic function (e.g., AND, OR, XOR, XNOR, inverters) or a storage function (flipflop or latch). The simplest cells are direct representations of the elemental NAND, NOR, and XOR boolean function, although cells of much greater complexity are commonly used (such as a 2-bit full-adder, or muxed D-input flipflop.) The cell's boolean logic function is called its logical view: functional behavior is captured in the form of a truth table or Boolean algebra equation (for combinational logic), or a state transition table (for sequential logic).

Usually, the initial design of a standard cell is developed at the transistor level, in the form of a transistor netlist or schematic view. The netlist is a nodal description of transistors, of their connections to each other, and of their terminals (ports) to the external environment. A schematic view may be generated with a number of different Computer Aided Design (CAD) or Electronic Design Automation(EDA) programs that provide a Graphical User Interface (GUI) for this netlist generation process. Designers use additional CAD programs such as SPICE or Spectre to simulate the electronic behavior of the netlist, by declaring input stimulus (voltage or current waveforms) and then calculating the circuit's time domain (analogue) response. The simulations verify whether the netlist implements the desired function and predict other pertinent parameters, such as power consumption or signal propagation delay.

Since the logical and netlist views are only useful for abstract (algebraic) simulation, and not device fabrication, the physical representation of the standard cell must be designed too. Also called the layout view, this is the lowest level of design abstraction in common design practice. From a manufacturing perspective, the standard cell's VLSI layout is the most important view, as it is closest to an actual "manufacturing blueprint" of the standard cell. The layout is organized into base layers, which correspond to the different structures of the transistor devices, and interconnect wiring layers and via layers, which join together the terminals of the transistor formations. The interconnect wiring layers are usually numbered and have specific via layers representing specific connections between each sequential layer. Non-manufacturing layers may be also be present in a layout for purposes of Design Automation, but many layers used explicitly for Place and route (PNR) CAD programs are often included in a separate but similar abstract view. The abstract view often contains much less information than the layout and may be recognizable as a Layout Extraction Format (LEF) file or an equivalent.

After a layout is created, additional CAD tools are often used to perform a number of common validations. A Design Rule Check (DRC) is done to verify that the design meets foundry and other layout requirements. A Parasitic EXtraction (PEX) then is performed to generate a PEX-netlist with parasitic properties from the layout. The nodal connections of that netlist are then compared to those of the schematic netlist with a Layout Vs Schematic (LVS) procedure to verify that the connectivity models are equivalent.

The PEX-netlist may then be simulated again (since it contains parasitic properties) to achieve more accurate timing, power, and noise models. These models are often characterized (contained) in a Synopsys Liberty format, but other Verilog formats may be used as well.

Finally, powerful Place and Route (PNR) tools may be used to pull everything together and synthesize (generate) Very Large Scale Integration (VLSI) layouts, in an automated fashion, from higher level design netlists and floor-plans.

Additionally, a number of other CAD tools may be used to validate other aspects of the cell views and models. And other files may be created to support various tools that utilize the standard cells for a plethora of other reasons. All of these files that are created to support the use of all of the standard cell variations are collectively known as a standard cell library.

For a typical Boolean function, there are many different functionally equivalent transistor netlists. Likewise, for a typical netlist, there are many different layouts that fit the netlist's performance parameters. The designer's challenge is to minimize the manufacturing cost of the standard cell's layout (generally by minimizing the circuit's die area), while still meeting the cell's speed and power performance requirements. Consequently, integrated circuit layout is a highly labor intensive job, despite the existence of design tools to aid this process.