right|thumb|300px|A rendering of a small standard cell with three metal layers ([[dielectric has been removed). The sand-colored structures are metal interconnect, with the vertical pillars being contacts, typically plugs of tungsten. The reddish structures are polysilicon gates, and the solid at the bottom is the crystalline silicon bulk.]]
In semiconductor design, standard-cell methodology is a method of designing application-specific integrated circuits (ASICs) with mostly digital-logic features. Standard-cell methodology is an example of design abstraction, whereby a low-level very-large-scale integration (VLSI) layout is encapsulated into an abstract logic representation (such as a NAND gate).
Cell-based methodology – the general class to which standard cells belong – makes it possible for one designer to focus on the high-level (logical function) aspect of digital design, while another designer focuses on the implementation (physical) aspect. Along with semiconductor manufacturing advances, standard-cell methodology has helped designers scale ASICs from comparatively simple single-function ICs (of several thousand gates), to complex multi-million gate system-on-a-chip (SoC) devices.
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 to simulate the electronic behavior of the netlist, by declaring input stimulus (voltage or current waveforms) and then calculating the circuit's time domain (analog) 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 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.
Library
A standard-cell library is a collection of low-level electronic logic functions such as AND, OR, NOT, flip-flops, latches, and buffers. These cells are realized as fixed-height, variable-width full-custom cells. The key aspect with these libraries is that they are of a fixed height, which enables them to be placed in rows, easing the process of automated digital layout. The cells are typically optimized full-custom layouts, which minimize delays and area.
A typical standard-cell library contains two main components:
- Library database - consists of a number of views often including layout, schematic, symbol, abstract, and other logical or simulation views. From this, various information may be captured in a number of formats including the Cadence LEF format, and the Synopsys Milkyway format, which contain reduced information about the cell layouts, sufficient for automated place and route tools.
- Timing abstract - generally in Liberty format, to provide functional definitions, timing, power, and noise information for each cell.
A standard-cell library may also contain the following additional components:
- A full layout of the cells
- SPICE models of the cells
- Verilog models or VHDL-VITAL models
- parasitic extraction models
- DRC rule decks
An example is a simple XOR logic gate, which can be formed from OR, NOT and AND gates.
Application of standard cell
Strictly speaking, a 2-input NAND or NOR function is sufficient to form any arbitrary Boolean function set. But in modern ASIC design, standard-cell methodology is practiced with a sizable library (or libraries) of cells. The library usually contains multiple implementations of the same logic function, differing in area and speed.