Standard cell

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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. Silicon chip 3d.png
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.

Application-specific integrated circuit

An application-specific integrated circuit is an integrated circuit (IC) customized for a particular use, rather than intended for general-purpose use. For example, a chip designed to run in a digital voice recorder or a high-efficiency bitcoin miner is an ASIC. Application-specific standard products (ASSPs) are intermediate between ASICs and industry standard integrated circuits like the 7400 series or the 4000 series.

Integrated circuit layout representation of an integrated circuit in terms of planar geometric shapes which correspond to the patterns of metal, oxide, or semiconductor layers that make up the components of the integrated circuit

Integrated circuit layout, also known IC layout, IC mask layout, or mask design, is the representation of an integrated circuit in terms of planar geometric shapes which correspond to the patterns of metal, oxide, or semiconductor layers that make up the components of the integrated circuit.

System on a chip type of integrated circuit

A system on a chip or system on chip is an integrated circuit that integrates all components of a computer or other electronic system. These components typically include a central processing unit (CPU), memory, input/output ports and secondary storage – all on a single substrate. It may contain digital, analog, mixed-signal, and often radio frequency signal processing functions, depending on the application. As they are integrated on a single electronic substrate, SoCs consume much less power and take up much less area than multi-chip designs with equivalent functionality. Because of this, SoCs are very common in the mobile computing and edge computing markets. Systems on chip are commonly used in embedded systems and the Internet of Things.

Contents

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).

The AND gate is a basic digital logic gate that implements logical conjunction - it behaves according to the truth table to the right. A HIGH output (1) results only if all the inputs to the AND gate are HIGH (1). If none or not all inputs to the AND gate are HIGH, a LOW output results. The function can be extended to any number of inputs.

The OR gate is a digital logic gate that implements logical disjunction – it behaves according to the truth table to the right. A HIGH output (1) results if one or both the inputs to the gate are HIGH (1). If neither input is high, a LOW output (0) results. In another sense, the function of OR effectively finds the maximum between two binary digits, just as the complementary AND function finds the minimum.

An adder is a digital circuit that performs addition of numbers. In many computers and other kinds of processors adders are used in the arithmetic logic units or ALU. They are also utilized in other parts of the processor, where they are used to calculate addresses, table indices, increment and decrement operators, and similar operations.

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 (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.

In electronic design, a netlist is a description of the connectivity of an electronic circuit. In its simplest form, a netlist consists of a list of the electronic components in a circuit and a list of the nodes they are connected to. A network (net) is a collection of two or more interconnected components.

SPICE is a general-purpose, open-source analog electronic circuit simulator. It is a program used in integrated circuit and board-level design to check the integrity of circuit designs and to predict circuit behavior.

Spectre is a SPICE-class circuit simulator. It provides the basic SPICE analyses and component models. It also supports the Verilog-A modeling language. Spectre comes in enhanced versions that also support RF simulation (SpectreRF) and mixed-signal simulation.

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 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.

Place and route is a stage in the design of printed circuit boards, integrated circuits, and field-programmable gate arrays. As implied by the name, it is composed of two steps, placement and routing. The first step, placement, involves deciding where to place all electronic components, circuitry, and logic elements in a generally limited amount of space. This is followed by routing, which decides the exact design of all the wires needed to connect the placed components. This step must implement all the desired connections while following the rules and limitations of the manufacturing process.

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.

In electronic design automation, parasitic extraction is calculation of the parasitic effects in both the designed devices and the required wiring interconnects of an electronic circuit: parasitic capacitances, parasitic resistances and parasitic inductances, commonly called parasitic devices, parasitic components, or simply parasitics.

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.

Synopsys is an American company. Synopsys' first and best-known product is Design Compiler, a logic-synthesis tool. Synopsys offers a wide range of other products used in the design of an application-specific integrated circuit. Products include logic synthesis, behavioral synthesis, place and route, static timing analysis, formal verification, hardware description language simulators as well as transistor-level circuit simulation. The simulators include development and debugging environments which assist in the design of the logic for chips and computer systems. In recent years Synopsys has also expanded into the application security market.

Verilog, standardized as IEEE 1364, is a hardware description language (HDL) used to model electronic systems. It is most commonly used in the design and verification of digital circuits at the register-transfer level of abstraction. It is also used in the verification of analog circuits and mixed-signal circuits, as well as in the design of genetic circuits. In 2009, the Verilog standard was merged into the SystemVerilog standard, creating IEEE Standard 1800-2009. Since then, Verilog is officially part of the SystemVerilog language. The current version is IEEE standard 1800-2017.

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, INVERT, 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:

  1. 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.
  2. 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:

An example is a simple XOR logic gate, which can be formed from OR, INVERT 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. This variety enhances the efficiency of automated synthesis, place, and route (SPR) tools. Indirectly, it also gives the designer greater freedom to perform implementation trade-offs (area vs. speed vs. power consumption). A complete group of standard-cell descriptions is commonly called a technology library.

Commercially available Electronic Design Automation (EDA) tools use the technology libraries to automate synthesis, placement, and routing of a digital ASIC. The technology library is developed and distributed by the foundry operator. The library (along with a design netlist format) is the basis for exchanging design information between different phases of the SPR process.

Synthesis

Using the technology library's cell logical view, the Logic Synthesis tool performs the process of mathematically transforming the ASIC's register-transfer level (RTL) description into a technology-dependent netlist. This process is analogous to a software compiler converting a high-level C-program listing into a processor-dependent assembly-language listing.

The netlist is the standard-cell representation of the ASIC design, at the logical view level. It consists of instances of the standard-cell library gates, and port connectivity between gates. Proper synthesis techniques ensure mathematical equivalency between the synthesized netlist and original RTL description. The netlist contains no unmapped RTL statements and declarations.

The high-level synthesis tool performs the process of transforming the C-level models (SystemC, ANSI C/C++) description into a technology-dependent netlist.

Placement

The placement tool starts the physical implementation of the ASIC. With a 2-D floorplan provided by the ASIC designer, the placer tool assigns locations for each gate in the netlist. The resulting placed gates netlist contains the physical location of each of the netlist's standard-cells, but retains an abstract description of how the gates' terminals are wired to each other.

Typically the standard cells have a constant size in at least one dimension that allows them to be lined up in rows on the integrated circuit. The chip will consist of a huge number of rows (with power and ground running next to each row) with each row filled with the various cells making up the actual design. Placers obey certain rules: Each gate is assigned a unique (exclusive) location on the die map. A given gate is placed once, and may not occupy or overlap the location of any other gate.

Routing

Using the placed-gates netlist and the layout view of the library, the router adds both signal connect lines and power supply lines. The fully routed physical netlist contains the listing of gates from synthesis, the placement of each gate from placement, and the drawn interconnects from routing.

DRC/LVS

Simulated lithographic and other fabrication defects visible in a small standard cell. Eda-fabrication.PNG
Simulated lithographic and other fabrication defects visible in a small standard cell.

Design Rule Check (DRC) and Layout Versus Schematic (LVS) are verification processes. Reliable device fabrication at modern deep-submicrometer (0.13 µm and below) requires strict observance of transistor spacing, metal layer thickness, and power density rules. DRC exhaustively compares the physical netlist against a set of "foundry design rules" (from the foundry operator), then flags any observed violations.

The LVS process confirms that the layout has the same structure as the associated schematic; this is typically the final step in the layout process. The LVS tool takes as an input a schematic diagram and the extracted view from a layout. It then generates a netlist from each one and compares them. Nodes, ports, and device sizing are all compared. If they are the same, LVS passes and the designer can continue. LVS tends to consider transistor fingers to be the same as an extra-wide transistor. Thus, 4 transistors (each 1 μm wide) in parallel, a 4-finger 1 μm transistor, or a 4 μm transistor are viewed the same by the LVS tool. The functionality of .lib files will be taken from SPICE models and added as an attribute to the .lib file.

Other cell-based methodologies

"Standard cell" falls into a more general class of design automation flows called cell-based design. Structured ASICs, FPGAs, and CPLDs are variations on cell-based design. From the designer's standpoint, all share the same input front end: an RTL description of the design. The three techniques, however, differ substantially in the details of the SPR flow (Synthesize, Place-and-Route) and physical implementation.

Complexity measure

For digital standard cell designs, for instance in CMOS, a common technology-independent metric for complexity measure is gate equivalents (GE).

See also

The standard cell areas in a CBIC are build-up of rows of standard cells, like a wall built-up of bricks

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Very Large Scale Integration process of creating an integrated circuit by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed

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In computer engineering, a hardware description language (HDL) is a specialized computer language used to describe the structure and behavior of electronic circuits, and most commonly, digital logic circuits.

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Silvaco

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OrCAD electronic design automation software

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Integrated circuit design Engineering process for electronic hardware

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Physical design (electronics)

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The Gajski-Kuhn chart depicts the different perspectives in VLSI hardware design. Mostly, it is used for the development of integrated circuits. Daniel Gajski and Robert Kuhn developed it in 1983. In 1985, Robert Walker and Donald Thomas refined it.

NanGate, Inc is a privately held US/Silicon Valley-based multinational corporation dealing in Electronic Design Automation (EDA) for electrical engineering and electronics. NanGate was founded in October 2004 by a group of semiconductor professionals with a background from Intel Corporation and Vitesse Semiconductor Corp. The company has received capital investments from a group of Danish business angels and venture capital companies. The company is today owned and controlled by its management following a management buy-out in 2012. NanGate markets a range of software products and design services for the design and optimization of standard cell libraries and application-specific integrated circuits. The market focus is standard cell library design and optimization for 14-28 nanometer CMOS processes.