Standard cell

Last updated
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.

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). [1] 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 (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. [1] 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.

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. [2]

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, 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: [3]

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. [3] 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. [3]

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. [2] 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. [2] 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

Related Research Articles

In electronics, a logic gate is an idealized or physical device implementing a Boolean function; that is, it performs a logical operation on one or more binary inputs and produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and unlimited fan-out, or it may refer to a non-ideal physical device.

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

Very large-scale integration (VLSI) is the process of creating an integrated circuit (IC) by combining millions of MOS transistors onto a single chip. VLSI began in the 1970s when MOS integrated circuit chips were widely adopted, enabling complex semiconductor and telecommunication technologies to be developed. The microprocessor and memory chips are VLSI devices. Before the introduction of VLSI technology, most ICs had a limited set of functions they could perform. An electronic circuit might consist of a CPU, ROM, RAM and other glue logic. VLSI lets IC designers add all of these into one chip.

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.

Application-specific integrated circuit Integrated circuit customized (typically optimized) for a specific task

An application-specific integrated circuit is an integrated circuit (IC) chip 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 product (ASSP) chips are intermediate between ASICs and industry standard integrated circuits like the 7400 series or the 4000 series. ASIC chips are typically fabricated using metal-oxide-semiconductor (MOS) technology, as MOS integrated circuit chips.

Electronic design automation (EDA), also referred to as electronic computer-aided design (ECAD), is a category of software tools for designing electronic systems such as integrated circuits and printed circuit boards. The tools work together in a design flow that chip designers use to design and analyze entire semiconductor chips. Since a modern semiconductor chip can have billions of components, EDA tools are essential for their design.

In digital circuit design, register-transfer level (RTL) is a design abstraction which models a synchronous digital circuit in terms of the flow of digital signals (data) between hardware registers, and the logical operations performed on those signals.

Formal equivalence checking process is a part of electronic design automation (EDA), commonly used during the development of digital integrated circuits, to formally prove that two representations of a circuit design exhibit exactly the same behavior.

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.

In electronics, logic synthesis is a process by which an abstract specification of desired circuit behavior, typically at register transfer level (RTL), is turned into a design implementation in terms of logic gates, typically by a computer program called a synthesis tool. Common examples of this process include synthesis of designs specified in hardware description languages, including VHDL and Verilog. Some synthesis tools generate bitstreams for programmable logic devices such as PALs or FPGAs, while others target the creation of ASICs. Logic synthesis is one aspect of electronic design automation.

VLSI Technology, Inc., was a company that designed and manufactured custom and semi-custom integrated circuits (ICs). The company was based in Silicon Valley, with headquarters at 1109 McKay Drive in San Jose. Along with LSI Logic, VLSI Technology defined the leading edge of the application-specific integrated circuit (ASIC) business, which accelerated the push of powerful embedded systems into affordable products.

In electronic design a semiconductor intellectual property core, IP core, or IP block is a reusable unit of logic, cell, or integrated circuit layout design that is the intellectual property of one party. IP cores may be licensed to another party or can be owned and used by a single party alone. The term is derived from the licensing of the patent and/or source code copyright that exist in the design. IP cores can be used as building blocks within application-specific integrated circuit (ASIC) designs or field-programmable gate array (FPGA) logic designs.

Integrated circuit design Engineering process for electronic hardware

Integrated circuit design, or IC design, is a subset of electronics engineering, encompassing the particular logic and circuit design techniques required to design integrated circuits, or ICs. ICs consist of miniaturized electronic components built into an electrical network on a monolithic semiconductor substrate by photolithography.

Physical verification is a process whereby an integrated circuit layout design is verified via EDA software tools to ensure correct electrical and logical functionality and manufacturability. Verification involves design rule check (DRC), layout versus schematic (LVS), XOR, antenna checks and electrical rule check (ERC).

The Layout Versus Schematic (LVS) is the class of electronic design automation (EDA) verification software that determines whether a particular integrated circuit layout corresponds to the original schematic or circuit diagram of the design.

Physical design (electronics)

In integrated circuit design, physical design is a step in the standard design cycle which follows after the circuit design. At this step, circuit representations of the components of the design are converted into geometric representations of shapes which, when manufactured in the corresponding layers of materials, will ensure the required functioning of the components. This geometric representation is called integrated circuit layout. This step is usually split into several sub-steps, which include both design and verification and validation of the layout.

A process design kit (PDK) is a set of files used within the semiconductor industry to model a fabrication process for the design tools used to design an integrated circuit. The PDK is created by the foundry defining a certain technology variation for their processes. It is then passed to their customers to use in the design process. The customers may enhance the PDK, tailoring it to their specific design styles and markets. The designers use the PDK to design, simulate, draw and verify the design before handing the design back to the foundry to produce chips. The data in the PDK is specific to the foundry's process variation and is chosen early in the design process, influenced by the market requirements for the chip. An accurate PDK will increase the chances of first-pass successful silicon.

A gate equivalent (GE) stands for a unit of measure which allows specifying manufacturing-technology-independent complexity of digital electronic circuits. For today's CMOS technologies, the silicon area of a two-input drive-strength-one NAND gate usually constitutes the technology-dependent unit area commonly referred to as gate equivalent. A specification in gate equivalents for a certain circuit reflects a complexity measure, from which a corresponding silicon area can be deduced for a dedicated manufacturing technology.

In the automated design of integrated circuits, signoff checks is the collective name given to a series of verification steps that the design must pass before it can be taped out. This implies an iterative process involving incremental fixes across the board using one or more check types, and then retesting the design. There are two types of sign-off's: front-end sign-off and back-end sign-off. After back-end sign-off the chip goes to fabrication. After listing out all the features in the specification, the verification engineer will write coverage for those features to identify bugs, and send back the RTL design to the designer. Bugs, or defects, can include issues like missing features, errors in design, etc. When the coverage reaches a maximum% then the verification team will sign it off. By using a methodology like UVM, OVM, or VMM, the verification team develops a reusable environment. Nowadays, UVM is more popular than others.

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 was a privately held US/Silicon Valley-based multinational corporation dealing in Electronic Design Automation (EDA) for electrical engineering and electronics until its acquisition by Silvaco, Inc. in 2018. 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.

References

  1. 1 2 A. Kahng et al.: "VLSI Physical Design: From Graph Partitioning to Timing Closure", Springer (2011), doi : 10.1007/978-90-481-9591-6, ISBN   978-90-481-9590-9, pp. 12-14.
  2. 1 2 3 A. Kahng et al.: "VLSI Physical Design: From Graph Partitioning to Timing Closure", Springer (2011), doi : 10.1007/978-90-481-9591-6, ISBN   978-90-481-9590-9, p. 10.
  3. 1 2 3 D. Jansen et al. "The Electronic Design Automation Handbook", Springer (2003), doi : 10.1007/978-0-387-73543-6, ISBN   978-14-020-7502-5, pp. 398-420.

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