What is Logic Design?

Definition

Digital logic design is the process of creating digital circuits that perform specific functions using logic gates and combinational/sequential logic structures. It is a design foundation upon which microprocessors, AI and memory chips, digital communication systems, and embedded controllers are built.

In modern digital silicon chip design, logic synthesis and EDA tools play a crucial role by automating and optimizing the transformation of high-level design descriptions into hardware implementations.

Key components of digital logic design include:

• Logic Gates: The fundamental elements or building blocks of digital logic design. Each gate performs a specific Boolean operation based on its inputs. Common gate functions include AND, OR, NOT, NAND, NOR, XOR, and XNOR.
• Combinational Circuits: Circuits whose outputs depend only on the current set of inputs. Examples include adders (arithmetic operations), multiplexers (data routing), encoders, decoders, and comparators.
• Sequential Circuits: Circuits whose outputs depend on both current inputs and the history of past inputs. These circuits incorporate memory elements such as flip-flops, latches, registers, and counters.
• Hardware Definition Languages (HDLs): Computer languages designed to describe digital logic designs in software constructs. When compiled/synthesized, instead of producing a software executable, it produces a digital logic circuit – gates and gate-to-gate connections that duplicate the functionality described in the HDL. Examples of HDLs are VHDL and Verilog.
• Register-Transfer Level (RTL) Descriptions: Specifies the behavior of the digital logic circuit in terms of the flow of data between registers and the logical operations performed on that data. RTL descriptions are written using HDLs.
• Integrated Circuits (ICs): Modern digital logic is often implemented in ICs, which can contain millions or billions of gates. Field Programmable Gate Arrays (FPGAs) and Application Specific Integrated Circuits (ASICs) are common platforms for complex digital logic designs.

Key applications of digital logic:

• Computing: CPUs, GPUs, AI, and memory modules.
• Communication: Routers, switches, and modems.
• Control Systems: Robotics, industrial automation, automotive electronics and ADAS.
• Consumer Electronics: Smartphones, tablets, smart TVs.

Digital logic design involves a structured engineering process, generally consisting of the following high-level steps:

Requirement Analysis:

The design process begins by defining the desired functionality of the target silicon chip or sub-block of a larger chip. This could be as simple as a binary adder or as complex as a high-performance CPU, GPU, or AI neural network chip. Requirements are specified in terms of inputs, outputs, performance, power consumption, and other constraints.

The requirements for a digital logic design involve architectural analysis and exploration to arrive at an optimal architecture for the design. This is a critical step as the chosen architecture is the core of the design. A suboptimal architecture will lead to a suboptimal logic design.

Logic Synthesis:

The automated process of converting a high-level, human-readable hardware description (written in HDLs) into a gate-level representation suitable for implementation on silicon (such as FPGAs or ASICs). This process considers design constraints, such as area, speed, and power consumption, to generate an optimized netlist – a file containing the logic gates and all their interconnections and connections to the "outside world". Logic synthesis is the cornerstone tool of logic design, making it possible to automate the creation of large designs by specifying a high-level language to describe the desired functionality. Key steps in logic synthesis are:

• Design Entry: The designer specifies the desired functionality using an HDL, usually at the RTL level.
• Elaboration: The tool reads the HDL and interprets and expands the design hierarchy.
• Optimization: The synthesized logic design is optimized for performance, area, or power, based on the constraints specified by the logic design engineer.
• Technology Mapping: The design is mapped to available gate types or cells in the target library, based on the silicon foundry that will be fabricating the silicon chip.
• Netlist Generation: The final output is a logic gate-level netlist—a detailed list of components and connections.

Simulation & Verification:

Before physical implementation, logic designs are simulated using specialized software tools that read in and process the netlist. Simulation allows engineers to verify correct operation (the desired functionality), test various input scenarios, and identify potential issues without building the design in silicon or even going through physical design. Different types of simulation include functional simulation and timing verification.

Years ago, digital logic design was largely manual – creating digital logic circuits by hand using truth tables and Boolean algebra, and constructing schematics, which shows the connections of each logic gate. With the advent of automated logic synthesis – creating the digital logic design automatically from RTL descriptions – EDA software could generate large complex designs by compiling the input RTL into actual gate netlists, as well as optimize them.

Key benefits include:

• Reliability and Precision: Logic synthesis software ensures the output logic design – no matter how large – will always implement the RTL specification exactly, eliminating human error. Verification software can confirm the logical correctness of the output design by comparing the RTL specification with the output gate-level netlist.
• Scalability and Modularity: Complex systems can be constructed by combining simple logic blocks. This modularity simplifies design, testing, and maintenance, enabling rapid development and integration of new features. Proven, verified RTL blocks (for example adders, comparators, ALUs, etc.) can be reused as part of other new designs.
• Speed and Efficiency: Logic synthesis software can create many thousands – or millions – of connected logic gates forming digital chip designs fast and efficiently, without manual work. The design engineer can specify constraints to the synthesis tool, such as timing, power, or area constraints to guide the software to create and optimal output netlist.
• Programmability and Flexibility: Since HDLs allow specifying a logic design through a high-level language, it is very straightforward for logic designers to make changes, add or remove logic, etc. and quickly re-synthesize. HDLs make logic design highly programmable and flexible.

Simulation & Verification:

Before physical implementation, logic designs are simulated using specialized software tools that read in and process the netlist. Simulation allows engineers to verify correct operation (the desired functionality), test various input scenarios, and identify potential issues without building the design in silicon or even going through physical design. Different types of simulation include functional simulation and timing verification.

In 1987, Synopsys introduced its first logic synthesis product, Design Compiler, which revolutionized digital chip design by compiling, or "synthesizing" high-level RTL descriptions into optimized gate-level implementations. Synopsys was the first to commercialize the technology and make it widely accessible to the semiconductor industry.

Today, Synopsys has combined the logic synthesis capabilities of Design Compiler with the physical design steps in the Fusion Compiler software product, which offers a complete silicon design solution. As a unified environment, Fusion Compiler provides design floorplanning, logic synthesis and design, physical design, optimization, and analysis, delivering a full silicon chip design journey.