DVD - Lecture 3a: Logic Synthesis - Part 1

Introduction to Logic Synthesis

Overview of Logic Synthesis

• Logic synthesis is the process that converts RTL (Register Transfer Level) designs into a technology-specific gate-level netlist, optimized for predefined constraints.

• The synthesis tool takes an RTL description and standard cell library, producing a netlist that details how gates are interconnected.

• This process applies not only to ASIC designs but also to FPGAs, mapping designs to lookup tables within the FPGA architecture.

Mathematical Representation

• A finished state machine is represented mathematically with inputs (X), outputs (Y), internal states (Z), and functions (Lambda and Delta).

• The synthesis transforms this state machine into a target circuit composed of Boolean gates and interconnecting wires.

Benefits of Logic Synthesis

Advantages

• Logic synthesis automates many design details, reducing bugs and improving productivity compared to manual design methods.

• It abstracts HDL descriptions from specific implementation technologies, facilitating easier transitions between different technologies.

• While logic synthesis offers significant advantages, there may be cases where designers possess prior knowledge that allows for better manual optimization.

Example Scenario

• An example illustrates how synthesizers can simplify complex combinatorial logic; what might be difficult for humans becomes straightforward for automated tools.

Goals of Logic Synthesis

Optimization Objectives

• Key goals include minimizing area (measured in literal count), maximizing performance (targeting clock frequency), and minimizing power consumption.

• Designers often need to balance these objectives by assigning weights based on project requirements—some may prioritize smaller area while others focus on higher performance.

Constraint Formulation

• Constraints are formulated as optimization problems, such as minimizing area while ensuring a minimum clock speed requirement.

Methods in Logic Synthesis

High-Level Description

• Various methods exist for logic synthesis including instantiation of primitive gates like AND or OR gates directly from Verilog code.

• More complex behavioral descriptions can be replaced with optimized architectures like carry lookahead adders through macro expansion or substitution.

Synthesis Flow in Hardware Design

Overview of Inference and Logic Optimization

• Inference involves recognizing specific patterns within the hardware description language (HDL), allowing the tool to substitute different operators effectively.

• The most common inference relates to sequential elements, where tools identify flip-flops based on constructs like "always at positive clock" blocks.

• Logic optimization occurs after reading the design, utilizing techniques for Boolean operation grouping and minimization.

• Structural reorganization allows for circuit retiming and other optimizations beyond simple Boolean logic adjustments.

Stages of Synthesis Process

Syntax Analysis and Library Definition

• The synthesis process begins with syntax analysis, checking HDL files for errors using commands like read HDL.

• The library definition stage informs the synthesis tool about technology specifics, including leaf cells and IP types through commands such as read Libs.

Elaboration and Binding

• Elaboration converts RTL into a Boolean structure while performing various optimizations related to computational Boolean algebra.

• Binding is necessary when encountering instantiated standard cells or IP blocks, directing the tool on how to handle these components.

Constraints Definition

• After elaboration, a structural model is created that allows for defining constraints regarding performance metrics like speed.

• Constraints are typically defined using SDC (Synopsis Design Constraints), with commands such as read SDC pointing to relevant files.

Mapping and Optimization Stages

Pre-Mapping Optimization

• Pre-mapping optimization involves mapping elaborated Boolean logic to generic internal cells representing standard cells in libraries.

Technology Mapping

• Technology mapping connects RTL descriptions with actual standard cells, incorporating real delays and limitations from the cell library.

Post-Mapping Optimization

• This stage iterates over designs applying heuristics and optimizations post-mapping. Cadence Genus utilizes three stages: generic sin map and synopt for this purpose.

Final Steps in Synthesis

Reporting Results

• After completing synthesis, various reports are generated focusing on timing analysis, size estimation, gate usage, etc., guiding further design decisions.

Exporting Results

• Once satisfied with results from synthesis processes, exporting data (e.g., netlist via write HDL) prepares it for physical implementation in place-and-route tools.