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Christopher Long
Christopher Long

SystemVerilog: A Powerful Language for Logic Design and Verification



Logic Design and Verification Using SystemVerilog




SystemVerilog is a Hardware Description Language (HDL) that enables designers to work at the higher levels of logic design abstractions that match the increased complexity of current day integrated circuit and field-programmable gate array (FPGA) designs. It also provides a rich set of features for verification, such as object-oriented design, assertions, randomization and coverage. In this article, we will explore the main aspects of SystemVerilog for design and verification, and how it can help you create more efficient and effective logic designs.




Logic Design and Verification Using SystemVerilog (Revised) Donald Thomas



Introduction




Logic design is the process of creating digital circuits that perform a specific function or task. Logic design can be done at different levels of abstraction, such as gate-level, register-transfer level (RTL), behavioral or algorithmic level. Logic verification is the process of ensuring that the logic design meets the specifications and requirements, such as functionality, performance, timing, power and area. Logic verification can be done using various methods, such as simulation, formal methods, emulation or prototyping.


What is SystemVerilog?




SystemVerilog is a Hardware Description Language (HDL) that extends the Verilog HDL with many enhancements and additions. It was standardized by IEEE in 2005 as IEEE 1800-2005, and has been revised several times since then. The latest version is IEEE 1800-2017. SystemVerilog can be used for both design and verification purposes, as it supports both structural and behavioral modeling of logic circuits. It also incorporates features from other languages, such as C++, Java and VHDL.


Why use SystemVerilog for design and verification?




SystemVerilog offers many benefits for logic design and verification, such as:



  • It allows designers to work at higher levels of abstraction, such as RTL or behavioral level, which can improve productivity and readability.



  • It provides new data types, literals, operators, procedures and statements that can simplify coding and enhance expressiveness.



  • It supports user-defined types and structures that can enable modularity and reusability.



  • It enables hierarchy and connectivity features that can facilitate top-down or bottom-up design methodologies.



  • It supports tasks and functions that can encapsulate logic functionality and improve code organization.



  • It introduces interfaces that can define communication protocols between modules and reduce wiring complexity.



  • It supports simple verification features that can help debug logic designs using print statements, assertions or breakpoints.



  • It enables clocking blocks that can define clocking domains and synchronization mechanisms.



  • It supports random stimulus generation that can test logic designs with various input scenarios.



  • It supports classes and object-oriented design that can enable encapsulation, inheritance, polymorphism and virtuality.



  • It supports interfaces in verification that can enable communication between testbenches and designs.



  • It supports covergroup coverage that can measure the effectiveness of verification by collecting statistics on the tested scenarios.



  • It supports queues and dynamic arrays that can store and manipulate variable-sized data.



  • It supports assertion-based verification that can check the properties and behavior of logic designs using formal methods.



  • It supports SystemVerilog assertions that can specify and monitor the assertions using a declarative language.



  • It supports direct programming interface that can interface with external software languages, such as C or C++.



  • It supports interprocess synchronization that can coordinate multiple threads or processes in verification.



How to learn SystemVerilog?




There are many resources available for learning SystemVerilog, such as books, online courses, tutorials, videos, blogs and forums. Some of the popular books on SystemVerilog are:



  • Logic Design and Verification Using SystemVerilog by Donald Thomas. This book provides an in-depth introduction to the main SystemVerilog features for design and verification, and demonstrates how they can be used for more efficient and effective logic design. It is suitable for students and designers who want to update their skills from Verilog or VHDL.



  • SystemVerilog for Verification: A Guide to Learning the Testbench Language Features by Chris Spear and Greg Tumbush. This book focuses on the verification aspects of SystemVerilog, such as classes, randomization, coverage, assertions and testbenches. It is suitable for verification engineers who want to learn how to use SystemVerilog for creating advanced testbenches.



  • SystemVerilog for Design and Verification Training by Cadence Design Systems. This is an online course that covers the main SystemVerilog features for design and verification, such as data types, operators, procedures, user-defined types, hierarchy, connectivity, interfaces, simple verification features, clocking blocks, random stimulus, classes, interfaces in verification, covergroup coverage, queues and dynamic arrays, assertion-based verification, SystemVerilog assertions, direct programming interface and interprocess synchronization. It is suitable for design and verification engineers who want to learn how to use SystemVerilog for creating efficient and effective logic designs.



SystemVerilog for Design




In this section, we will explore the main features of SystemVerilog for design purposes, such as data types and literals, procedures, statements and operators, user-defined types and structures, hierarchy and connectivity, static arrays, tasks and functions, and interfaces.


Data types and literals




SystemVerilog introduces new data types and literals that can simplify coding and enhance expressiveness. Some of the new data types are:



  • logic: This is a 4-state data type that can represent 0, 1, X (unknown) or Z (high impedance). It can be used for both combinational and sequential logic.



  • bit: This is a 2-state data type that can represent 0 or 1. It can be used for binary data or arithmetic operations.



  • enum: This is an enumerated data type that can define a set of named constants. It can be used for symbolic representation of states or values.



  • struct: This is a structured data type that can group multiple variables of different types into a single variable. It can be used for modularity and reusability.



  • union: This is a union data type that can store one of multiple variables of different types in a single variable. It can be used for memory optimization or type conversion.



Some of the new literals are:



  • '0, '1, 'x, 'z: These are unsized literals that can represent 0, 1, X or Z values of any size.



  • 'b, 'o, 'd, 'h: These are base specifiers that can define binary, octal, decimal or hexadecimal values.



  • '...: This is a concatenation operator that can combine multiple values into a single value.



  • 'n...: This is a replication operator that can repeat a value n times.



  • "...": This is a string literal that can represent a sequence of characters. It can be used for text data or string manipulation.



Here are some examples of using the new data types and literals:


```verilog // Declare a logic variable and assign a value logic a; a = 1'b1; // Declare a bit variable and assign a value bit b; b = 'b1010; // Declare an enum variable and define the constants enum RED, GREEN, BLUE color; color = RED; // Declare a struct variable and group multiple variables struct logic [3:0] data; bit valid; bit [1:0] parity; packet; packet = '4'hA, 1'b1, 2'b01; // Declare a union variable and store one of multiple variables union logic [7:0] byte; logic [3:0] nibble [2]; data; data.byte = 8'hFF; ``` Procedures, statements and operators




SystemVerilog introduces new procedures, statements and operators that can simplify coding and enhance expressiveness. Some of the new procedures are:



  • always_comb: This is a procedural block that executes whenever any of its input variables change. It can be used for modeling combinational logic.



  • always_ff: This is a procedural block that executes at the positive or negative edge of a clock or a reset signal. It can be used for modeling sequential logic.



  • always_latch: This is a procedural block that executes whenever any of its input variables change, except when it is disabled by a reset signal. It can be used for modeling latches.



  • final: This is a procedural block that executes only once at the end of the simulation. It can be used for displaying final results or closing files.



Some of the new statements are:



  • assert: This is a statement that checks a condition and reports an error if it is false. It can be used for debugging or verification purposes.



  • assume: This is a statement that assumes a condition to be true and constrains the random variables accordingly. It can be used for verification purposes.



  • cover: This is a statement that counts how many times a condition is true. It can be used for verification purposes.



  • restrict: This is a statement that restricts the values of random variables to satisfy a condition. It can be used for verification purposes.



  • unique case: This is a statement that specifies that only one of the case items can match the expression. It can be used for avoiding ambiguity or synthesis issues.



  • priority case: This is a statement that specifies that the case items are evaluated in order of priority. It can be used for avoiding ambiguity or synthesis issues.



  • foreach: This is a statement that iterates over the elements of an array or a queue. It can be used for simplifying loops.



  • do-while: This is a statement that executes a loop body at least once and then repeats while a condition is true. It can be used for simplifying loops.



Some of the new operators are:



  • ?:: This is a conditional operator that evaluates an expression based on a condition. It can be used for simplifying if-else statements.



  • inside: This is a membership operator that checks if an expression is inside a set of values. It can be used for simplifying case statements or randomization constraints.



  • with: This is an array reduction operator that applies an operation to the elements of an array that satisfy a condition. It can be used for simplifying array operations.



  • '...: This is an assignment pattern operator that assigns values to the members of an aggregate data type. It can be used for simplifying assignments.



  • '->, '=>, ', ': These are implication operators that check the logical implication between two expressions. They can be used for writing assertions.



Here are some examples of using the new procedures, statements and operators:


```verilog // Declare an always_comb block to model a 4-bit adder always_comb begin sum = a + b; carry = (a + b) >> 4; end // Declare an always_ff block to model a D flip-flop with reset always_ff @(posedge clk or negedge rst) begin if (rst) q q == 0); // Declare an assume statement to assume that a and b are always positive assume property (@(posedge clk) a > 0 && b > 0); // Declare a cover statement to count how many times a and b are equal cover property (@(posedge clk) a == b); // Declare a restrict statement to restrict the values of a and b to be less than 16 restrict a 10); // Declare a conditional expression to assign the maximum of a and b to max max = (a > b) ? a : b; // Declare an inside expression to check if op is valid if (op inside 2'b00, 2'b01, 2'b10) $display("Valid op"); // Declare a with expression to sum the elements of an array that are even sum = arr.sum() with (item % 2 == 0); // Declare an assignment pattern to assign values to a struct variable packet = 'data:4'hA, valid:1'b1, parity:2'b01; ``` 71b2f0854b


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