Case Statement SystemVerilog: A Comprehensive Guide to Using Case Statements in SystemVerilog

Niranjana R

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SystemVerilog is a hardware description language that is used to model digital circuits. It is an extension of Verilog and includes many features that make it easier to write complex designs. One of the key features of SystemVerilog is the case statement, which is used to select one of several possible actions based on the value of a variable.

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Understanding the case statement in SystemVerilog is essential for writing complex digital designs. There are several types of case statements, including case, casez, and casex, each with its own syntax and usage. The case statement is similar to a switch statement in C, but it has some important differences. For example, the case statement can be used with any data type, not just integers, and it can include ranges and don’t-care values.

In this article, we will explore the syntax and usage of case statements in SystemVerilog. We will also provide several examples of how case statements can be used in digital designs. Finally, we will discuss some common pitfalls and how to avoid them when using case statements.

Key Takeaways

  • The case statement is a key feature of SystemVerilog that is used to select one of several possible actions based on the value of a variable.
  • There are several types of case statements in SystemVerilog, each with its own syntax and usage.
  • When using case statements, it is important to be aware of common pitfalls and how to avoid them.

Understanding Case Statement in SystemVerilog

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In SystemVerilog, the case statement is used to check if the given expression matches one of the other expressions in the list and branches accordingly. It is typically used to implement a multiplexer. The if-else construct may not be suitable if there are many conditions to be checked and would synthesize into a priority encoder instead of a multiplexer.

The case statement can be used with different types of expressions, including integers, enumerated types, and strings. The syntax for the case statement is as follows:

case (expression)
  value1: statement1;
  value2: statement2;
  default: statement3;
endcase

In the above syntax, the expression is the variable or expression that is being evaluated. The value1 and value2 are the possible values that the expression can take. The statement1 and statement2 are the statements that will be executed if the expression matches the value1 or value2, respectively. The statement3 is the statement that will be executed if the expression does not match any of the values.

It is important to note that the case statement is case-sensitive. This means that if the expression is a string, it must match the case of the values in the case statement. To avoid this issue, the casez statement can be used instead, which ignores the case of the values.

Another important feature of the case statement is the ability to use wildcard characters. The ? character can be used to match any single bit, and the x character can be used to match any unknown bit. For example, the following code will execute statement1 if the two most significant bits of expression are 10, and statement2 if the two most significant bits are 11:

case (expression[3:2])
  2’b10: statement1;
  2’b11: statement2;
endcase

In conclusion, the case statement is a powerful construct in SystemVerilog that allows for efficient implementation of multiplexers and other conditional logic. Its ability to handle different types of expressions, case sensitivity, and wildcard characters makes it a versatile tool for hardware design.

Types of Case Statements

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In SystemVerilog, there are three types of case statements: Normal case, Wildcard case, and Priority case. Each type of case statement has its own specific syntax and use cases.

Normal Case

The normal case statement in SystemVerilog is used to compare a single expression against a set of possible values. The syntax for the normal case statement is as follows:

case (expression)
  value1: statement1;
  value2: statement2;
  …
  default: default_statement;
endcase

In the above code, the expression is evaluated and compared to each value. If a match is found, the corresponding statement is executed. If no match is found, the default statement is executed.

Wildcard Case

The wildcard case statement in SystemVerilog is used to compare a single expression against a set of possible values, as well as wildcard values. The syntax for the wildcard case statement is as follows:

casez (expression)
  value1: statement1;
  value2: statement2;
  …
  default: default_statement;
endcase

In the above code, the expression is evaluated and compared to each value. If a match is found, the corresponding statement is executed. If the expression matches a wildcard value, the corresponding statement is also executed. If no match is found, the default statement is executed.

Priority Case

The priority case statement in SystemVerilog is used to compare a single expression against a set of possible values, as well as priority values. The syntax for the priority case statement is as follows:

unique case (expression)
  value1: statement1;
  value2: statement2;
  …
  default: default_statement;
endcase

In the above code, the expression is evaluated and compared to each value. If a match is found, the corresponding statement is executed. If the expression matches a priority value, the corresponding statement is also executed. If multiple matches are found, only the first match is executed. If no match is found, the default statement is executed.

Overall, the type of case statement used in SystemVerilog depends on the specific use case and the type of comparison needed. The normal case statement is used for exact matches, the wildcard case statement is used for matches with wildcard values, and the priority case statement is used for matches with priority values.

Syntax and Usage of Case Statements

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Syntax

The case statement is a conditional statement used in SystemVerilog to select one of several code blocks to execute. It is similar to a switch statement in C and C++. The syntax of the case statement is as follows:

case (expression)
    value1: statement1;
    value2: statement2;
    value3: statement3;
    default: statement4;
endcase

The expression is evaluated and compared to each of the value expressions. If the expression matches value1, then statement1 is executed. If the expression matches value2, then statement2 is executed, and so on. If the expression does not match any of the value expressions, then statement4 is executed. The default case is optional. If it is not included, then no action is taken if the expression does not match any of the value expressions.

The case statement can also be written using casex or casez instead of a case. These statements are similar to the case, but they allow for wildcard characters in the value expressions. casex treats x and z as don’t care, while casez treats z as don’t care and x as unknown.

Usage

The case statement is commonly used in SystemVerilog to implement a multiplexer. It can also be used to implement a state machine. When using a case statement to implement a state machine, each value expression corresponds to a state, and the statement associated with that value expression corresponds to the actions to be taken in that state.

One important consideration when using a case statement is the order of the value expressions. The case statement evaluates the value expressions in the order they are written. If two or more value expressions match the expression, then the statement associated with the first matching value expression is executed. Therefore, it is important to order the value expressions in the case statement so that the most specific expressions come first and the more general expressions come last.

Another consideration when using a case statement is that the value expressions must be constant expressions. This means that they cannot depend on any variables or signals that may change during simulation. If a value expression depends on a variable or signal, then a generated statement can be used to create multiple case statements, each with a different value expression.

Case Statement Examples

In this section, we will provide a few examples to help you understand how to use the case statement in SystemVerilog. We will cover two examples, Example 1 and Example 2.

Example 1

Let’s say we want to implement a simple state machine with three states, A, B, and C. We will use a case statement to transition between the states based on the current state and the input signal.

module state_machine (
    input clk,
    input reset,
    input [1:0] input_signal,
    output reg [1:0] state
);

always @(posedge clk) begin
    if (reset) begin
        state <= 2’b00;
    end else begin
        case ({state, input_signal})
            2’b00_00: state <= 2’b00;
            2’b00_01: state <= 2’b01;
            2’b01_10: state <= 2’b10;
            2’b10_11: state <= 2’b00;
            default: state <= state;
        endcase
    end
end

endmodule

In this example, we use a case statement to transition between the states based on the current state and the input signal. The case statement checks the concatenation of the current state and the input signal and assigns the new state accordingly. The default case assigns the current state to the new state if none of the other cases match.

Example 2

Let’s say we want to implement a priority encoder with four inputs. We will use a case statement to assign the output signal based on the highest priority input.

module priority_encoder (
    input [3:0] input_signal,
    output reg [1:0] output_signal
);

always @(*) begin
    casez (input_signal)
        4’b000_: output_signal = 2’b00;
        4’b001_: output_signal = 2’b01;
        4’b01__: output_signal = 2’b10;
        4’b1___: output_signal = 2’b11;
        default: output_signal = 2’b00;
    endcase
end

endmodule

In this example, we use a case statement to assign the output signal based on the highest priority input. The case statement checks the input signal and assigns the output signal based on the highest priority input. The casez statement uses the “don’t care” symbol (_) to simplify the code and make it easier to read. The default case assigns the output signal to 00 if none of the other cases match.

These examples demonstrate how to use the case statement in SystemVerilog to implement different types of logic. By using the case statement, we can simplify our code and make it easier to read and understand.

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