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Continuous Assignments in Verilog

Welcome to our article on continuous assignments in Verilog. In this guide, we will explore the concept of continuous assignments, their syntax, and usage in Verilog. Whether you are a beginner or an experienced Verilog designer, understanding continuous assignments is essential for accurate hardware description and efficient simulation.

Table of Contents

What are Continuous Assignments in Verilog?

In Verilog, continuous assignments play a crucial role in hardware description and simulation. They allow us to describe the behavior of hardware components and their connections in a concise and efficient manner. Continuous assignments are used to assign values continuously to signals, wires, or registers, ensuring that the output is constantly updated based on the changes in the input.

By utilizing continuous assignments, we can define the logical relationships between different components in a hardware design. We can specify the connections and dependencies between signals, facilitating the simulation and understanding of complex electronic systems. Verilog’s continuous assignments simplify the process of hardware description, enabling designers to express circuit behavior in a way that closely resembles the functionality of the physical hardware.

With continuous assignments, we can update the values of signals in real-time, allowing for dynamic responses to input changes. This capability is particularly useful in modeling combinational logic circuits and building blocks for system-on-chip designs. Continuous assignments also contribute to the modularity and readability of Verilog code by providing a clear and concise representation of interconnections between different hardware elements.

Take a look at the visual representation below for a better understanding of how continuous assignments work in Verilog:

As seen in the diagram, continuous assignments establish connections and propagate data between components, ensuring the accurate representation of the circuit’s behavior. This approach enables efficient simulation and verification of hardware designs, aiding in the identification of potential issues or bugs early in the development process.

Syntax and Usage of Continuous Assignments

In Verilog, continuous assignments play a crucial role in hardware description and simulation. They allow us to define and connect signals in a concise and efficient manner, providing a clear representation of the system’s behavior. In this section, we will explore the syntax and correct usage of continuous assignments in Verilog, empowering you to effectively implement them in your designs.

The syntax of a continuous assignment in Verilog follows the simple form of: assign output_signal = input_expression ;

The assign keyword signifies the beginning of a continuous assignment statement. Next, we specify the output signal that we want to assign a value to using the output_signal . This can be a single wire, output port, or bus. Lastly, we provide the input_expression that determines the value for the output signal based on other signals or expressions.

Let’s take a look at an example to better understand the syntax:

“`verilog module ExampleModule(input A, input B, output C); wire D;

assign D = A & B; assign C = D | A; endmodule “`

In the example above, we have a module with two input ports, A and B , and an output port C . We also have an intermediate wire named D . The first continuous assignment statement assigns the result of the logical AND operation between A and B to D . The second continuous assignment statement assigns the result of the logical OR operation between D and A to C .

Remember, continuous assignments in Verilog are always active and continuously update the assigned signals as soon as changes occur in the input signals. This ensures synchronization and concurrency within the design.

Continuous assignments are commonly used in Verilog to connect and model combinational logic. They are particularly useful when describing multiplexers, arithmetic circuits, and other complex digital systems.

Verilog Operators for Continuous Assignments

Verilog provides a range of operators that can be used within continuous assignments to express different behaviors and computations. These operators include simple logic operators such as AND ( & ), OR ( | ), and XOR ( ^ ), as well as more complex arithmetic operators like addition ( + ) and subtraction ( - ).

When working with continuous assignments, it is important to have a good understanding of the available operators and their precedence to ensure accurate and predictable behavior in your Verilog designs.

Now that we have explored the syntax and usage of continuous assignments in Verilog, let’s move on to the next section where we will compare continuous assignments with procedural assignments in Verilog, highlighting their differences and preferred usage.

Continuous Assignments vs. Procedural Assignments

When working with Verilog, it is important to understand the differences between continuous assignments and procedural assignments. While both assignment types serve a purpose in hardware description and simulation, they have distinct characteristics that make them suitable for different scenarios.

Continuous assignments are used to represent instantaneous connections in a Verilog design. They are typically used for connecting wires and other continuous signals. Continuous assignments use the “ = ” operator and are concurrent, meaning they are continuously evaluated and updated based on their inputs. These assignments are executed outside procedural blocks and can span across modules, enabling efficient modeling of combinational logic and inter-module connectivity.

Procedural assignments, on the other hand, are used within procedural blocks to describe behavior that evolves over time. They are executed sequentially and allow for modeling of registers, memory elements, and complex sequential logic. Procedural assignments use the “ <= ” nonblocking assignment operator and are commonly used in procedural blocks such as always blocks and initial blocks.

One key difference between continuous assignments and procedural assignments is their level of abstraction. Continuous assignments provide a high-level abstraction by describing the connectivity between hardware components in a concise and straightforward manner. They are suitable for representing combinational logic and interconnections that do not require any timing control.

Procedural assignments, on the other hand, offer a lower-level of abstraction as they allow for detailed control of timing and state transitions. They are used for modeling more complex behavior that involves clocking, synchronization, and sequential execution. Procedural assignments are essential for describing registers, memory, state machines, and other time-dependent behavior.

In terms of performance, continuous assignments can be more efficient than procedural assignments in some cases. Continuous assignments allow for concurrent evaluation, which can lead to better simulation performance and reduced memory usage. However, procedural assignments offer more flexibility and control over timing, making them necessary for certain design requirements.

In summary, the choice between continuous assignments and procedural assignments depends on the specific requirements of your Verilog design. Continuous assignments are ideal for representing interconnections and combinational logic, while procedural assignments are essential for modeling sequential behavior and detailed timing control. Understanding the differences and appropriate use cases for these assignment types will enable you to effectively design and simulate complex Verilog designs.

Benefits of Using Continuous Assignments

In Verilog, continuous assignments offer numerous advantages that contribute to the efficiency of simulation and ease of design modifications, ultimately leading to improved productivity.

1. Simulation Efficiency

Continuous assignments in Verilog enable faster simulations by eliminating the need for time-consuming procedural blocks. Instead, they provide a direct and continuous connection between signals, allowing changes in one signal to propagate to dependent signals immediately. This eliminates unnecessary delays and improves simulation accuracy.

2. Concise and Readable Code

Using continuous assignments results in more concise and readable code. Complex hardware connections and signal assignments are represented in a single line, reducing overall code length and enhancing code readability. This makes it easier for designers to understand and modify the code, leading to efficient debugging and maintenance.

3. Design Flexibility

Continuous assignments offer greater flexibility in hardware design modifications. Since these assignments are not bound by procedural constraints, designers can easily make changes without worrying about timing or sequencing issues. This allows for more agile design iterations and faster prototyping.

4. Consistency and Simplicity

Continuous assignments provide a consistent and simple approach to hardware description. The behavior of signals remains constant throughout the simulation, allowing designers to focus on the functional aspects rather than managing complex timing control structures. This simplifies the overall design process and improves design comprehension.

5. Enhanced Debugging and Analysis

Continuous assignments facilitate easier debugging and analysis of signal behavior. With the direct and continuous connection between signals, it becomes straightforward to trace signal transitions and identify potential issues or anomalies. This accelerates the debugging process and enhances the overall quality of the design.

6. Improved Productivity

By incorporating continuous assignments, designers can significantly improve their productivity. The advantages of simulation efficiency, concise code, design flexibility, consistency, and enhanced debugging all contribute to a more efficient design process. This allows designers to focus more on innovation and optimizing the design rather than spending excessive time on mundane tasks.

7. Compatibility with Hierarchical Designs

Continuous assignments seamlessly integrate with hierarchical designs, enabling the modularization and reuse of code. Signals can be connected across different modules using continuous assignments, simplifying the design hierarchy and promoting code reusability. This adds flexibility and scalability to the design process.

8. Industry Standard

Continuous assignments are widely adopted and considered an industry standard for hardware description. By using continuous assignments, designers align themselves with established practices, making their code more accessible and understandable to other professionals in the field.

Overall, the advantages of continuous assignments in Verilog, including simulation efficiency, concise and readable code, design flexibility, consistency, enhanced debugging, improved productivity, compatibility with hierarchical designs, and industry recognition, make them an essential tool for efficient and effective hardware design and simulation.

Best Practices for Implementing Continuous Assignments

Implementing continuous assignments in Verilog requires adherence to certain coding guidelines and best practices to ensure clean and maintainable code. By following these practices, you can avoid common pitfalls and issues that may arise during development.

1. Use Descriptive Signal and Variable Names

When defining signals and variables in your continuous assignments, use clear and descriptive names that accurately represent their purpose and functionality. This makes your code more readable and understandable, facilitating collaboration and maintenance in the long run.

2. Maintain Consistent Formatting

Consistent formatting improves code readability and makes it easier to spot errors or inconsistencies. Indentation, line spacing, and proper use of whitespace can greatly enhance the readability of your continuous assignments. Consider using an automated formatter or adhering to a specific style guide to maintain consistency throughout your codebase.

3. Group Related Assignments

Grouping related continuous assignments together improves code organization and makes it easier to understand the overall behavior of your Verilog design. By organizing your assignments logically, you can quickly identify signal dependencies and ensure correct initialization and connectivity.

4. Comment Your Code

Adding comments to your continuous assignments helps other developers (including your future self) understand the purpose and intent of the code. Commenting can provide valuable context, especially for complex assignments or when implementing specific functionality. Use clear and concise comments to explain non-obvious logic or rationale behind the code.

5. Avoid Latches and Unintended Feedback

Continuous assignments should not introduce latches or unintended feedback loops in your design. Ensure that you avoid any undefined behavior or combinational loops that can occur due to improper signal assignments or incomplete sensitivity lists. Verifying your code using linting and simulation tools can help detect and resolve such issues.

6. Test Your Continuous Assignments

Thoroughly testing your continuous assignments is crucial to ensure their correct behavior under different scenarios. Create comprehensive testbenches that cover all possible input and boundary conditions. Simulation tools can assist in verifying the correctness of your continuous assignments and help identify any potential errors or unintended consequences.

By following these best practices, you can streamline the development process and minimize the risk of errors in your Verilog code. Well-implemented continuous assignments result in efficient hardware description and simulation, ultimately leading to successful designs.

In conclusion, continuous assignments in Verilog play a crucial role in hardware description and simulation. They allow us to succinctly describe the behavior of hardware components and their connections, resulting in efficient and accurate designs. By using continuous assignments, we can simplify the coding process, improve simulation efficiency, and enhance productivity.

Throughout this article, we have explored the syntax, usage, and benefits of continuous assignments in Verilog. We have discussed how they differ from procedural assignments and highlighted the situations where they are most appropriate. Additionally, we have provided best practices for implementing continuous assignments, ensuring clean and maintainable code.

As engineers and designers, it is important to understand the power and versatility of continuous assignments in Verilog. By utilizing this feature effectively, we can create hardware designs that are both reliable and efficient. Whether you are a beginner or an experienced professional, continuous assignments should be an integral part of your Verilog coding arsenal.

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Verilog Continuous Assignment Statements Tutorial

Continuous assignment statements are an essential aspect of Verilog that allows you to assign values to signals without using procedural blocks. Unlike procedural assignments found in always blocks, continuous assignments are used for modeling combinational logic. In this tutorial, we will explore continuous assignment statements in Verilog and learn how to use them to describe the behavior of combinational circuits efficiently.

Introduction to Continuous Assignment Statements

Continuous assignment statements in Verilog are used to specify the relationship between input and output signals in a combinational circuit. They allow you to assign a value to a signal continuously, meaning the assignment is continuously evaluated as the inputs change. Continuous assignments are used outside procedural blocks and are ideal for describing combinational logic or interconnections between signals.

Example of Continuous Assignment Statements:

Another example:, steps to use continuous assignment statements.

To use continuous assignment statements in Verilog, follow these steps:

• Identify the combinational logic relationship between input and output signals.
• Use the 'assign' keyword to create a continuous assignment statement.
• Specify the output signal on the left-hand side and the combinational logic expression on the right-hand side of the assignment.
• Ensure that the right-hand side expression does not contain any procedural constructs, as continuous assignments are not allowed to contain procedural statements.
• Continuous assignments are evaluated in parallel with no explicit sequencing, making them suitable for combinational logic modeling.

Common Mistakes with Continuous Assignment Statements

• Using procedural statements such as if-else or case statements within continuous assignments.
• Missing the 'assign' keyword before the continuous assignment statement, leading to syntax errors.
• Attempting to use continuous assignments for modeling sequential logic, which is not their intended use.
• Using continuous assignments for outputs in modules with procedural assignments, leading to unexpected behavior.
• Not considering the propagation delays of combinational logic when using continuous assignments, which may affect simulation results.

Frequently Asked Questions (FAQs)

• Q: Can I use continuous assignments inside an always block? A: No, continuous assignments are not allowed inside always blocks. They are used outside procedural blocks to model combinational logic.
• Q: What is the difference between continuous assignments and procedural assignments? A: Continuous assignments are evaluated continuously for combinational logic, while procedural assignments in always blocks are used for modeling sequential logic that executes based on clock edges or event triggers.
• Q: Can I use continuous assignments for bidirectional signals? A: No, continuous assignments can only be used for assigning values to output or wire signals, not bidirectional signals or registers.
• Q: How do continuous assignments affect the simulation time of a Verilog design? A: Continuous assignments add negligible overhead to the simulation time as they represent combinational logic and are evaluated in parallel with no explicit sequencing.
• Q: Can I use continuous assignments for modeling arithmetic operations? A: Yes, continuous assignments can be used to model arithmetic operations in combinational logic. For example, you can use continuous assignments to describe the addition or subtraction of signals.
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Verilog: Continuous & Procedural Assignments

Continuous Assignment

Continuous assignment is used to drive a value on to a net in dataflow modeling. The net can be a vector or scalar, indexed part select, constant bit or part select of a vector. Concatenation is also supported with scalar vector types.

module Conti_Assignment (addr1,addr2,wr,din,valid1,valid2,dout); input [31:0] addr1,addr2; input [31:0] din; output [31:0] dout; input valid1,valid2,wr;

wire valid; wire [31:0] addr;

//Net (scalar) continuous assignment assign valid = valid1 | valid2;

//Vector continuous assignment assign addr[31:0] = addr1[31:0] ^ addr2[31:0];

//Part select & Concatenation in Continuous assignment assign dout[31:0] = (valid & wr) ? {din[31:2],2'b11} : 32'd0;

Regular & Implicit Assignment

Regular continuous assignment means, the declaration of a net and its continuous assignments are done in two different statements. But in implicit assignment, continuous assignment can be done on a net when it is declared itself. In the below example, `valid` is declared as wire during the assignment. If signal name is used to the left of the continuous assignment, an implicit net declaration will be inferred. In the below code `dout` is not declared as net, but it is inferred during assignment.

module Implicit_Conti_Assignment (addr1,addr2,wr,din,valid1,valid2,dout); input [31:0] addr1,addr2; input [31:0] din; output [31:0] dout; input valid1,valid2,wr;

//Net (scalar) Implict continuous assignment wire valid = (valid1 | valid2);

//Implicit net declaration -dout assign dout[31:0] = (valid & wr) ? {din[31:2],2'b11} : 32'd0;

Procedural Assignment

We have already seen that continuous assignment updates net, but procedural assignment update values of reg, real, integer or time variable. The constant part select, indexed part select and bit select are possible for vector reg.

There are two types of procedural assignments called blocking and non-blocking. Blocking assignment, as the name says, gets executed in the order statements are specified. The "=" is the symbol used for blocking assignment representation. Non-blocking assignment allows scheduling of assignments. It will not block the execution. The symbol "<=" is used for non-blocking assignment representation and mainly used for concurrent data transfers.

Following example shows the differences in the simulation result by using blocking and non-blocking assignments.

/* module Nonblocking_Assignment (addr1,addr2,wr,din,valid1,valid2,data,aout); input [31:0] addr1,addr2; input [31:0] din; output [31:0] data,aout; input valid1,valid2,wr;

reg [31:0] data,aout, addr; reg valid;

always @(addr1,addr2,wr,din,valid1,valid2) begin valid <= (valid1 | valid2); addr <= (addr1[31:0] | addr2[31:0]); data <= (valid & wr) ? {din[31:2],2'b11} : 32'd0; aout <= wr ? addr: {addr1[15:0],addr2[31:16]}; end initial $monitor($time,"NON-BLOCKING: Values valid1=%b, valid2=%b, wr=%b, addr1=%d, addr2=%d, data=%d, aout=%d", valid1,valid2,wr,addr1,addr2,data,aout); endmodule */ module Blocking_Assignment (addr1,addr2,wr,din,valid1,valid2,data,aout); input [31:0] addr1,addr2; input [31:0] din; output [31:0] data,aout; input valid1,valid2,wr;

always @(addr1,addr2,wr,din,valid1,valid2) begin valid = (valid1 | valid2); addr = (addr1[31:0] | addr2[31:0]); data = (valid & wr) ? {din[31:2],2'b11} : 32'd0; aout = wr ? addr : {addr1[15:0],addr2[31:16]}; $monitor($time,"BLOCKING: Values valid1=%b, valid2=%b, wr=%b, addr1=%d, addr2=%d, data=%d, aout=%d", valid1,valid2,wr,addr1,addr2,data,aout); end endmodule

module test; reg valid1,valid2,wr; reg [31:0] addr1,addr2,din; wire [31:0] data,aout;

Blocking_Assignment Block_Assign(addr1,addr2,wr,din,valid1,valid2,data,aout);

//Nonblocking_Assignment Nonblock_Assign(addr1,addr2,wr,din,valid1,valid2,data,aout);

initial begin valid1 = 0; valid2 = 0; addr1 = 32'd12; addr2 = 32'd36; din = 32'd198; wr = 1;

#5 valid1 = 1; #10 valid1 = 0; valid2 = 1; #10 addr1 = 32'd0; addr2 = 32'd0; #5 wr = 0; #12 wr = 1;

/* ncsim> run 0NON-BLOCKING: Values valid1=0, valid2=0, wr=1, addr1= 12, addr2= 36, data= X, aout= x 5NON-BLOCKING: Values valid1=1, valid2=0, wr=1, addr1= 12, addr2= 36, data= 0, aout= 44 15NON-BLOCKING: Values valid1=0, valid2=1, wr=1, addr1= 12, addr2= 36, data= 199, aout= 44 25NON-BLOCKING: Values valid1=0, valid2=1, wr=1, addr1= 0, addr2= 0, data= 199, aout= 44 30NON-BLOCKING: Values valid1=0, valid2=1, wr=0, addr1= 0, addr2= 0, data= 0, aout= 0 42NON-BLOCKING: Values valid1=0, valid2=1, wr=1, addr1= 0, addr2= 0, data= 199, aout= 0 ncsim: *W,RNQUIE: Simulation is complete. */

/* ncsim> run 0BLOCKING: Values valid1=0, valid2=0, wr=1, addr1= 12, addr2= 36, data= 0, aout= 44 5BLOCKING: Values valid1=1, valid2=0, wr=1, addr1= 12, addr2= 36, data= 199, aout= 44 15BLOCKING: Values valid1=0, valid2=1, wr=1, addr1= 12, addr2= 36, data= 199, aout= 44 25BLOCKING: Values valid1=0, valid2=1, wr=1, addr1= 0, addr2= 0, data= 199, aout= 0 30BLOCKING: Values valid1=0, valid2=1, wr=0, addr1= 0, addr2= 0, data= 0, aout= 0 42BLOCKING: Values valid1=0, valid2=1, wr=1, addr1= 0, addr2= 0, data= 199, aout= 0 ncsim: *W,RNQUIE: Simulation is complete. ncsim> exit */

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Using a continous assignment in a Verilog procedure?

Is it possible and/or useful to ever use a continuous assignment in a Verilog procedure? For example, would there ever be any reason to put an assign inside of an always block?

For example this code:

Furthermore would it be possible to generate sequential logic with this approach?

• system-verilog

It is called procedural continuous assignment . It is the use of an assign or force (and their corresponding counterparts deassign and release ) within procedural block. A new continuous assignment process is created when the line is reached in the procedural block. assign can be applied to register types such as reg , integer , and real . force can be applied to registers and nets (i.e. wire s). It has been part of the LRM since 1364-1995.

• IEEE Std 1364-1995 § 9.3
• IEEE Std 1364-2001 § 9.3
• IEEE Std 1364-2005 § 9.3
• IEEE Std 1800-2005 § 25.3
• IEEE Std 1800-2009 § 10.6
• IEEE Std 1800-2012 § 10.6

Procedural continuous assignments are synthesizable, by most tools. However it is recommend to limit the use to behavior modeling of an analog block, test bench files, or fixing RTL<->gate functional mismatches.

• always @* assign data_in = Data; is functional the same as always @* data_in = Data;
• always @(posedge clk) assign data_in = Data; is functional the same as: always @(posedge clk) enable = 1; always @* if (enable==1) data_in = Data;

A valid use of procedural continuous assignment would be should be applited to the following:

It will synthesize to a flop with an asynchronous set and reset with priority to reset. In simulation however the model is inaccurate if rst_n and set_n are both low then rst_n goes high. q should go to 1 the the asynchronous set is still enabled, but nothing to trigger in in the sensitivity list. This is a well documented issue with Verilog. It is the one case procedural continuous assignment are allowed in RTL when used with the translate off keyword your synthesizer. The release / deassign allows the the register/wire to be assigned in the usual manner.

OR (currently valid but discouraged)

Using assign / deassign in this manner is being considered to be depreciated in future IEEE 1800 release. IEEE Std 1800-2005 § 25.3, IEEE Std 1800-2009 § C.4.2 and IEEE Std 1800-2012 § C.4.2 recognizes assign used this way causes confusion and is the source of errors. Use force / release if procedural continuous assignment as needed.

In generate using procedural continuous assignment (with force / release ) should only be used if absolutely necessary. Alternative approaches are more reliable.

Misuse of procedural continuous assignment and solutions:

Combinational logic on reg :

Combinational logic on wire :

Sequential logic:

Solution ( assuming original functionality is wrong ):

Solution ( assuming original functionality is correct ):

• 1 There is nothing unsynthesizable about a procedural continuous assignment. always @sel if (sel) assign out = in1 else assign out = in2; is perfectly synthesizable as a mux. The real problem was that that Verilog chose to use the same syntax/keyword for two features that are too similar. That was the cause of much confusion. –  dave_59 Commented May 19, 2014 at 21:18
• 1 +1. I learned something. Looks like most tools support it; not altera and yosys as examples. Still not recommended. It causes to much confusion. –  Greg Commented May 19, 2014 at 21:50
• @Greg What is the differnce between the solutions to the "Sequential logic" example? Do they behave differently? –  Moberg Commented May 21, 2014 at 9:05
• 1 The "assume original is wrong" calculates the combinational logic before setting the flop, lets call this Comb2Flop. The "assume original is correct" assigns the flop and uses it as a mux selector for combinational logic, lets call this Flop2Comb. The difference being reg2 in Flop2Comb is asynchronous so a change a/b/c/x/y/z unrelated to the clock could change reg2 's value. reg2 is only updated on a posedge clk for Flop2Comb. Note: procedural continuous assignment in real designs are not common so I'm assuming any use of it is a newbie mistake. –  Greg Commented May 21, 2014 at 17:00

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A continuous assignment drives a value into a net.

Description:

Continuous assignments model combinational logic. Each time the expression changes on the right-hand side, the right-hand side is re-evaluated, and the result is assigned to the net on the left-hand side.

The implicit continuous assignment combines the net declaration (see Net data type) and continuous assignment into one statement. The explicit assignment require two statements: one to declare the net (see Net data type), and one to continuously assign a value to it.

Continuous assignments are not the same as procedural continuous assignments. Continuous assignments are declared outside of procedural blocks. They automatically become active at time zero, and are evaluated concurrently with procedural blocks, module instances, and primitive instances.

Net data type , Procedural continuous assignment

• The Verilog-AMS Language
• Continuous Assigns

Continuous Assigns 

A module may have any number of continuous assign statements. Continuous assign statements are used to drive values on to wires. For example:

This is referred to as a continuous assign because the wire on the left-hand side of the assignment operator is continuously driven with the value of the expression on the right hand side. The target of the assign statement must be a wire. The continuous assign statement is not a procedural statement and so must be used at the module level; it cannot be placed in an initial or always process.

You can add delay to a continuous assign statement as follows:

In this case, the value of a changes 10 units of time after the expression b & c changes. Continuous assign statement implement inertial delay, meaning that continuous assign statements swallow glitches. This is illustrated below with the assumption that the unit of time is 1ns.

It is possible to specify up to three delay values on a continuous assignment:

When you specify more than one:

The first delay refers to the transition to the 1 value (rise delay).

The second delay refers to the transition to the 0 value (fall delay).

The third delay refers to the transition to the high-impedance value.

When a value changes to the unknown (x) value, the delay is the smallest of the delays specified.

If only two delays are specified, then the delay to high-impedance is the smallest of the two values specified.

Continuous Assignment and Combinational Logic in SystemVerilog

In this post, we primarily talk about the concept of continuous assignment in SystemVerilog . We also look at how we can use this in conjunction with the SystemVerilog operators to model basic combinational logic circuits .

However, continuous assignment is a feature which is entirely inherited from verilog so anyone who is already familiar with verilog can skip this post.

There are two main classes of digital circuit which we can model in SystemVerilog – combinational and sequential .

Combinational logic is the simplest of the two, consisting solely of basic logic gates, such as ANDs, ORs and NOTs. When the circuit input changes, the output changes almost immediately (there is a small delay as signals propagate through the circuit).

In contrast, sequential circuits use a clock and require storage elements such as flip flops . As a result, output changes are synchronized to the circuit clock and are not immediate.

In the rest of this post, we talk about the main techniques we can use to design combinational logic circuits in SystemVerilog.

In the next post, we will discuss the techniques we use to  model basic sequential circuits .

Continuous Assignment in SystemVerilog

In verilog based designs, we use continuous assignment to drive data on verilog net types . As a result of this, we use continuous assignment to model combinational logic circuits.

In SystemVerilog, we often use the logic data type rather than the verilog net or reg types. This is because the behavior of the logic type is generally more intuitive than the reg and wire types.

Despite this, we still make use of continuous assignment in SystemVerilog as it provides a convenient way of modelling combinational logic circuits.

We can use continuous assignment with either the logic type or with net types such as wire.

In SystemVerilog, we can actually use two different methods to implement continuous assignment.

The first of these is known as explicit continuous assignment. This is the most commonly used method for continuous assignment in SystemVerilog.

In addition, we can also use implicit continuous assignment, or net declaration assignment as it is also known. This method is less common but it can allow us to write less code.

Let's look at both of these techniques in more detail.

• Explicit Continuous Assignment

We normally use the assign keyword when we want to use continuous assignment in SystemVerilog. This approach is known as explicit continuous assignment.

The SystemVerilog code below shows the general syntax for continuous assignment using the assign keyword.

In this construct, we use the <variable> field to give the name of the signal which we are assigning data to. As we mentioned earlier, we can only use continuous assignment to assign data to net or logic type variables.

The <value> field can be a fixed value or we can create an expression using the SystemVerilog operators we discussed in a previous post.

When we use continuous assignment, the <variable> value changes whenever one of the signals in the <value> field changes state.

The code snippet below shows the most basic example of continuous assignment in SystemVerilog. In this case, whenever the b signal changes states, the value of a is updated so that it is equal to b.

• Net Declaration Assignment

We can also use implicit continuous assignment in our SystemVerilog designs. This approach is also commonly known as net declaration assignment in SystemVerilog.

When we use net declaration assignment, we place a continuous assignment in the statement which declares our signal. This can allow us to reduce the amount of code we have to write.

To use net declaration assignment in SystemVerilog, we use the = symbol to assign a value to a signal when we declare it.

The code snippet below shows the general syntax we use for net declaration assignment.

The variable and value fields have the same function for both explicit continuous assignment and net declaration assignment.

As an example, the SystemVerilog code below shows how we would use net declaration assignment to assign the value of b to signal a.

Modelling Combinational Logic Circuits in SystemVerilog

We use continuous assignment and the SystemVerilog operators to model basic combinational logic circuits in SystemVerilog.

In order to show we would do this, let's look at the very basic example of a three input and gate as shown below.

In order to model this circuit in SystemVerilog, we must use the assign keyword to drive the data on to the and_out output.

We can then use the bit wise and operator (&) to model the behavior of the and gate.

The code snippet below shows how we would model this three input and gate in SystemVerilog.

This example shows how simple it is to design basic combinational logic circuits in SystemVerilog. If we need to change the functionality of the logic gate, we can simply use a different SystemVerilog bit wise operator .

If we need to build a more complex combinational logic circuit, it is also possible for us to use a mixture of different bit wise operators.

To demonstrate this, let's consider the basic circuit shown below as an example.

In order to model this circuit in SystemVerilog, we need to use a mixture of the bit wise and (&) and or (|) operators. The code snippet below shows how we would implement this circuit in SystemVerilog.

Again, this code is relatively straight forward to understand as it makes use of the SystemVerilog bit wise operators which we discussed in the last post.

However, we need to make sure that we use brackets to model more complex logic circuit. Not only does this ensure that the circuit operates properly, it also makes our code easier to read and maintain.

Modelling Multiplexors in SystemVerilog

Multiplexors are another component which are commonly used in combinational logic circuits.

In SystemVerilog, there are a number of ways we can model these components.

One of these methods uses a construct known as an always block which we will discuss in detail in the next post. Therefore, we will not discuss this approach to modelling multiplexors in this post.

However, we will look at the other methods we can use to model multiplexors in the rest of this post.

• SystemVerilog Conditional Operator

As we talked about in a previous post, there is a conditional operator in SystemVerilog . This functions in the same way as the conditional operator in the C programming language.

To use the conditional operator, we write a logical expression before the ? operator which is then evaluated to see if it is true or false.

The output is assigned to one of two values depending on whether the expression is true or false.

The SystemVerilog code below shows the general syntax which the conditional operator uses.

From this example, it is clear how we can create a basic two to one multiplexor using this operator.

However, let's look at the example of a simple 2 to 1 multiplexor as shown in the circuit diagram below.

The code snippet below shows how we would use the conditional operator to model this multiplexor in SystemVerilog.

• Nested Conditional Operators

Although this is not common, we can also write code to build larger multiplexors by nesting conditional operators.

To show how this is done, let's consider a basic 4 to 1 multiplexor as shown in the circuit below.

In order to model this in SystemVerilog using the conditional operator, we treat the multiplexor circuit as if it were a pair of two input multiplexors.

This means one multiplexor will select between inputs A and B whilst the other selects between C and D. Both of these multiplexors use the LSB of the address signal as the address pin.

The SystemVerilog code shown below demonstrates how we would implement this.

To create the full four input multiplexor, we would then need another multiplexor.

This multiplexor then takes the output of the other two multiplexors and uses the MSB of the address signal to select between.

The code snippet below shows the simplest way to do this. This code uses the signals mux1 and mux2 which we defined in the last example.

However, we could easily remove the mux1 and mux2 signals from this code and instead use nested conditional operators.

This reduces the amount of code that we would have to write without affecting the functionality.

The code snippet below shows how we would do this.

As we can see from this example, when we use conditional operators to model multiplexors in verilog, the code can quickly become difficult to understand. Therefore, we should only use this method to model small multiplexors.

• Arrays as Multiplexors

It is also possible for us to use basic SystemVerilog arrays to build simple multiplexors.

In order to do this, we combine all of the multiplexor inputs into a single array type and use the address to point at an element in the array.

In order to get a better idea of how this works in practise, let's consider a basic four to one multiplexor as an example.

The first thing we must do is combine our input signals into an array. There are two ways in which we can do this.

Firstly, we can declare an array and then assign all of the individual bits, as shown in the SystemVerilog code below.

Alternatively we can use the SystemVerilog concatenation operator , which allows us to assign the entire array in one line of code.

In order to do this, we use a pair of curly braces - { } - and list the elements we wish to include in the array inside of them.

When we use the concatenation operator we can also declare and assign the variable in one statement.

The SystemVerilog code below shows how we can use the concatenation operator to populate an array.

As SystemVerilog is a loosely typed language , we can use the two bit addr signal as if it were an integer type. This signal then acts as a pointer that determines which of the four elements to select.

The code snippet below demonstrates this method in practise.

What is the difference between implicit and explicit continuous assignment?

When we use implicit continuous assignment we assign the variable a value when we declare. In contrast, when we use explicit continuous assignment we use the assign keyword to assign a value.

Write the code for a 2 to 1 multiplexor using any of the methods discussed in this post.

Write the code for circuit below using both implicit and explicit continuous assignment.

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Verilog assign statement

Hardware schematic.

Signals of type wire or a similar wire like data type requires the continuous assignment of a value. For example, consider an electrical wire used to connect pieces on a breadboard. As long as the +5V battery is applied to one end of the wire, the component connected to the other end of the wire will get the required voltage.

In Verilog, this concept is realized by the assign statement where any wire or other similar wire like data-types can be driven continuously with a value. The value can either be a constant or an expression comprising of a group of signals.

Assign Syntax

The assignment syntax starts with the keyword assign followed by the signal name which can be either a single signal or a concatenation of different signal nets. The drive strength and delay are optional and are mostly used for dataflow modeling than synthesizing into real hardware. The expression or signal on the right hand side is evaluated and assigned to the net or expression of nets on the left hand side.

Delay values are useful for specifying delays for gates and are used to model timing behavior in real hardware because the value dictates when the net should be assigned with the evaluated value.

• LHS should always be a scalar or vector net or a concatenation of scalar or vector nets and never a scalar or vector register.
• RHS can contain scalar or vector registers and function calls.
• Whenever any operand on the RHS changes in value, LHS will be updated with the new value.
• assign statements are also called continuous assignments and are always active

In the following example, a net called out is driven continuously by an expression of signals. i1 and i2 with the logical AND & form the expression.

If the wires are instead converted into ports and synthesized, we will get an RTL schematic like the one shown below after synthesis.

Continuous assignment statement can be used to represent combinational gates in Verilog.

The module shown below takes two inputs and uses an assign statement to drive the output z using part-select and multiple bit concatenations. Treat each case as the only code in the module, else many assign statements on the same signal will definitely make the output become X.

Assign reg variables

It is illegal to drive or assign reg type variables with an assign statement. This is because a reg variable is capable of storing data and does not require to be driven continuously. reg signals can only be driven in procedural blocks like initial and always .

Implicit Continuous Assignment

When an assign statement is used to assign the given net with some value, it is called explicit assignment. Verilog also allows an assignment to be done when the net is declared and is called implicit assignment.

Combinational Logic Design

Consider the following digital circuit made from combinational gates and the corresponding Verilog code.

Combinational logic requires the inputs to be continuously driven to maintain the output unlike sequential elements like flip flops where the value is captured and stored at the edge of a clock. So an assign statement fits the purpose the well because the output o is updated whenever any of the inputs on the right hand side change.

After design elaboration and synthesis, we do get to see a combinational circuit that would behave the same way as modeled by the assign statement.

See that the signal o becomes 1 whenever the combinational expression on the RHS becomes true. Similarly o becomes 0 when RHS is false. Output o is X from 0ns to 10ns because inputs are X during the same time.

Click here for a slideshow with simulation example !

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Procedural continuous assignments

Assign and deassign, force and release.

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Placing values onto variables and nets are called assignments. There are three necessary forms: An assignment has two parts, right-hand side (RHS) and left-hand side (LHS) with an equal symbol (=) or a less than-equal symbol (<=) in between. Assignment Type Left-hand Side Procedural Continuous Procedural Continuous The RHS can contain any expression that evaluates to a final value while the LHS indicates a variable or net to which RHS's value is being assigned. Procedural Assignment Procedural assignments occur within procedures such as initial, always, task , and functions are used to place values onto variables. The variable will hold the value until the next assignment to the same variable. The value will be placed onto the variable when the simulation executes this statement during simulation time. This can be modified and controlled the way we want by using control flow statements such as if-else-if, looping , and case statement mechanisms. Variable Declaration Assignment An initial value can be placed onto a variable at the time of its declaration. The assignment does not have the duration and holds the value until the next assignment to the same variable happens. NOTE: The variable declaration assignments to an array are not allowed. If the variable is initialized during declaration and at 0 times in an initial block as shown below, the order of evaluation is not guaranteed, and hence can have either 8'h05 or 8'hee. Continuous Assignment This is used to assign values onto scalar and vector nets. And it happens whenever there is a change in the RHS. It provides a way to model combinational logic without specifying an interconnection of gates and makes it easier to drive the net with logical expressions. Whenever b or c changes its value, the whole expression in RHS will be evaluated and updated with the new value. Net Declaration Assignment This allows us to place a continuous assignment on the same statement that declares the net. NOTE: Only one declaration assignment is possible because a net can be declared only once. Procedural continuous assignment. These are procedural statements that allow expressions to be continuously assigned to variables or nets. And these are the two types. 1. Assign deassign: It will override all procedural assignments to a variable and deactivate it using the same signal with deassign . The value of the variable will remain the same until the variable gets a new value through a procedural or procedural continuous assignment. The LHS of an assign statement cannot be a part-select, bit-select, or an array reference, but it can be a variable or a combination of the variables. 2. Force release: These are similar to the assign deassign statements but can also be applied to nets and variables. The LHS can be a bit-select of a net, part-select of a net, variable, or a net but cannot be the reference to an array and bit or part select of a variable. The force statement will override all other assignments made to the variable until it is released using the release keyword. Send your Feedback to [email protected] Help Others, Please Share Learn Latest Tutorials Transact-SQL Reinforcement Learning R Programming React Native Python Design Patterns Python Pillow Python Turtle Preparation Verbal Ability Interview Questions Company Questions Trending Technologies Artificial Intelligence Cloud Computing Data Science Machine Learning B.Tech / MCA Data Structures Operating System Computer Network Compiler Design Computer Organization Discrete Mathematics Ethical Hacking Computer Graphics Software Engineering Web Technology Cyber Security C Programming Control System Data Mining Data Warehouse 442 IMAGES Verilog: Continuous Assignment Solved 1. Using continuous assignments, write a Verilog PPT Solved Using Verilog continuous assignment statements, write PPT HDL Verilog:Online Lecture 9:Unit 2:Dataflow modelling,Continuous assignments and delays, simulation COMMENTS Verilog Assignments This will override all procedural assignments to a variable and is deactivated by using the same signal with deassign. The value of the variable will remain same until the variable gets a new value through a procedural or procedural continuous assignment. Using Continuous Assignment to Model Combinational Logic in Verilog The verilog code below shows the general syntax for continuous assignment using the assign keyword. assign <variable> = <value>; The <variable> field in the code above is the name of the signal which we are assigning data to. We can only use continuous assignment to assign data to net type variables. Continuous Assignments in Verilog In Verilog, continuous assignments play a crucial role in hardware description and simulation. They allow us to define and connect signals in a concise and efficient manner, providing a clear representation of the system's behavior. In this section, we will explore the syntax and correct usage of continuous assignments in Verilog, empowering ... Verilog Continuous Assignment Statements Tutorial Continuous assignment statements in Verilog are used to specify the relationship between input and output signals in a combinational circuit. They allow you to assign a value to a signal continuously, meaning the assignment is continuously evaluated as the inputs change. Continuous assignments are used outside procedural blocks and are ideal ... Verilog: Continuous & Procedural Assignments Continuous assignment is used to drive a value on to a net in dataflow modeling. The net can be a vector or scalar, indexed part select, constant bit or part select of a vector. Concatenation is also supported with scalar vector types. module Conti_Assignment (addr1,addr2,wr,din,valid1,valid2,dout); input [31:0] addr1,addr2; input [31:0] din ... Using a continous assignment in a Verilog procedure? It is called procedural continuous assignment. It is the use of an assign or force (and their corresponding counterparts deassign and release) within procedural block. A new continuous assignment process is created when the line is reached in the procedural block. assign can be applied to register types such as reg, integer, and real. force can ... Continuous Assignment Description: Continuous assignments model combinational logic. Each time the expression changes on the right-hand side, the right-hand side is re-evaluated, and the result is assigned to the net on the left-hand side. The implicit continuous assignment combines the net declaration (see Net data type) and continuous assignment into one statement. Continuous Assigns Continuous assign statements are used to drive values on to wires. For example: assign a = b & c; This is referred to as a continuous assign because the wire on the left-hand side of the assignment operator is continuously driven with the value of the expression on the right hand side. The target of the assign statement must be a wire. PDF L3: Introduction to Verilog (Combinational Logic) L3: 6.111 Spring 2004 Introductory Digital Systems Laboratory 6 Continuous (Dataflow) Assignment Continuous assignments use the assignkeyword A simple and natural way to represent combinational logic Conceptually, the right-hand expression is continuously evaluated as a function of arbitrarily-changing inputs…just like dataflow PDF Intro to Verilog use Verilog's operators and continuous assignment statements: Conceptually assign's are evaluated continuously, so whenever a value used in the RHS changes, the RHS is re-evaluated and the value of the wire/bus specified on the LHS is updated. This type of execution model is called "dataflow" since evaluations Continuous Assignment and Combinational Logic in SystemVerilog Explicit Continuous Assignment. We normally use the assign keyword when we want to use continuous assignment in SystemVerilog. This approach is known as explicit continuous assignment. The SystemVerilog code below shows the general syntax for continuous assignment using the assign keyword. assign <variable> = <value>; Verilog assign statement Verilog assign statement. Signals of type wire or a similar wire like data type requires the continuous assignment of a value. For example, consider an electrical wire used to connect pieces on a breadboard. As long as the +5V battery is applied to one end of the wire, the component connected to the other end of the wire will get the required ... Verilog Continuous assignments provide a way of modeling combinational logic at a higher level of abstraction than Gate-Level logic. It allows the use of Boolean logic rather than gate connections. The left-hand side of an assignment is a variable to which the right-side value is to be assigned and must be a scalar or vector net or concatenation of both. Verilog Continuous Assignment Statement In verilog, continuous assignment statement is implemmented with assign statement or with wire declaration. We will first consider the assign statement. The left-hand side of an assignment is a variable to which the right-side value is to be assigned. The left hand side must be a scalar or vector net or concatenation of both. Procedural continuous assignments The continuous assignment is used to drive net data type variables using the 'assign' statements whereas procedural assignments are used to drive reg data type variables using initial and always block statements. Verilog also provides a third type of assignment i.e. procedural continuous assignment that drives net or reg data type variables ... Verilog Assignments And these are the two types. 1. Assign deassign: It will override all procedural assignments to a variable and deactivate it using the same signal with deassign. The value of the variable will remain the same until the variable gets a new value through a procedural or procedural continuous assignment. Verilog Continuous assignments provide a way of modeling combinational logic at a higher level of abstraction than Gate-Level logic. It allows the use of Boolean logic rather than gate connections. The left-hand side of an assignment is a variable to which the right-side value is to be assigned and must be a scalar or vector net or concatenation of both. essay homework paper phd project resume review study thesis