continuous and procedural assignment in verilog

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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.

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.

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.

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Verilog Assignments

Variable declaration assignment, net declaration assignment, assign deassign, force release.

• Procedural continuous

Legal LHS values

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.

The RHS can contain any expression that evaluates to a final value while the LHS indicates a net or a variable to which the value in RHS is being assigned.

Procedural Assignment

Procedural assignments occur within procedures such as always , initial , task and functions and 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 at some point during simulation time. This can be controlled and modified the way we want by the use of control flow statements such as if-else-if , case statement and looping mechanisms.

An initial value can be placed onto a variable at the time of its declaration as shown next. The assignment does not have a duration and holds the value until the next assignment to the same variable happens. Note that variable declaration assignments to an array are not allowed.

If the variable is initialized during declaration and at time 0 in an initial block as shown below, the order of evaluation is not guaranteed, and hence can have either 8'h05 or 8'hee.

Procedural blocks and assignments will be covered in more detail in a later section.

Continuous Assignment

This is used to assign values onto scalar and vector nets and 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, then the whole expression in RHS will be evaluated and a will be updated with the new value.

This allows us to place a continuous assignment on the same statement that declares the net. Note that because a net can be declared only once, only one declaration assignment is possible for a net.

Procedural Continuous Assignment

• assign ... deassign
• force ... release

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. The LHS of an assign statement cannot be a bit-select, part-select or an array reference but can be a variable or a concatenation of variables.

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/part select of a variable. The force statment will override all other assignments made to the variable until it is released using the release keyword.

Procedural continuous assignments

Till now we have seen two types of assignments i.e. continuous assignment and procedural assignment .

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 for a certain period of time by overriding the existing assignments.

There are two types of procedural continuous assignments

assign and deassign

Force and release.

The assign and deassign statements control reg type variable values by overriding existing procedural assignments for a limited time period. After the execution of the deassign statement, another procedural or procedural continuous assignment can change the variable value once again, till then the previous value can hold.

The d1 = 3 is assigned at #5 time units and deassign at #10 time units.The d1 = 3 retains till next assignment d1 = 7 happens at 20 time units.

The force and release statements control net and reg data type variable values by overriding existing procedural, continuous or procedural continuous assignments for a limited time period. After the execution of the release statement for the reg data type variable, another procedural or procedural continuous assignment can change the variable value once again, till then the previous value can hold. The value of the previous continuous assignment retains in the case of the net data type variable.

The d1 belongs to the reg data type and d2 belongs to the net data type. Both variables are forced at #5 time units and released at #10 time units Once, it is released, 

• The d1 value remains the same (d1 = 3) until it is changed to d1 = 7 at 20 time units.
• The d2 value holds a previously assigned value using continuous assignment (d2 = 2).

Verilog Tutorials

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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In the previous blog, Procedural Assignments, we looked at procedural assignments. So let’s start a new blog with more details about the study about procedural continuous assignment.

Previously, we learned that a register is assigned a value via procedural assignments. The value remains in the register until another procedural assignment assigns another value to it.

whereas in continuous procedural assignments, reactions are different. They are procedural statements that allow expression values to be constantly pushed into registers or nets for brief periods of time.

Existing assignments to a register or net are overridden by procedural continuous assignments. They are a valuable addition to the standard procedural assignment statement.

assign and deassign

The first form of procedural continuous assignment is expressed by the terms assign and deassign.

Only a register or a concatenation of registers can be used on the left side of procedural continuous assignments. It cannot be a net or register array component or bit select.

The impact of regular procedural assignments is overridden by procedural continuous assignments. Procedural continuous assignments are typically used for timed intervals.

A simple example is shown below by using assign and deassign statements.

We overrode the assignment on q in the preceding example and applied new values to it. After the deassign, the register variables keep the continually given value until they are altered by a subsequent procedural assignment.

force and release

To represent the second version of procedural continuous assignments, the keywords force and release are used. They can be used to overrule register and net assignments.

Force and release statements are commonly used in interactive debugging to force some registers or nets to a value while noting the effect on other registers and nets. It is not advised to utilise force and release statements within design blocks.

They should only exist in stimuli or as debug statements.

force and release on registers

Any procedural assignments or procedural continuous assignments on a register are overridden by force until the register is released.

After being released, the register variables will retain the forced value, which may then be altered by a subsequent procedural assignment.

We may use the following to temporarily override the values of “o” in below example.

force and release on nets

Any ongoing assignments on nets are overridden until the net is freed. When the net is released, it quickly returns to its regular drive value.

A net can be made to represent an expression or a value.

In the preceding example, a new phrase is pushed onto the net between times 50 and 100. When the force statement is active from time 50 to time 100, the phrase a | b & c is revalued and assigned to out anytime the values of signals a, b, or c change.

Thus, the force statement operates similarly to a continuous assignment, except that it is only active for a short duration.

Let’s look at a good example, and don’t worry about the not understanding part because it will be clarified in a later blog; instead, concentrate on how the assign statement works.

4-Bit Carry Lookahead Adder in Verilog

It is worth noting that the carry lookahead adder output (o result) is one bit larger than both adder inputs. This is due to the fact that two N bit vectors added together can yield a result of N+1 size. For instance, b”11″ + b”11″ = b”110″. 3 + 3 = 6 in decimal.

The carry lookahead adder described above makes use of a Verilog parameter to allow for different implementations of the same code. This increases the code’s versatility and reusability.

Using the parameter, the code generates a generate statement that instantiates as many full-adders as the WIDTH parameter specifies.

This code demonstrates the power of parameters and generate statements in creating code that is compact but highly malleable.

It can be used for inputs of any width. Simply set the width appropriately for his or her specific application, and the tools will generate the appropriate amount of logic.

The waveform of the above example is shown in the below figure 1.

Fig 1 4-Bit Carry Lookahead Adder waveform

As a result of reading this blog, we now have a better understanding of procedural continuous assignment.

So let’s summarise the whole blog with a few questions that will make the blog more interesting.

• What do you mean by “procedural continuous assignment”?
• How many types of procedural continuous assignment are there?
• How to use “assign” and “deassign” assignments?
• What is force and release assignment?
• How are force and release assignments used on registers?
• How are force and release assignments used on nets?

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Introduction to SystemVerilog pp 745–757 Cite as

Procedural and Continuous Assignments

• Ashok B. Mehta 2  
• First Online: 07 July 2021

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This chapter will delve into the nuances of procedural and continuous assignments. It discusses features such as blocking and non-blocking procedural assignment, assign/deassign, force-release, etc.

Electronic Supplementary Material The online version of this chapter ( https://doi.org/10.1007/978-3-030-71319-5_23 ) contains supplementary material, which is available to authorized users.

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Using Continuous Assignment to Model Combinational Logic in Verilog

In this post, we talk about continuous assignment in verilog using the assign keyword. We then look at how we can model basic logic gates and multiplexors in verilog using continuous assignment.

There are two main classes of digital circuit which we can model in verilog – 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 this post, we talk about the techniques we can use to design combinational logic circuits in verilog. In the next post, we will discuss the techniques we use to model basic sequential circuits .

Continuous Assignment in Verilog

We use continuous assignment to drive data onto verilog net types in our designs. As a result of this, we often use continuous assignment to model combinational logic circuits.

We can actually use two different methods to implement continuous assignment in verilog.

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

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 verilog. This approach is known as explicit continuous assignment.

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

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.

The <value> field can be a fixed value or we can create an expression using the verilog operators we discussed in a previous post. We can use either variable or net types in this expression.

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 verilog. 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 verilog designs. This approach is also commonly known as net declaration assignment in verilog.

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

Modelling Combinational Logic Circuits in Verilog

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

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

To model this circuit in verilog, we use the assign keyword to drive the data on to the and_out output. This means that the and_out signal must be declared as a net type variable, such as a wire.

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 verilog.

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

To model this circuit in verilog, 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 verilog.

Again, this code is relatively straight forward to understand as it makes use of the verilog 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 Verilog

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

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

One of these methods uses a construct known as an always block . We normally use this construct to model sequential logic circuits, which is the topic of the next post in this series. Therefore, we will look at this approach in more detail the next blog post.

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

• Verilog Conditional Operator

As we talked about in a previous blog, there is a conditional operator in verilog . 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 verilog 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 verilog.

• 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.

To model this in verilog 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.

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

This takes the outputs from the first two multiplexors and uses the MSB of the address signal to select between them.

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 verilog arrays to build simple multiplexors.

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.

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 verilog code below.

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

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, as long as we use a net type.

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

As verilog 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. As the mux output is a wire, we must use continuous assignment in this instance.

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. 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 we discussed.

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

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Verilog Tutorial

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.

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• 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.

A procedural continuous assignment assigns a value to a register.

Description:

A procedural continuous assignments overrides any other procedural assignment. After the procedural continuous assignment is executed, it remains in force on the assigned register or net until it is deassigned, or until another procedural continuous assignment is made to the same register or net.

The keywords assign and deassign can be used for registers, or a concatenation of registers only. It can not be used for memories and bit- or part-select of a register.

Deassign and release de-activate a procedural continuous assignment. The register value remains after the de-activation until a new value is assigned.

A procedural continuous assignment is not the same as a continuous assignment. Procedural continuous assignments are declared inside procedural blocks.

Continuous assignment

alexmiculescu (Member) asked a question.

• module arithmetic_logic_unit
• input [ 3 : 0 ] opcode ,
• input [ 15 : 0 ] operand0 ,
• input [ 15 : 0 ] operand1 ,
• input operand_carry ,
• output reg [ 15 : 0 ] result ,
• output reg zero ,
• output reg carry
• always @(*) begin
• case ( opcode )
• 4 'h3: begin // ADD
• // {carry,result} = operand0 \+ operand1 \+ operand_carry;
• // zero = (result == 16' b0 ) ? 1 'b1 : 1' b0 ;
• 4 'h4: begin // SUB
• {carry,result} = operand0 - operand1 - operand_carry;
• zero = (result == 16' b0 ) ? 1 'b1 : 1' b0 ;
• 4 'h5: begin // AND
• result = operand0 & operand1;
• carry = 1 'b0;
• 4' h6 : begin // OR
• result = operand0 | operand1 ;
• zero = ( result == 16 'b0) ? 1' b1 : 1 'b0;
• carry = 1' b0 ;
• 4 'h7: begin // XOR
• result = operand0 ^ operand1;
• 4' h8 : begin // ROL
• result = { operand0 [ 14 : 0 ], operand0 [ 15 ] };
• carry = operand0 [ 15 ];
• 4' h9 : begin // ROR
• result = { operand0 [ 0 ], operand0 [ 15 : 1 ]};
• carry = operand0 [ 0 ];
• default: begin
• result = 16' b0 ;
• zero = 1' b0 ;
• General Discussion

muzaffer (Member)

u4223374 (Member)

• assign result = ( opcode == 4 'h3) ? operand0 \+ operand1 \+ operand_carry : (opcode == 4' h4 ) ? operand0 - operand1 - operand_carry : ( opcode == 4 'h5) ? operand0 & operand1 : (opcode == 4' h6 ) ? operand0 | operand1 : ( opcode == 4 'h7) operand0 ^ operand1 : (opcode = 4' h8 ) ? { operand0 [ 14 , 0 ], operand1 [ 15 ]} : ( opcode == 4 'h9) ? {operand0[0],operand0[15:1]} : 16' b0 ;

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