conditional assignment verilog

Verilog Conditional Operator

Just what the heck is that question mark doing.

Have you ever come across a strange looking piece of Verilog code that has a question mark in the middle of it? A question mark in the middle of a line of code looks so bizarre; they’re supposed to go at the end of sentences! However in Verilog the ? operator is a very useful one, but it does take a bit of getting used to.

The question mark is known in Verilog as a conditional operator though in other programming languages it also is referred to as a ternary operator , an inline if , or a ternary if . It is used as a short-hand way to write a conditional expression in Verilog (rather than using if/else statements). Let’s look at how it is used:

Here, condition is the check that the code is performing. This condition might be things like, “Is the value in A greater than the value in B?” or “Is A=1?”. Depending on if this condition evaluates to true, the first expression is chosen. If the condition evaluates to false, the part after the colon is chosen. I wrote an example of this. The code below is really elegant stuff. The way I look at the question mark operator is I say to myself, “Tell me about the value in r_Check. If it’s true, then return “HI THERE” if it’s false, then return “POTATO”. You can also use the conditional operator to assign signals , as shown with the signal w_Test1 in the example below. Assigning signals with the conditional operator is useful!

Nested Conditional Operators

There are examples in which it might be useful to combine two or more conditional operators in a single assignment. Consider the truth table below. The truth table shows a 2-input truth table. You need to know the value of both r_Sel[1] and r_Sel[0] to determine the value of the output w_Out. This could be achieved with a bunch of if-else if-else if combinations, or a case statement, but it’s much cleaner and simpler to use the conditional operator to achieve the same goal.

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?: conditional operator in Verilog

Compact conditional operators.

Many Verilog designs make use of a compact conditional operator:

A comman example, shown below, is an “enable” mask. Suppose there is some internal signal named a . When enabled by en== 1 , the module assigns q = a , otherwise it assigns q = 0 :

The syntax is also permitted in always blocks:

Assigned Tasks

This assignment uses only a testbench simulation, with no module to implement. Open the file src/testbench.v and examine how it is organized. It uses the conditional operator in an always block to assign q = a^b (XOR) when enabled, else q= 0 .

Run make simulate to test the operation. Verify that the console output is correct. Then modify the testbench to use an assign statement instead of an always block . Change the type of q as appropriate for the assign statement.

Turn in your work using git :

Indicate on Canvas that your assignment is done.

Verilog Conditional Statements Tutorial

Conditional statements are crucial in Verilog as they enable you to make decisions and create conditional behaviors in your designs. They allow you to execute specific blocks of code based on certain conditions. In this tutorial, we will explore different types of conditional statements in Verilog and learn how to use them effectively.

Introduction to Conditional Statements

Conditional statements in Verilog provide a way to control the flow of your code based on certain conditions. They are essential for implementing decision-making logic and creating complex behaviors in digital designs. The three primary conditional statements in Verilog are if , else if , and case .

Verilog if Statement

The if statement is used to check a single condition and execute a block of code if the condition evaluates to true. If the condition is false, the code inside the if block is skipped. The syntax of the if statement is as follows:

Verilog else if Statement

The else if statement allows you to check additional conditions if the previous conditions in the if and else if blocks are false. The code inside the first matching condition block is executed, and the rest are skipped. The syntax of the else if statement is as follows:

Verilog case Statement

The case statement is used to perform multi-way decisions based on the value of an expression. It is similar to the switch-case statement in many programming languages. The syntax of the case statement is as follows:

Common Mistakes with Verilog Conditional Statements

• Using blocking assignments inside the conditional blocks, leading to incorrect behavior.
• Not considering all possible cases in the case statement, causing incomplete behavior.
• Mixing different data types in conditionals without proper typecasting.
• Using the assignment operator "=" instead of the equality operator "==" in conditions.
• Overlooking operator precedence in complex conditions, leading to unexpected results.

Frequently Asked Questions (FAQs)

• Q: Can I have nested conditional statements in Verilog? A: Yes, you can nest conditional statements (e.g., if inside else or case inside case ) to create complex decision structures.
• Q: Can I use non-constant expressions in case statements? A: No, Verilog requires constant expressions in case statements, so each case value must be a constant or a constant expression.
• Q: Is the case statement only for sequential logic? A: No, the case statement can be used for both combinational and sequential logic depending on how it is used in the code.
• Q: How does Verilog handle multiple matching cases in a case statement? A: Verilog executes the first matching case it encounters in a case statement and skips the rest.
• Q: Can I use multiple conditions in an if statement? A: Yes, you can use logical operators (e.g., &&, ||) to combine multiple conditions in an if statement.

Conditional statements in Verilog are essential for implementing decision-making logic and creating complex behaviors in digital designs. The if , else if , and case statements enable you to control the flow of your code based on specific conditions. By understanding how to use these conditional statements correctly, you can create efficient and reliable Verilog designs for various applications.

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• Conditional Statements in Verilog

16 Sep 2021

Conditional statements in a programming language help to branch the execution of code depending on some condition. In simpler words, using these statements, the execution of specific code lines can be controlled using some conditions. Verilog provides two types of conditional statements, namely

• Case statement

Let us explore both statements in detail.

If-else is the most straightforward conditional statement construct. For the compiler, this statement means that “If some condition is true, execute code inside if or else execute codes inside else.” The condition evaluates to be true if the argument is a non-zero number. In if-else, it is not necessary to always have an else block.

If-else-if construct is used when more than one condition needs to be checked. All the conditions are checked serially, and if none of the conditions is true, then the else block is executed.

Case Statements

Case statements are generally used when the condition is an equivalence check, which means “==” or “===”. Case statement makes the code more readable wrt to if-else if there are many conditions to check. As the name indicates, this statement will switch a particular case based on some arguments.

Again, to account for the four-state variables, there are three case statement variants: case , casez , and casex . Let us look at them in detail and also find out the difference.

In the case statement, exact matching is done. If even a single bit does not match the cases defined, the default case will be executed.

In the previous statement, we have seen that a case will be executed if it matches precisely with the argument. However, in many cases, we would not want to match the cases exactly. Consider a simple example where the master uses a select signal to select a particular slave amongst many (say, four slaves). Thus, the select line would be a 4-bit signal. A single bit needs to be high to select any slave, and the rest of the bits can be anything. Thus, instead of writing multiple cases for a slave, we can use casez. In casez, the z or ‘?’ is does not care, which means it will not be considered while comparing the case with the argument.

Casex is a bit more flexible than casez. In casex, even X is not considered during case comparison. Thus, X , Z , ? all are considered as do not care in casex. Let us see the difference between all these case statements using an example.

In the below example, the same operations are performed in different always block but using a different case statement. In the output, it can be seen that, if the value of the argument, i.e., op_code is X , then default case is executed in the case and casez construct, but for casex , the X is considered as do not care, and thus the first case is executed. When the op_code is Z , then only in the case construct the default case is executed, and for the casex and casez , Z is considered as do not care, and the 1st case is executed.

All three case statements are synthesizable, but it is not recommended to use casex and casez in designs. It is because the output after synthesis and during the simulation can differ in casex and casez. Case statements come in behavioural modelling (we will learn more about in future topics), and for complex behavioural logics, the synthesis can be different in different tools.

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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.

Modeling Concurrent Functionality in Verilog

• First Online: 01 March 2019

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• Brock J. LaMeres 2  

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This chapter presents a set of built-in operators that will allow basic logic expressions to be modeled within a Verilog module. This chapter then presents a series of combinational logic model examples.

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LaMeres, B.J. (2019). Modeling Concurrent Functionality in Verilog. In: Quick Start Guide to Verilog. Springer, Cham. https://doi.org/10.1007/978-3-030-10552-5_3

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• The Verilog-AMS Language
• Initial and Always Processes
• Assignment Statements

Assignment Statements 

Blocking assignment .

A blocking assignment evaluates the expression on its right hand side and then immediately assigns the value to the variable on its left hand side:

It is also possible to add delay to a blocking assignment. For example:

In this case, the expression on the right hand side is evaluated and the value is held for 10 units of time. During this time, the execution of the code is blocked in the middle of the assignment statement. After the 10 units of time, the value is stored in the variable on the left

Nonblocking Assignment 

A nonblocking assignment evaluates the expression on its right hand side without immediately assigning the value to the variable on the left. Instead the value is cached and execution is allowed to continue onto the next statement without performing the assignment. The assignment is deferred until the next blocking statement is encountered. In the example below, on the positive edge of clk the right-hand side of the first nonblocking assignment is evaluated and the value cached without changing a. Then the right-hand side of the second nonblocking assignment statement is evaluated is also cached without changing b. Execution continues until it returns to the event statement, once there the execution of the process blocks until the next positive edge of the clk. Just before the process blocks, the cached values finally assigned to the target variables. In this way, the following code swaps the values in a and b on every positive edge of clk:

Adding delay to nonblocking assignments is done as follows:

Using nonblocking assignment with delay in this manner is a way of implementing transport delay , as shown below:

Blocking versus Nonblocking Assignment 

Nonblocking statements allow you to schedule assignments without blocking the procedural flow. You can use the nonblocking procedural statement whenever you want to make several register assignments within the same time step without regard to order or dependence upon each other. It means that nonblocking statements resemble actual hardware more than blocking assignments.

Generally you would use nonblocking assignment whenever assigning to variables that are shared between multiple initial or always processes if the statements that access the variable could execute at the same time. Doing so resolves race conditions.

Blocking assignment is used to assign to temporary variables when breaking up large calculations into multiple assignment statements. For example:

Procedural Continuous Assignment 

Two types of continuous assignment are available in initial and always processes: assign and force .

The target of an assign statement must be a register or a concatenation of registers. The value is continuously driven onto its target and that value takes priority over values assigned in procedural assignments. Once a value is assigned with an assign statement, it can only be changed with another assign statement or with a force statement. Execution of deassign releases the continuous assignment, meaning that the value of the register can once again be changed with procedural assignments. For example, the following implements a D-type flip-flop with set and reset:

Assign statements are used to implement set and reset because they dominate over the non-blocking assignment used to update q upon positive edges of the clock c . If instead a simple procedural assignment were used instead, then a positive edge on the clock could change q even if r or s were high.

A force statement is similar to assign , except that it can be applied to both registers and nets. It overrides all other assignments until the release statement is executed. Force is often used in testbenches to eliminate initial x-values in the DUT or to place it in a particular state. For example:

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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