verilog a bus assignment

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Verilog: How to assign the output of a module to a bus which have different width

Say I have a module foo with a bus input A and a bus output B:

And foo is instantiated inside top module, and I want something like below(likely to have syntax error):

What is the syntax correct and most elegant way to do that?

Use concats, something like the following. Use a temporary signal as a filling for the unconnected slice.

• Is there a way not to use "tmp" to skip the bits in the middle? –  user3618363 Commented Jul 11, 2019 at 23:35

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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.

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System Verilog: syntax qualifying every bit of a bus

There must be something obvious I'm missing and frankly I'm too ashamed to ask my co-workers. Say I have these signals:

One way qualify every bit with valid is:

If I wanted to explicitly zero out the bus whenever don't have valid, I could do:

Another thing can do is use a "for loop" to assign each bit.

But I feel like I'm missing something very obvious in terms of a one liner. It's not any bitwise operator. Any help?

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Blocking and Non-blocking Assignments in Verilog

Introduction to blocking and non-blocking assignments in verilog programming language.

Hello, fellow Verilog enthusiasts! In this blog post, I will introduce you to the concepts of Blocking and Non-blocking Assignments in Verilog . These assignments are crucial for defining how your code interacts with signals and executes within your digital designs.

What are Blocking and Non-blocking Assignments in Verilog Programming Language?

In Verilog, blocking and non-blocking assignments determine how variables receive values and manage the execution of statements within procedural blocks. These assignments play a crucial role in modeling and simulating digital circuits, ensuring that your designs perform as intended.

1. Blocking Assignments

Characteristics of blocking assignments:, 1. sequential execution:.

Each blocking assignment statement blocks or halts the execution of subsequent statements until it finishes. This ensures that operations occur in a specific sequence, which is crucial when the result of one operation is used in the next.

For example, if multiple calculations depend on the results of previous ones, blocking assignments ensure that these dependencies are respected.

2. Immediate Update:

The value of the variable is updated immediately after the assignment statement is executed. This immediate update is ideal for combinational logic, where the output should directly reflect changes in the input.

3. Combinational Logic:

2. non-blocking assignments.

Non-blocking assignments use the <= operator and allow for concurrent execution of statements within the same procedural block. They schedule the updates to occur at the end of the current time step or clock cycle, enabling multiple assignments to happen simultaneously. This behavior is essential for modeling sequential logic, where the updates to variables occur at specific times, often driven by clock edges.

Characteristics of Non-Blocking Assignments:

1. concurrent execution:.

Non-blocking assignments do not block the execution of subsequent statements. All non-blocking assignments within the same procedural block are executed in parallel, but the actual updates to the variables occur at the end of the current time step or clock cycle.

2. Deferred Update:

The value of the variable is not updated immediately. Instead, the assignment schedules the update to occur at the end of the time step, allowing other operations to proceed based on the old value before the update takes effect.

3. Sequential Logic:

Non-blocking assignments are ideal for modeling sequential circuits such as flip-flops and registers, where the value changes in sync with clock edges or specific events.

Why we need Blocking and Non-blocking Assignments in Verilog Programming Language?

Blocking and non-blocking assignments are fundamental in Verilog for modeling different aspects of digital circuits. Blocking assignments are ideal for combinational logic and scenarios requiring sequential execution, providing immediate updates and simplifying code flow.

Blocking assignments are essential when you need sequential execution of statements. They play a crucial role in:

1. Modeling Combinational Logic

Sequential Execution: Blocking assignments ensure that operations occur in a specified order, which is crucial when one operation’s result is needed for subsequent operations within the same procedural block.

2. Simplifying Code

3. initializations in testbenches.

Setup and Initialization: In testbenches, blocking assignments are commonly used to set initial values for simulation variables, configure test conditions, or initialize registers and memories at the start of the simulation.

1. Modeling Sequential Logic

Avoiding Race Conditions: Non-blocking assignments defer updates to prevent race conditions and ensure consistent variable updates within a clock cycle.

2. Synchronizing with Clock Edges

Timing Accuracy: Use non-blocking assignments to model behavior that changes with clock edges, ensuring updates occur at specific times and reflect real hardware operation. This timing accuracy is critical for designing reliable and functional sequential circuits.

3. Improving Simulation Accuracy

Example of blocking and non-blocking assignments in verilog programming language.

Here are examples of blocking and non-blocking assignments in Verilog, illustrating their usage in different contexts:

1. Blocking Assignments Example

Explanation:, 2. non-blocking assignments example.

Non-blocking assignments use the <= operator and allow for concurrent execution of statements within a procedural block. They model sequential logic and ensure updates occur at the end of the current time step or clock cycle.

Non-Blocking Assignments ( <= ) are used for concurrent execution, which is essential for sequential logic where updates need to be synchronized with clock edges and occur at the end of a time step or clock cycle.

Advantages of Blocking and Non-blocking Assignments in Verilog Programming Language

1. advantages of blocking assignments, 1.1 sequential execution:, 1.2 simplicity in combinational logic:.

Straightforward Coding: For combinational logic where the order of operations is crucial, blocking assignments make it easier to express complex logic without dealing with concurrency issues.

1.3 Ease of Initialization:

1.4 immediate updates:, 2. advantages of non-blocking assignments, 2.1 modeling sequential logic:.

Accurate Timing: Non-blocking assignments are essential for accurately modeling sequential circuits like flip-flops and registers, where updates should occur at the end of a clock cycle. This timing accuracy is critical for ensuring correct behavior in synthesized hardware.

2.2 Concurrency:

2.3 avoiding race conditions:.

Consistent Updates: By deferring updates until the end of the time step, non-blocking assignments help prevent race conditions, where the order of operations could lead to unpredictable results.

2.4 Synchronized Behavior:

Clock Edge Sensitivity: Non-blocking assignments are ideal for modeling behaviors that need to synchronize with clock edges, ensuring that all updates occur simultaneously and consistently across all registers.

2.5 Simulation Accuracy:

Disadvantages of blocking and non-blocking assignments in verilog programming language, 1. disadvantages of blocking assignments, 1.1 potential for simulation issues:, 1.2 limited for sequential logic:.

Inaccurate Timing: Blocking assignments are not suitable for modeling sequential logic where updates should occur at specific clock edges, potentially leading to incorrect behavior in designs that rely on precise timing.

1.3 Race Conditions:

Concurrency Issues: When used in parallel processes, blocking assignments can lead to race conditions where the order of execution affects the final result, making it challenging to manage concurrent operations.

1.4 Complex Debugging:

2. disadvantages of non-blocking assignments, 2.1 increased complexity:.

Understanding Concurrency: Non-blocking assignments introduce concurrency, which can make code more complex to understand and debug, particularly in designs with multiple interacting processes.

2.2 Potential for Unintended Behavior:

2.3 less immediate feedback:, 2.4 synthesis challenges:.

Complex Synthesis: While non-blocking assignments are essential for modeling sequential logic, they can sometimes complicate synthesis processes, especially if not used correctly, leading to inefficient hardware implementations.

Equivalent Technical Articles

Conditional statements in c programming, do-while loop in c programming, while loop in c programming language.

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What is the "+:" operator called in Verilog?

I am going through a Verilog test case, and I found this statement:

What is the "+:" operator known as? I tried to find this on google, but I didn't get any relevant answer.

2 Answers 2

That syntax is called an indexed part-select . The first term is the bit offset and the second term is the width. It allows you to specify a variable for the offset, but the width must be constant.

Example from the SystemVerilog 2012 LRM:

Lastly i got the source page for this, this is called as Indexed Vector part Select ("+:") .

To explain it a bit more

With Indexed vector part select, which is added in Verilog 2000, you can select a part of bus rather then selecting whole bus.

44*8 part is starting point of part select variable and 64 is the width of part select andis constant.It means that if initially we have initialized

we are selecting a particular part of PQR using

that is PQR_AR[352+:64] or it means that we are taking a part from 352 to 415 out of 0 to 415.

• \$\begingroup\$ From dwikle answer derived from LRM, when we initialize a bus like: input [415:0] PQR; Then, PQR_AR[44*8 +: 64] should be PQR_AR[352:288] and not PQR_AR[415:352]. Please correct me if I am going in wrong direction. \$\endgroup\$ –  ABX Commented Dec 1, 2015 at 5:09
• \$\begingroup\$ @ABX I agree about the direction, except it should be PQR_AR[352:289] to be a 64 bit wide bus \$\endgroup\$ –  peterbc Commented May 22, 2019 at 8:50

Not the answer you're looking for? Browse other questions tagged verilog or ask your own question .

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