Continuous Assignment in Verilog

Summary

This video script delves into Verilog's continuous assignment, a fundamental concept for building complex digital circuits. It contrasts continuous and procedural assignments, focusing on the former's role in combinational logic. The script introduces the 'assign' keyword and gate shorthand notation, essential for creating circuits with less code. An example of a multiplexer implementation using continuous assignment is provided, demonstrating how it simplifies design compared to gate primitives. The session also emphasizes the importance of verifying RTL for correctness and teases the next topic: procedural assignment for sequential logic.

Takeaways

• 📘 Verilog's gate primitives are great for beginners, but for complex circuits, a deeper understanding of the language is necessary.
• 🔄 Assignment in Verilog is fundamental, involving placing values onto nets, wires, registers, or variables, and consists of a left-hand side and a right-hand side.
• 🏗️ Gate primitives used in Verilog are a form of assignment where the output is the left-hand side and the input expression is the right-hand side.
• 🔑 There are two types of assignments in Verilog: continuous and procedural, with continuous assignment being the focus of this session.
• 🔄 Continuous assignment is used for combinational logic circuits and updates the output whenever there's a change on the right-hand side.
• 🕒 Procedural assignment, to be covered later, is used for sequential logic and updates based on specific conditions or signals.
• 🔑 The 'assign' keyword is fundamental to continuous assignment in Verilog.
• 🛠️ Gate shorthand notation, including bitwise and logical operators, allows for compact representation of complex logic operations.
• 🌐 Continuous assignment can simplify the creation of complex circuits like multiplexers, which would be cumbersome to build using only gate primitives.
• 🔗 Continuous assignment can also be used to connect different nets and buses without additional components, simplifying the design of digital systems.
• 🔍 It's important to check the RTL implementation to ensure that the continuous assignment is correctly implemented.

Q & A

What is the main focus of the lab session described in the transcript?

The main focus of the lab session is to enhance understanding of Verilog as a language by exploring assignment, a fundamental element that powers most of what Verilog does.

What are the two main types of assignment in Verilog mentioned in the transcript?

The two main types of assignment in Verilog are continuous and procedural assignments.

How does continuous assignment differ from procedural assignment?

Continuous assignment assigns values to nets and happens whenever there is a change on the right-hand side, continuously driving the left-hand side. Procedural assignment assigns values to registers or variables only under specified conditions, such as on the edge of a clock or another signal.

What is the purpose of the 'assign' keyword in Verilog?

The 'assign' keyword in Verilog is used to create continuous assignment expressions, which generate circuits by assigning values to nets or wires based on expressions on the right-hand side.

What is gate shorthand notation in Verilog?

Gate shorthand notation in Verilog refers to the use of individual operators representing each logic gate, allowing for the construction of complex combinational logic circuits using single assignment statements.

How does the bitwise operator differ from the logical operator in Verilog?

Bitwise operators in Verilog perform operations on a bit-by-bit basis, allowing operations on all bits of a bus individually. Logical operators, on the other hand, operate on multi-bit values as a single entity and produce a single bit output.

What is the significance of the example of a multiplexer in the context of the lab session?

The example of a multiplexer demonstrates how continuous assignment can be used to simplify Verilog designs by easily adding complex elements to circuits, such as selecting a signal based on the status of select bits.

Why is it important to check the RTL when using continuous assignment in Verilog?

It is important to check the RTL when using continuous assignment in Verilog to ensure that the implemented design is correct, as it can be easy to make mistakes with gate shorthand notation.

What will be the focus of the next session according to the transcript?

The next session will focus on procedural assignment, which will allow the introduction of sequential logic to the designs.

How does the transcript describe the process of building a digital circuit using Verilog gate primitives?

The transcript describes the process of building a digital circuit using Verilog gate primitives as an entry-level introduction that can become cumbersome for complex circuits, necessitating a deeper understanding of Verilog and the use of continuous assignment for more efficient design.

Outlines

00:00

💻 Introduction to Verilog Assignments and Continuous Assignment

This paragraph introduces the concept of assignments in Verilog, which is fundamental to the language and crucial for building digital circuits. It explains that assignments involve placing values onto nets, wires, registers, or variables using the left-hand side (LHS) and right-hand side (RHS) of an equation. The paragraph differentiates between two types of assignments: continuous and procedural. Continuous assignments are the focus of this session, and they assign values to nets whenever there's a change on the RHS, akin to wiring a combinational circuit. Procedural assignments, which will be covered later, assign values to registers or variables under specific conditions, such as clock edges, and are used for sequential and behavioral Verilog. The paragraph also introduces the 'assign' keyword for continuous assignments and the concept of gate shorthand notation, which simplifies the creation of complex combinational logic circuits using bitwise and logical operators.

05:02

🔌 Enhancing Circuit Design with Continuous Assignment

The second paragraph delves into the practical application of continuous assignment in Verilog for designing more complex digital systems. It discusses the inefficiency of using gate primitives for larger circuits, such as multiplexers with many inputs, and how continuous assignment can simplify this process. The paragraph explains how continuous assignment can be used to connect different nets and buses without routing them through components, which is particularly useful for building multiplexers. It illustrates how a multiplexer can be created using continuous assignment by selecting a signal based on the status of select bits. The example provided shows how to use bus addressing to connect individual wires in a bus to different places, which can be scaled up to create larger multiplexers. The paragraph concludes by emphasizing the importance of using continuous assignment for its versatility in building modules and the need to check the RTL to ensure correctness, with a预告 of the next session focusing on procedural assignment for sequential logic.

Keywords

💡Verilog

Verilog is a hardware description language used to model and design digital systems. In the context of the video, Verilog is the primary tool for generating digital circuits, and the script discusses the need to move beyond basic gate primitives to more complex circuit designs using Verilog's capabilities.

💡Gate Primitives

Gate primitives refer to the basic building blocks of digital circuits, such as AND, OR, NOT, and NAND gates. The script mentions that while these have served as a good introduction, more complex circuits require a deeper understanding of Verilog.

💡Hierarchical Design

Hierarchical design is a method of structuring digital systems by breaking them down into smaller, manageable modules. The video script implies that this approach has been used in the past, but the focus is now shifting towards more advanced techniques within Verilog.

💡Assignment

Assignment in Verilog is the process of placing values onto nets, wires, registers, or variables. It is fundamental to Verilog programming and is described as having a left-hand side (the target) and a right-hand side (the value to be assigned). The script explains that assignments are central to building digital circuits in Verilog.

💡Continuous Assignment

Continuous assignment in Verilog is used to assign values to nets and happens whenever there is a change on the right-hand side of the assignment. It is likened to wiring up a combinational circuit with jumper wires, ensuring the output is continuously updated with changes in the circuit. The script emphasizes its use in building combinational logic circuits at a higher level of abstraction.

💡Procedural Assignment

Procedural assignment, while not the main focus of the session, is mentioned as a method of assigning values to registers or variables under specified conditions, such as on the edge of a clock signal. This type of assignment is crucial for sequential and behavioral Verilog design and will be explored in more detail in a future session.

💡Combinational Circuit

A combinational circuit is a type of digital circuit where the output is determined solely by the current inputs, without any memory of past inputs. The script discusses how continuous assignment allows for the creation of such circuits at a higher level of abstraction.

💡Gate Shorthand

Gate shorthand refers to the use of Verilog operators that represent logic gates. These operators allow for the construction of complex logic expressions in a concise manner. The script explains that these operators are used to build equations that the compiler translates into representative circuits.

💡Bitwise Operators

Bitwise operators in Verilog perform operations on individual bits of binary values. The script mentions that these operators enable operations to be performed on all bits of a bus individually, which is essential for creating complex combinational logic circuits.

💡Logical Operators

Logical operators in Verilog compare entire multi-bit values as a single entity. The script explains that logical AND checks if two buses are identical, while logical OR checks if any bit in a bus is non-zero, which is useful for validating buses as a whole.

💡Multiplexer

A multiplexer is a digital component that selects one of many input signals and forwards the selected input to a single output line. The script provides an example of how continuous assignment can be used to implement a multiplexer, illustrating the concept with a simple 2-input multiplexer and discussing the scalability to larger multiplexers.

Highlights

Introduction to Verilog gate primitives for generating digital circuits.

Transition to a more robust understanding of Verilog for complex circuit design.

Fundamental concept of assignment in Verilog, involving left-hand and right-hand sides.

Explanation of gate primitives as a form of assignment in Verilog.

Differentiation between continuous and procedural assignments in Verilog.

Continuous assignment for nets, triggered by changes on the right-hand side.

Procedural assignment for registers or variables under specified conditions.

Overview of how continuous assignment simplifies combinational circuit design.

Introduction of the 'assign' keyword for creating continuous assignment expressions.

Gate shorthand notation for representing logic gates and operations in Verilog.

Bitwise operators for performing bit-by-bit comparisons in Verilog.

Logical operators for comparing entire multi-bit values in Verilog.

Building complex combinational logic circuits using single assignment statements.

Example of using continuous assignment to implement a multiplexer in Verilog.

Scalability of multiplexer design using continuous assignment for larger bus sizes.

Importance of checking RTL implementation for correctness in Verilog designs.

Advantages of using continuous assignment over gate primitives for module building.

Anticipation of the next session focusing on procedural assignment and sequential logic.

Transcripts

we've now spent several weeks working

with Verilog gate primitives to generate

digital circuits this is served well as

an entry-level introduction to Vera log

and hierarchical design but we need

something a bit more robust if we want

to start building more complex circuits

it's time to step up our understanding

of Vera log as a language and take a

look at what we're actually doing when

we write it in this lab session we'll be

looking at assignment which is one of

the fundamental elements of Vera log and

powers pretty much everything that it

does the act of placing values onto nets

or wires registers or variables in Vera

log is known as assigning each

assignment has two parts a left-hand

side and a right-hand side with an equal

symbol between them the right hand side

can contain any expression that

evaluates to a final value while the

left hand side indicates the net or

variable to which the value on the right

hand side is being assigned sounds

complicated

but we've already been doing this the

gate primitives that we've been using up

until now assign statements just in a

different format the left-hand side is

the output whilst the right-hand side

expression is a combination of the

inputs and the selected gate so when we

create a logic gate in our design we're

assigning the output wire to be a result

of the expression generated by

performing whatever function on the

input wires

there are two main types of assignment

continuous and procedural we'll just be

focusing on continuous assignment this

session but I'll give an overview of

both now and we'll be looking at

procedural assignment in more detail in

the next session continuous assignment

assigns values to nets and happens

whenever there is a change on the right

hand side as a result the left-hand side

is continuously driven by the right this

can be thought of as wiring up a

combinational circuit with jumper wires

whatever changes there are in the

circuit the output will be affected

procedural assignment assigns values to

registers or variables only under

specified conditions usually on the edge

of a clock or another signal it doesn't

matter how much the right-hand side

changes the left-hand side only updates

when the conditional signal triggers

latching in the new result the register

or variable will then hold this value

until it's assigned again this

facilitates sequential and behavioral

véra log through things like if and case

statements which we'll look at in the

final lab session of this course we'll

just be looking continuous assignment in

this session continuous assignment

allows us to build combinational logic

circuits at a higher level of

abstraction than with gate primitives it

allows us to significantly reduce the

amount of code we have to write but as a

result is much less verbose we can't

necessarily just glance it and form the

results in circuit in our minds the

fundamental element of continuous

assignment is the assign keyword we use

it create these assignment expressions

and thus generate our circuits the left

hand side of the expression is our

output which could be a module output or

an internal wire and the right-hand side

is the logic which drives that output as

you can see from this example we aren't

limited to a single gaper line anymore

and therefore can theoretically create

an entire circuit with just a single

line of code in order to generate these

expressions we need to learn the

notation which is known as gate

shorthand we have individual operators

representing each logic gate as well as

an operator to generate things like NAND

and nor by building equations using

these operators the core

compiler will build us a representative

circuit these operators are known as

bitwise operators the comparisons they

perform will take place on a bit by bit

basis so if you were to and two 4-bit

wires together the result would be a

4-bit value comparing bit zero to bit

zero bit won't a bit one and so on and

so forth

this allows us to perform operations on

all bits on a bus individually if we

want to validate a bus as a whole we can

use logical operators to compare entire

terms to each other logical operators

operate on multi bit values as a single

entity and only produce a single bit

output logical and allows us to compare

two buses to see if they're identical to

each other whereas logical or would tell

us simply if a bus has a non zero value

so using this gate shorthand notation we

can build up complex combinational logic

circuits with just a single assigned

statement for each sum of products

function I'm now going to give you an

example of how continuous assignment can

be used in circuit development and

introduce the implementation of another

fundamental digital electronics

component a multiplexer

up until now we've only really

implemented basic combinational circuits

in Verilog theoretically we can build

anything using gate primitives in

hierarchical design but as we've seen

these designs can get complicated very

quickly now that we've got a better

understanding of continuous assignment

we can use it to easily add more complex

elements to our circuits and start to

build up full digital electronic systems

we could easily build a simple 2 input

multiplexer from gate primitives by

looking at the truth table as you can

see it's just a couple of an gates a nor

gate and an inverter however once we

start introducing more inputs the

circuit very quickly scales up in

complexity whilst a 4 input multiplexer

is still a relatively simple circuit in

terms of connections it's going to be a

long process to manually connect the

gate primitives together in complex

digital systems we can easily find

ourselves wanting to use 16 and 32-bit

multiplexers so writing these in using

gate primitives simply isn't feasible

however we can use another aspect of

continuous assignment to make things a

bit easier one of the things we can do

with continuous assignment is connect

different nets and buses together

without necessarily routing them through

any components as you know from previous

weeks we can address individual nets on

a bus in a similar way to how you would

access array elements in C++ we can use

this addressing to connect individual

wires in a bus to different places or

even separate wide buses into narrower

ones all multiplexers really do is

select a signal based on the status of

the Select bit so we can use that

information to build a simple

multiplexer using continuous assignment

we'll take the input of our multiplexer

to be a bus and assign the output to be

whichever bit on the bus select is

pointing to because this is continuous

assignment the output is updated every

time there is a change in signal on the

right-hand side of the expression so as

select changes different nets on the a

bus will be connected to the output as

we know from earlier lab sessions we

address an entire bus just by referring

to its name so we can easily scale this

module up to a 4 or 8 bit multiplexer

just by changing the number of

puts the assignment line itself doesn't

change the RTL if the result shows that

we've implemented our multiplexer

correctly on the device however this

design is still implemented in lookup

tables despite being such a fundamental

component of digital electronics that

aren't actually any general-purpose

multiplexers for use in this video we've

covered continuous assignment and how it

can be used to simplify our verilock

designs going ahead you should use it as

much as possible as it is a far more

versatile way to build modules however

you should always check the RTL as you

go along it's very easy to get the gate

shorthand notation wrong next week we'll

be looking a procedure assignment to

start adding sequential logic to our

designs