DVD - Lecture 2c: Simple Verilog Examples

Summary

This video tutorial introduces basic Verilog programming concepts, including writing simple modules like 'Hello World' and practical examples of combinational and sequential logic. It covers the creation of multiplexers using assign statements, always blocks, and case statements, explaining the differences between reg and wire. The video also dives into flip-flops, asynchronous resets, and latches. Key topics like arithmetic operations, signal concatenation, and handling of signed/unsigned numbers in Verilog are discussed, along with best practices for writing test benches, including clock generation using macros.

Takeaways

• 😀 Basic Verilog programs start with a 'module' and an 'initial' block, where 'display' prints text and 'finish' ends the simulation.
• 😀 In Verilog, you can create a simple multiplexer (MUX) using an 'assign' statement, 'always' block, or a 'case' statement to handle combinational logic.
• 😀 Always blocks in Verilog are used for sequential and combinational logic, with 'reg' used for outputs in sequential logic, and 'wire' for combinational logic.
• 😀 An 'always' block with a conditional expression (if-else) requires an 'else' clause to avoid unintended latches in combinational logic.
• 😀 Sequential logic, like a D flip-flop, uses 'always @(posedge clock)' and non-blocking assignments (<=) for registering inputs like D.
• 😀 Asynchronous resets in flip-flops are handled with 'neg' reset, and the reset signal is not synchronized to the clock.
• 😀 Latches are level-sensitive and do not use a clock signal. If an 'else' clause is missing in a latch description, the output retains the previous state.
• 😀 Verilog supports basic arithmetic operations (addition, subtraction, shift) on multi-bit vectors, but division is more complex and should be used carefully.
• 😀 Verilog handles signed and unsigned numbers. To specify signed values, use the 'signed' keyword, and for signed constants, add 's' before the number.
• 😀 The difference between 'reg' and 'wire' in Verilog: 'reg' is used inside 'always' blocks, while 'wire' is used for continuous assignments like 'assign'.

Q & A

What is the purpose of the 'Hello World' example in Verilog?

The 'Hello World' example in Verilog serves as an introductory demonstration of how to write a basic Verilog program. It doesn't perform any functional operation but shows the syntax and structure of a Verilog module, including using system calls like 'display' to print a message and 'finish' to terminate the simulation.

What are the three ways to write a multiplexer (MUX) in Verilog?

The three ways to write a multiplexer (MUX) in Verilog are: 1) Using an 'assign' statement with the ternary operator, 2) Using an 'always' block with a blocking assignment, and 3) Using an 'always' block with a 'case' statement.

What is the key difference between using a 'reg' and a 'wire' in Verilog?

'reg' and 'wire' are both used for declaring signals, but the key difference lies in their usage: a 'reg' is used for signals that store values (e.g., in sequential logic inside 'always' blocks), while a 'wire' is used for combinational logic (e.g., in 'assign' statements). A 'reg' can hold a value over time, whereas a 'wire' only represents a connection.

Why is it important to include an 'else' statement in a combinational 'always' block?

Including an 'else' statement in a combinational 'always' block ensures that all possible cases are handled. Without it, if the condition isn't met, the signal may be left undefined, which could result in unintended behavior or the creation of a latch — something that is typically undesirable in combinational logic.

What does an asynchronous reset in a D flip-flop do?

An asynchronous reset in a D flip-flop immediately resets the output (Q) to a defined state (e.g., 0) when the reset signal is activated, regardless of the clock's state. This allows for an immediate response to the reset signal, unlike synchronous resets that depend on a clock edge.

How does a latch differ from a flip-flop in Verilog?

A latch is level-sensitive and will change its output based on the 'enable' signal. If 'enable' is high, the output follows the input. If 'enable' is low, the output retains its previous value. In contrast, a flip-flop is edge-triggered, meaning it only changes its output on a specific clock edge, such as the rising or falling edge.

What is the purpose of the 'always' block in Verilog?

The 'always' block in Verilog is used to describe the behavior of a circuit that changes over time. It can be used for both combinational and sequential logic. The contents of the block are executed whenever the sensitivity list conditions are met (e.g., a clock edge or a change in an input signal).

What is the function of the 'assign' statement in Verilog?

'assign' is used to describe combinational logic in Verilog. It assigns a value to a wire based on the logic expression on the right-hand side. The 'assign' statement is used for continuous assignments, meaning the value of the wire is updated whenever the inputs to the expression change.

How does Verilog handle signed and unsigned numbers?

In Verilog, vectors are treated as unsigned by default. To handle signed numbers, the 'signed' keyword is used when declaring a wire or register. This enables two's complement representation for arithmetic operations, allowing for the representation and manipulation of negative numbers.

What is a macro in Verilog and how is it used?

A macro in Verilog is defined using the 'define' directive and allows for code substitution. It is commonly used to define constants, such as clock periods, to ensure consistency throughout the code. For example, a macro like 'clock period 10' would replace all occurrences of 'clock' in the code with the value 10 during compilation.