Verilog Control Blocks

Hardware behavior cannot be implemented without conditional statements and other ways to control the flow of logic. Verilog has a set of mechanisms and control flow blocks.

if-else-if

This conditional statement is used to decide whether certain statements will be executed or not. It is very similar to the if-else-if statements in C language. If the expression evaluates to true, then the first statement will be executed.

If the expression evaluates to false and if an else part exists, the else part will be executed.

Syntax

Following is the most simplified syntax of the if-else-if conditional statement:

The else part of an if-else is optional, and it can create confusion. To avoid this confusion, it's easier to always associate the else to the previous if that lacks an else. Another way is to enclose statements within a begin-end block. The last else part handles the none-of-the-above or default case where none of the other conditions were satisfied.

Loops provide a way of executing single or multiple statements within a block one or more number of times. In Verilog, there are four different types of looping statements.

1. Forever loop

This loop will continuously execute the statements within the block.

Syntax

Example

After the execution of the above example, it produces the following data.

2. Repeat loop

This will execute statements a fixed number of times. If the expression evaluates to an X or Z, it will be treated as zero and not executed.

Syntax

Example

The above code generates the following outcome.

3. ile loop

This will execute statements as long as an expression is true and will exit once the condition becomes false. If the condition is false from the start, statements will not be executed at all.

Syntax

Example

Run the above code, and we will get the following output.

4. For loop

For loop controls the statements using a three-step process:

• Initialize a loop counter variable.
• Evaluate the expression, usually involving the loop counter variable.
• Increment loop counter variable so that the expression will become false at a later time, and the loop will exit.

Syntax

Example

After execution the for loop code, the output looks like