Exploring Basic Verilog Code Examples

In this blog post, we’ll delve into some fundamental Verilog code examples that are essential for understanding digital design concepts. Whether you’re new to Verilog or looking to refresh your knowledge, these code snippets will serve as a handy reference for building logic circuits.

1. Logic Gates
2. Half Adder
3. Flip Flops

Happy Coding!

Mastering Verilog: Part 8 - Understanding break and continue Statements.

As we progress in mastering Verilog, we’ve already delved into several foundational concepts. Now, let’s explore a pivotal element in loop control: the break and continue statements. These statements play a key role in enhancing the efficiency and flexibility of loops, making them indispensable in Verilog programming.

Before we delve into break and continue statements, let’s first illuminate the functionality provided by the $display keyword in Verilog:

• The $display system task in Verilog serves as a powerful tool for generating formatted text output during simulation. Widely utilized for debugging purposes, it facilitates the monitoring of signals within your Verilog code. Let’s delve into an overview of how the $display task operates:

Syntax:

$display(format_string, variable1, variable2, …);
format_string: Specifies the format of the output. It can include placeholders like %d for decimal, %b for binary, %h for hexadecimal, %s for string, %f for real numbers, and more.
variable1, variable2, …: Variables whose values will be substituted into the format string.

Example:

module DisplayExample;
reg [3:0] count = 4'b0000;
reg [7:0] data = 8'b10101010;

initial begin
$display(“Count: %d, Data: %b”, count, data);
$stop;
end
endmodule

In this example, %d is used to display the decimal value of count, and %b is used to display the binary value of data.

Output:

Count: 0, Data: 10101010
Common Format Specifiers:
%d: Decimal format.
%b: Binary format.
%h: Hexadecimal format.
%s: String format.
%f: Real number format.

By combining format specifiers with text in the format string, you can craft detailed output messages using the $display task. Utilize $display statements strategically to gain insights into the behavior of your Verilog code during simulation.

1. Understanding Break Statement

When a break statement is encountered in Verilog, it acts as a mechanism to prematurely terminate a loop based on a specified condition. This statement facilitates an immediate exit from the loop, irrespective of the current state of the loop condition.

Syntax:

while (condition) begin
// Loop body
if (break_condition) begin
break;
end
// Other statements
end

Example:

module BreakExample;
reg [3:0] count = 4'b0000;

initial begin
while (count < 10) begin
$display(“Count: %d”, count);
count = count + 1;

if (count == 5) begin
$display(“Breaking loop at count 5”);
break;
end
end
$display(“Loop exited due to break statement”);
$stop;
end
endmodule

Explanation:

This Verilog module defines a loop that increments the count variable from 0 to 9. Inside the loop, there’s an if statement that checks if count is equal to 5. If it is, the loop is terminated using the break statement.

Now, let’s predict the output based on the code logic:

1. Initially, count is 0.
2. The loop executes, displaying “Count: 0” and incrementing count to 1.
3. This process continues until count reaches 4.
4. When count becomes 5, the condition count == 5 is true, and the loop is terminated with the message “Breaking loop at count 5”.
5. Finally, the output “Loop exited due to break statement” is displayed.

So, the expected output of this Verilog code would be:

Count: 0
Count: 1
Count: 2
Count: 3
Count: 4
Breaking loop at count 5
Loop exited due to break statement

2. Exploring Continue Statement

In contrast, the continue statement in Verilog serves the purpose of skipping the remaining code within the current iteration of a loop and directly proceeding to the next iteration. This functionality proves especially beneficial when there’s a need to bypass specific iterations based on predefined conditions without terminating the loop entirely.

Syntax:

while (condition) begin
// Loop body
if (continue_condition) begin
continue;
end
// Other statements
end

Example:

module ContinueExample;
reg [3:0] count = 4'b0000;

initial begin
while (count < 5) begin
count = count + 1;

if (count == 3) begin
$display(“Skipping count 3”);
continue;
end

$display(“Count: %d”, count);
end
$stop;
end
endmodule

Explanation:

This Verilog module defines a loop that increments the count variable from 0 to 4. Inside the loop, there’s an if statement that checks if count is equal to 3. If it is, the loop skips the rest of the iteration using the continue statement.

Now, let’s predict the output based on the code logic:

1. Initially, count is 0.
2. The loop executes, incrementing count to 1 and displaying “Count: 1”.
   count becomes 2, and “Count: 2” is displayed.
3. When count becomes 3, the condition count == 3 is true, and the loop skips the rest of this iteration, displaying “Skipping count 3”.
4. The loop continues with count as 4 and displays “Count: 4”.
5. The loop exits as count becomes 5.

So, the expected output of this Verilog code would be:

Count: 1
Count: 2
Skipping count 3
Count: 4

Conclusion

In conclusion, mastering Verilog involves not only understanding fundamental concepts but also leveraging advanced features like the $display system task, break statements, and continue statements. The $display task empowers developers to monitor and debug Verilog code effectively, while break and continue statements offer precise control over loop execution, enhancing code efficiency and flexibility. By strategically incorporating these tools into Verilog designs and experimenting with various scenarios, developers can unlock the full potential of Verilog programming, leading to robust and optimized digital systems.

Stay tuned for more insights and advanced topics in our mastering Verilog series!

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Mastering Verilog: Part 7- Understanding Looping Statements.

Welcome back to our series on mastering Verilog! So far, we’ve covered essential topics like modules, operators, data types, assign statements, initial and always blocks, and conditional statements. Now, let’s dive into the world of looping statements in Verilog.

Looping statements in Verilog allow you to repeat a block of code multiple times, making your designs more efficient and scalable. Verilog provides several looping constructs, including for, while, repeat, and forever loops, each serving specific purposes. All looping statements can only be written inside procedural (initial and always) blocks.

Let us consider each looping statement one by one:

1. For Loop

The for loop in Verilog is used to execute a block of code a specified number of times. It follows this syntax:

for (initialization; condition; increment) begin
// code to execute in each iteration
end

Here’s an example of a for loop in Verilog:

module ForLoopExample (
input wire [7:0] data_in,
output wire [7:0] data_out
);

reg [7:0] sum;

always @(*) begin
sum = 8'b00000000;
for (int i = 0; i < 8; i = i + 1) begin
sum = sum + data_in[i];
end
end

assign data_out = sum;
endmodule

Explanation:
In this code:

1. data_in is an 8-bit input.
2. sum is an 8-bit register initialized to all zeros.
3. The always @(*) block indicates that the block should execute whenever there is a change in the inputs (data_in in this case).
4. The for loop iterates from i = 0 to i < 8, incrementing i by 1 in each iteration.
5. In each iteration of the loop, sum is updated by adding the value of data_in[i] to it.
6. Initially, sum is set to 00000000 (in binary) or 0 in decimal.
7. In the first iteration of the loop (i = 0), data_in[0] is added to sum.
8. In the second iteration (i = 1), data_in[1] is added to sum, and so on until the loop completes all 8 iterations (i = 7).
9. At the end of the loop, sum will contain the sum of all elements in data_in. The output data_out will be the sum of all elements in data_in.

2. While Loop

The while loop in Verilog repeats a block of code as long as a specified condition remains true. Its syntax is as follows:

while (condition) begin
// code to execute as long as condition is true
end

Here’s an example of a while loop in Verilog:

module WhileLoopExample (
input wire [7:0] data_in,
output wire [7:0] data_out
);

reg [7:0] inverted_data;

always @(*) begin
int i = 0;
while (i < 8) begin
inverted_data[i] = ~data_in[i];
i = i + 1;
end
end

assign data_out = inverted_data;
endmodule

Explanation:
In this code:

1. data_in is an 8-bit input.
2. inverted_data is an 8-bit register used to store the inverted bits of data_in.
3. The always @(*) block indicates that the block should execute whenever there is a change in the inputs (data_in in this case).
4. The while loop iterates as long as i is less than 8 (i.e., while i is in the range 0 to 7).
5. In each iteration of the loop, the bit at position i in data_in is inverted using the ~ operator and stored in the corresponding position in inverted_data.
6. The loop increments i by 1 in each iteration until i reaches 8, at which point the loop exits.
7. The output data_out will be the value of inverted_data, which contains the inverted bits of data_in. For example, if data_in is 10101010 (in binary), data_out will be 01010101, where each bit is inverted.

3. Repeat Loop

The repeat loop in Verilog repeats a block of code a specified number of times. Its syntax is as follows:

repeat (count) begin
// code to repeat count times
end

Here’s an example of a repeat loop in Verilog:

module RepeatLoopExample (
input wire [3:0] count,
output wire [3:0] result
);

reg [3:0] temp_result;

always @(*) begin
temp_result = 4'b0000;
repeat (count) begin
temp_result = temp_result + 1;
end
end

assign result = temp_result;
endmodule

Explanation:
In this code:

1. count is a 4-bit input representing the number of times the loop should repeat.
2. result is a 4-bit output that will store the final result after the loop.
   temp_result is a 4-bit register used to accumulate the result within the loop.
3. The always @(*) block indicates that the block should execute whenever there is a change in the inputs (count in this case).
4. The repeat loop is used to repeat a block of code a specified number of times, which is determined by the value of count. Here’s how the loop works in this example:
5. temp_result is initialized to 0000.
6. The repeat loop executes count times.
7. In each iteration of the loop, temp_result is incremented by 1 (temp_result = temp_result + 1).
8. After the loop completes all repetitions, the final value of temp_result is assigned to result.
9. For instance, if count is 5 (binary 0101), the repeat loop will execute 5 times, and result will be 0101 (binary) or 5 (decimal), as temp_result is incremented by 1 in each iteration.

4. Forever Loop

The forever loop in Verilog executes a block of code indefinitely. Its syntax is as follows:

forever begin
// code to execute indefinitely
end

Here’s an example of a while loop in Verilog:

module ForeverLoopExample (
input wire clk,
input wire start,
output wire [3:0] counter_out
);

reg [3:0] counter;

always @(posedge clk) begin
if (start) begin
counter <= 4'b0000;
forever begin
counter <= counter + 1;
end
end
end

assign counter_out = counter;
endmodule

Explanation:
In this code:

1. clk is a clock input.
2. start is a control input that triggers the beginning of the counting process.
3. counter_out is a 4-bit output that will store the counter’s value.
4. The forever loop is used to continuously execute a block of code indefinitely, which is useful for tasks that need to run continuously without a predefined end. Here’s how the forever loop works in this example:
5. The always @(posedge clk) block triggers on the positive edge of the clock (clk), indicating synchronous behavior.
6. When the start signal is asserted (start == 1), the counting process begins.
7. Inside the forever loop, counter is continuously incremented by 1 (counter <= counter + 1).
8. This example demonstrates a simple counter that starts counting when the start signal is asserted. The counter_out output continuously reflects the value of the counter register, incrementing by 1 on each clock cycle after the start signal is asserted. The forever loop ensures that the counting operation continues indefinitely until there is a change in the design or a specific condition to stop the counting process.

Conclusion

Looping statements such as for, while, repeat, and forever are indispensable tools in Verilog for executing repetitive tasks efficiently and effectively. By mastering these looping constructs, you can enhance the functionality and performance of your Verilog designs. Stay tuned for more insights and examples in our ongoing journey to master Verilog programming!

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