Use of forever and always statements

verilog
chitranna picture chitranna · Apr 11, 2013 · Viewed 32.7k times · Source

Both the following codes generate a clock. I need to know if there is any use of forever loop other than clock generation? I have only come across forever in clock generation. If it only serves this purpose, isn't it useless?

initial begin
clk = 0;
forever begin
#5 clk = ~clk;
end
end

initial begin
clk = 0 ;
always begin 
# 5 clk = ~clk;
end
end

Answer

Andy picture Andy · Apr 11, 2013

Your second code snippet is actually a syntax error. The difference between forever and always is that always can exist as a "module item", which is the name that the Verilog spec gives to constructs that may be written directly within a module, not contained within some other construct. initial is also a module item. always blocks are repeated, whereas initial blocks are run once at the start of the simulation.

forever is a procedural statement that can only be used in a procedural context. So it is legal to write initial forever or always forever, but not just forever.

The situation where forever becomes quite important is within tasks, which are procedural contexts, so use of always is not allowed. (Functions are procedural contexts as well, but may not contain delays, which makes it unlikely that forever will come in useful.

Related questions

How to declare and use 1D and 2D byte arrays in Verilog?

How to declare and use 1D and 2D byte arrays in Verilog? eg. how to do something like byte a_2D[3][3]; byte a_1D[3]; // using 1D for (int i=0; i< 3; i++) { a_1D[i] = (byte)i; } // using 2D for (…

Read More
Verilog: How to instantiate a module

If I have a Verilog module 'top' and a verilog module 'subcomponent' how do I instantiate subcomponent in top? top: module top( input clk, input rst_n, input enable, input [9:0] data_rx_1, input [9:0] data_rx_2, output [9:0] data_tx_2 ); subcomponent: module …

Read More
Using wire or reg with input or output in Verilog

When you declare something as input or output, how do you know if you have to also declare it as a reg or a wire?

Read More