Multiple clock generation [Verilog] [Using fork-join]

Question

I generated the following code :

module clk_gen();
    reg clk, clk1 , clk2, clk3 , clk4 ;  

always 
    fork 
       #5 clk = ~clk ;
       #1 clk1 = ~clk1 ;
       #3 clk2 = ~clk2 ;
       #4 clk3 = ~clk3 ;
       #3 clk4 = ~clk4 ;
    join


 initial begin
    clk = 1'b0 ;  
    clk1 = 1'b0 ;  
    clk2 = 1'b0 ;  
    clk3 = 1'b0 ;  
    clk4 = 1'b0 ;  
    $monitor ("Time : %0t ,RealTime : %0t ,Clk Value : %0d,Clk1 Value : %0d,Clk2 Value : %0d,Clk3 Value : %0d,Clk4 Value : %0d,$time,$realtime,clk,clk1,clk2,clk3,clk4) ;

    #30 $finish ;
 end

endmodule : clk_gen

I am not able to generate the clocks using the fork-join statement.All clocks generated are of the same frequency i.e of #5 delay.

Answer 1

join will wait till all thread finish, so actually for the longest one (#5). This is why all clocks are synchronized at 5 cycles.

you do not need a fork there. just use multiple always statements.

always 
   #5 clk = ~clk ;
always
   #1 clk1 = ~clk1 ;
always    
   #3 clk2 = ~clk2 ;
   ...

you can potentially use fork/join like this:

initial begin
    fork
        forever #5 clk = ~clk;
        forever #4 clk1 = ~clk1;
        ...
    join 
 end

which would be the same as using mulitpile initial blocks

 initial forever #5 clk = ~clk;

Answer 2

This way of clock generation can't be done thorugh any of the fork-join methods (fork-join, fork-join_any, fork-join_none).

The reason is that

• With fork-join, it will wait for the longest thread to be completed, and hence the period will be equal to the longest thread delay.
• With fork-join_any, it will be completed as soon as the 1st shortest delay thread gets completed, but however after that it will spawn multiple threads and hence the period will be equal to the shortest thread delay.
• With fork-join_none, infinite threads will be generated.

Hence ideal solution is to make separate always blocks for each clocks.

always #5 clk = ~clk ;
always #1 clk1 = ~clk1 ;
always #3 clk2 = ~clk2 ;
always #4 clk3 = ~clk3 ;
always #3 clk4 = ~clk4 ;