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  ../images/main/bullet_green_ball.gif Continuous Assignment Statements

Continuous assignment statements drive nets (wire data type). They represent structural connections.

  • They are used for modeling Tri-State buffers.
  • They can be used for modeling combinational logic.
  • They are outside the procedural blocks (always and initial blocks).
  • The continuous assign overrides any procedural assignments.
  • The left-hand side of a continuous assignment must be net data type.

syntax : assign (strength, strength) #(delay) net = expression;

   

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  ../images/main/bulllet_4dots_orange.gif Example - One bit Adder
   

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  1 module adder_using_assign ();
  2 reg a, b;
  3 wire sum, carry;
  4 
  5 assign  #5  {carry,sum} = a+b; 
  6 
  7 initial begin
  8   $monitor (" A = %b  B = %b CARRY = %b SUM = %b",a,b,carry,sum);
  9    #10  a = 0;
 10   b = 0;
 11    #10   a = 1;
 12    #10   b = 1;
 13    #10   a = 0;
 14    #10   b = 0;
 15    #10  $finish;
 16 end
 17 
 18 endmodule 
You could download file adder_using_assign.v here
   

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  ../images/main/bulllet_4dots_orange.gif Example - Tri-state buffer
   

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  1 module tri_buf_using_assign();
  2 reg data_in, enable;
  3 wire pad;
  4 
  5 assign pad = (enable) ? data_in : 1'bz;
  6 
  7 initial begin
  8   $monitor ("TIME = %g ENABLE = %b DATA : %b PAD %b", 
  9     $time, enable, data_in, pad);
 10    #1  enable = 0;
 11    #1  data_in = 1;
 12    #1  enable = 1;
 13    #1  data_in = 0;
 14    #1  enable = 0;
 15    #1  $finish;
 16 end
 17 
 18 endmodule
You could download file tri_buf_using_assign.v here
   

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  ../images/main/bullet_green_ball.gif Propagation Delay

Continuous Assignments may have a delay specified; only one delay for all transitions may be specified. A minimum:typical:maximum delay range may be specified.

   

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  ../images/main/bulllet_4dots_orange.gif Example - Tri-state buffer
   

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  1 module tri_buf_using_assign_delays();
  2 reg data_in, enable;
  3 wire pad;
  4 
  5 assign #(1:2:3) pad = (enable) ? data_in : 1'bz;
  6 
  7 initial begin
  8   $monitor ("ENABLE = %b DATA : %b PAD %b",enable, data_in,pad);
  9    #10  enable = 0;
 10    #10  data_in = 1;
 11    #10  enable = 1;
 12    #10  data_in = 0;
 13    #10  enable = 0;
 14    #10  $finish;
 15 end
 16 
 17 endmodule
You could download file tri_buf_using_assign_delays.v here
   

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Copyright 1998-2014

Deepak Kumar Tala - All rights reserved

Do you have any Comment? mail me at:deepak@asic-world.com