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  ../images/main/bullet_green_ball.gif Verilog HDL Abstraction Levels
   

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  • Behavioral Models : Higher level of modeling where behavior of logic is modeled.
  • RTL Models : Logic is modeled at register level
  • Structural Models : Logic is modeled at both register level and gate level.
   

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  ../images/main/bullet_green_ball.gif Procedural Blocks

Verilog behavioral code is inside procedure blocks, but there is an exception: some behavioral code also exist outside procedure blocks. We can see this in detail as we make progress.

   

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There are two types of procedural blocks in Verilog:

   

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  • initial : initial blocks execute only once at time zero (start execution at time zero).
  • always : always blocks loop to execute over and over again; in other words, as the name suggests, it executes always.
   

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  ../images/main/bulllet_4dots_orange.gif Example - initial
   

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  1 module initial_example();
  2 reg clk,reset,enable,data;
  3 
  4 initial begin
  5  clk = 0;
  6  reset = 0;
  7  enable = 0;
  8  data = 0;
  9 end
 10 
 11 endmodule
You could download file initial_example.v here
   

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In the above example, the initial block execution and always block execution starts at time 0. Always block waits for the event, here positive edge of clock, whereas initial block just executed all the statements within begin and end statement, without waiting.

   

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  ../images/main/bulllet_4dots_orange.gif Example - always
   

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  1 module always_example();
  2 reg clk,reset,enable,q_in,data;
  3 
  4 always @ (posedge clk)
  5 if (reset)  begin
  6    data <= 0;
  7 end else if (enable) begin   
  8    data <= q_in;
  9 end
 10 
 11 endmodule
You could download file always_example.v here
   

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In an always block, when the trigger event occurs, the code inside begin and end is executed; then once again the always block waits for next event triggering. This process of waiting and executing on event is repeated till simulation stops.

   

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  ../images/main/bulllet_4dots_orange.gif Procedural Assignment Statements
   

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  • Procedural assignment statements assign values to reg, integer, real, or time variables and can not assign values to nets (wire data types)
  • You can assign to a register (reg data type) the value of a net (wire), constant, another register, or a specific value.
   

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  ../images/main/bullet_star_pink.gif Example - Bad procedural assignment
   

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  1 module initial_bad();
  2 reg clk,reset;
  3 wire enable,data;
  4 
  5 initial begin
  6  clk = 0;
  7  reset = 0;
  8  enable = 0;
  9  data = 0;
 10 end
 11 
 12 endmodule
You could download file initial_bad.v here
   

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  ../images/main/bullet_star_pink.gif Example - Good procedural assignment
   

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  1 module initial_good();
  2 reg clk,reset,enable,data;
  3 
  4 initial begin
  5  clk = 0;
  6  reset = 0;
  7  enable = 0;
  8  data = 0;
  9 end
 10 
 11 endmodule
You could download file initial_good.v here
   

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  ../images/main/bulllet_4dots_orange.gif Procedural Assignment Groups

If a procedure block contains more than one statement, those statements must be enclosed within

   

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  • Sequential begin - end block
  • Parallel fork - join block
   

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When using begin-end, we can give name to that group. This is called named blocks.

   

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  ../images/main/bullet_star_pink.gif Example - "begin-end"
   

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  1 module initial_begin_end();
  2 reg clk,reset,enable,data;
  3 
  4 initial begin
  5  $monitor(
  6    "%g clk=%b reset=%b enable=%b data=%b", 
  7    $time, clk, reset, enable, data);
  8   #1   clk = 0;
  9   #10  reset = 0;
 10   #5   enable = 0;
 11   #3   data = 0;
 12   #1  $finish;
 13 end
 14 
 15 endmodule
You could download file initial_begin_end.v here
   

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Begin : clk gets 0 after 1 time unit, reset gets 0 after 11 time units, enable after 16 time units, data after 19 units. All the statements are executed sequentially.

   

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Simulator Output

   

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 0 clk=x reset=x enable=x data=x
 1 clk=0 reset=x enable=x data=x
 11 clk=0 reset=0 enable=x data=x
 16 clk=0 reset=0 enable=0 data=x
 19 clk=0 reset=0 enable=0 data=0
   

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  ../images/main/bullet_star_pink.gif Example - "fork-join"
   

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  1 module initial_fork_join();
  2 reg clk,reset,enable,data;
  3 
  4 initial begin
  5  $monitor("%g clk=%b reset=%b enable=%b data=%b", 
  6    $time, clk, reset, enable, data);
  7  fork
  8     #1   clk = 0;
  9     #10  reset = 0;
 10     #5   enable = 0;
 11     #3   data = 0;
 12  join
 13   #1  $display ("%g Terminating simulation", $time);
 14  $finish;
 15 end
 16 
 17 endmodule
You could download file initial_fork_join.v here
   

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Fork : clk gets its value after 1 time unit, reset after 10 time units, enable after 5 time units, data after 3 time units. All the statements are executed in parallel.

   

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Simulator Output

   

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 0 clk=x reset=x enable=x data=x
 1 clk=0 reset=x enable=x data=x
 3 clk=0 reset=x enable=x data=0
 5 clk=0 reset=x enable=0 data=0
 10 clk=0 reset=0 enable=0 data=0
 11 Terminating simulation
   

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  ../images/main/bulllet_4dots_orange.gif Sequential Statement Groups

The begin - end keywords:

   

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  • Group several statements together.
  • Cause the statements to be evaluated sequentially (one at a time)
    • Any timing within the sequential groups is relative to the previous statement.
    • Delays in the sequence accumulate (each delay is added to the previous delay)
    • Block finishes after the last statement in the block.
   

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  ../images/main/bullet_star_pink.gif Example - sequential
   

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  1 module sequential();
  2 
  3 reg a;
  4 
  5 initial begin
  6   $monitor ("%g a = %b", $time, a);
  7    #10  a = 0;
  8    #11  a = 1;
  9    #12  a = 0;
 10    #13  a = 1;
 11    #14  $finish;
 12 end
 13 
 14 endmodule
You could download file sequential.v here
   

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Simulator Output

   

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 0 a = x
 10 a = 0
 21 a = 1
 33 a = 0
 46 a = 1
   

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  ../images/main/bulllet_4dots_orange.gif Parallel Statement Groups

The fork - join keywords:

   

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  • Group several statements together.
  • Cause the statements to be evaluated in parallel (all at the same time).
    • Timing within parallel group is absolute to the beginning of the group.
    • Block finishes after the last statement completes (Statement with highest delay, it can be the first statement in the block).
   

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  ../images/main/bullet_star_pink.gif Example - Parallel
   

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  1 module parallel();
  2 
  3 reg a;
  4 
  5 initial 
  6 fork
  7   $monitor ("%g a = %b", $time, a);
  8    #10  a = 0;
  9    #11  a = 1;
 10    #12  a = 0;
 11    #13  a = 1;
 12    #14  $finish;
 13 join
 14 
 15 endmodule
You could download file parallel.v here
   

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Simulator Output

   

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 0 a = x
 10 a = 0
 11 a = 1
 12 a = 0
 13 a = 1
   

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  ../images/main/bullet_star_pink.gif Example - Mixing "begin-end" and "fork - join"
   

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  1 module fork_join();
  2 
  3 reg clk,reset,enable,data;
  4 
  5 initial  begin
  6   $display ("Starting simulation");
  7   $monitor("%g clk=%b reset=%b enable=%b data=%b", 
  8     $time, clk, reset, enable, data);
  9   fork : FORK_VAL
 10      #1  clk = 0;
 11      #5  reset = 0;
 12      #5  enable = 0;
 13      #2  data = 0;
 14   join
 15    #10  $display ("%g Terminating simulation", $time);
 16   $finish;
 17 end
 18 
 19 endmodule
You could download file fork_join.v here
   

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Simulator Output

   

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 0 clk=x reset=x enable=x data=x
 1 clk=0 reset=x enable=x data=x
 2 clk=0 reset=x enable=x data=0
 5 clk=0 reset=0 enable=0 data=0
 15 Terminating simulation
   

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  ../images/main/bulllet_4dots_orange.gif Blocking and Nonblocking assignment

Blocking assignments are executed in the order they are coded, hence they are sequential. Since they block the execution of next statment, till the current statement is executed, they are called blocking assignments. Assignment are made with "=" symbol. Example a = b;

   

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Nonblocking assignments are executed in parallel. Since the execution of next statement is not blocked due to execution of current statement, they are called nonblocking statement. Assignments are made with "<=" symbol. Example a <= b;

   

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Note : Correct way to spell 'nonblocking' is 'nonblocking' and not 'non-blocking'.

   

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  ../images/main/bullet_star_pink.gif Example - blocking and nonblocking
   

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  1 module blocking_nonblocking();
  2 
  3 reg a,b,c,d;
  4 // Blocking Assignment
  5 initial begin
  6    #10  a = 0;
  7    #11  a = 1;
  8    #12  a = 0;
  9    #13  a = 1;
 10 end
 11 
 12 initial begin
 13    #10  b <= 0;
 14    #11  b <= 1;
 15    #12  b <= 0;
 16    #13  b <= 1;
 17 end
 18 
 19 initial begin
 20    c = #10 0;
 21    c = #11 1;
 22    c = #12 0;
 23    c = #13 1;
 24 end
 25 
 26 initial begin
 27    d <=  #10  0;
 28    d <=  #11  1;
 29    d <=  #12  0;
 30    d <=  #13  1;
 31 end
 32 
 33 initial begin
 34   $monitor("TIME = %g A = %b B = %b C = %b D = %b",$time, a, b, c, d);
 35    #50  $finish;
 36 end
 37 
 38 endmodule
You could download file blocking_nonblocking.v here
   

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Simulator Output

   

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 TIME = 0 A = x B = x C = x D = x
 TIME = 10 A = 0 B = 0 C = 0 D = 0
 TIME = 11 A = 0 B = 0 C = 0 D = 1
 TIME = 12 A = 0 B = 0 C = 0 D = 0
 TIME = 13 A = 0 B = 0 C = 0 D = 1
 TIME = 21 A = 1 B = 1 C = 1 D = 1
 TIME = 33 A = 0 B = 0 C = 0 D = 1
 TIME = 46 A = 1 B = 1 C = 1 D = 1
   

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  ../images/main/bullet_star_pink.gif Waveform
   

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../images/verilog/blocking_nonblocking.gif
   

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  ../images/main/bulllet_4dots_orange.gif assign and deassign

The assign and deassign procedural assignment statements allow continuous assignments to be placed onto registers for controlled periods of time. The assign procedural statement overrides procedural assignments to a register. The deassign procedural statement ends a continuous assignment to a register.

   

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  ../images/main/bullet_star_pink.gif Example - assign and deassign
   

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  1 module assign_deassign ();
  2 
  3 reg clk,rst,d,preset;
  4 wire q;
  5 
  6 initial begin
  7   $monitor("@%g clk %b rst %b preset %b d %b q %b", 
  8     $time, clk, rst, preset, d, q);
  9   clk = 0;
 10   rst = 0;
 11   d  = 0;
 12   preset = 0;
 13    #10  rst = 1;
 14    #10  rst = 0;
 15   repeat (10) begin
 16     @ (posedge clk);
 17     d <= $random;
 18     @ (negedge clk) ;
 19     preset <= ~preset;
 20   end
 21    #1  $finish;
 22 end
 23 // Clock generator
 24 always  #1  clk = ~clk;
 25 
 26 // assign and deassign q of flip flop module
 27 always @(preset)
 28 if (preset) begin
 29   assign U.q = 1; // assign procedural statement
 30 end else begin
 31  deassign U.q;    // deassign procedural statement
 32 end
 33 
 34 d_ff U (clk,rst,d,q);
 35 
 36 endmodule
 37 
 38 // D Flip-Flop model
 39 module d_ff (clk,rst,d,q);
 40 input clk,rst,d;
 41 output q;
 42 reg q;
 43 
 44 always @ (posedge clk)
 45 if (rst) begin
 46   q <= 0;
 47 end else begin
 48   q <= d;
 49 end
 50 
 51 endmodule
You could download file assign_deassign.v here
   

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Simulator Output

   

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 @0  clk 0 rst 0 preset 0 d 0 q x
 @1  clk 1 rst 0 preset 0 d 0 q 0
 @2  clk 0 rst 0 preset 0 d 0 q 0
 @3  clk 1 rst 0 preset 0 d 0 q 0
 @4  clk 0 rst 0 preset 0 d 0 q 0
 @5  clk 1 rst 0 preset 0 d 0 q 0
 @6  clk 0 rst 0 preset 0 d 0 q 0
 @7  clk 1 rst 0 preset 0 d 0 q 0
 @8  clk 0 rst 0 preset 0 d 0 q 0
 @9  clk 1 rst 0 preset 0 d 0 q 0
 @10 clk 0 rst 1 preset 0 d 0 q 0
 @11 clk 1 rst 1 preset 0 d 0 q 0
 @12 clk 0 rst 1 preset 0 d 0 q 0
 @13 clk 1 rst 1 preset 0 d 0 q 0
 @14 clk 0 rst 1 preset 0 d 0 q 0
 @15 clk 1 rst 1 preset 0 d 0 q 0
 @16 clk 0 rst 1 preset 0 d 0 q 0
 @17 clk 1 rst 1 preset 0 d 0 q 0
 @18 clk 0 rst 1 preset 0 d 0 q 0
 @19 clk 1 rst 1 preset 0 d 0 q 0
 @20 clk 0 rst 0 preset 0 d 0 q 0
 @21 clk 1 rst 0 preset 0 d 0 q 0
 @22 clk 0 rst 0 preset 1 d 0 q 1
 @23 clk 1 rst 0 preset 1 d 1 q 1
 @24 clk 0 rst 0 preset 0 d 1 q 1
 @25 clk 1 rst 0 preset 0 d 1 q 1
 @26 clk 0 rst 0 preset 1 d 1 q 1
 @27 clk 1 rst 0 preset 1 d 1 q 1
 @28 clk 0 rst 0 preset 0 d 1 q 1
 @29 clk 1 rst 0 preset 0 d 1 q 1
 @30 clk 0 rst 0 preset 1 d 1 q 1
 @31 clk 1 rst 0 preset 1 d 1 q 1
 @32 clk 0 rst 0 preset 0 d 1 q 1
 @33 clk 1 rst 0 preset 0 d 1 q 1
 @34 clk 0 rst 0 preset 1 d 1 q 1
 @35 clk 1 rst 0 preset 1 d 0 q 1
 @36 clk 0 rst 0 preset 0 d 0 q 1
 @37 clk 1 rst 0 preset 0 d 1 q 0
 @38 clk 0 rst 0 preset 1 d 1 q 1
 @39 clk 1 rst 0 preset 1 d 1 q 1
 @40 clk 0 rst 0 preset 0 d 1 q 1
   

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  ../images/main/bulllet_4dots_orange.gif force and release

Another form of procedural continuous assignment is provided by the force and release procedural statements. These statements have a similar effect on the assign-deassign pair, but a force can be applied to nets as well as to registers.

   

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One can use force and release while doing gate level simulation to work around reset connectivity problems. Also can be used insert single and double bit errors on data read from memory.

   

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  ../images/main/bullet_star_pink.gif Example - force and release
   

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  1 module force_release ();
  2 
  3 reg clk,rst,d,preset;
  4 wire q;
  5 
  6 initial begin
  7   $monitor("@%g clk %b rst %b preset %b d %b q %b", 
  8     $time, clk, rst, preset, d, q);
  9   clk = 0;
 10   rst = 0;
 11   d  = 0;
 12   preset = 0;
 13    #10  rst = 1;
 14    #10  rst = 0;
 15   repeat (10) begin
 16     @ (posedge clk);
 17     d <= $random;
 18     @ (negedge clk) ;
 19     preset <= ~preset;
 20   end
 21    #1  $finish;
 22 end
 23 // Clock generator
 24 always  #1  clk = ~clk;
 25 
 26 // force and release of flip flop module
 27 always @(preset)
 28 if (preset) begin
 29   force U.q = preset; // force procedural statement
 30 end else begin
 31   release U.q;    // release procedural statement
 32 end
 33 
 34 d_ff U (clk,rst,d,q);
 35 
 36 endmodule
 37 
 38 // D Flip-Flop model
 39 module d_ff (clk,rst,d,q);
 40 input clk,rst,d;
 41 output q;
 42 wire q;
 43 reg q_reg;
 44 
 45 assign q = q_reg;
 46 
 47 always @ (posedge clk)
 48 if (rst) begin
 49   q_reg <= 0;
 50 end else begin
 51   q_reg <= d;
 52 end
 53 
 54 endmodule
You could download file force_release.v here
   

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Simulator Output

   

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 @0  clk 0 rst 0 preset 0 d 0 q x
 @1  clk 1 rst 0 preset 0 d 0 q 0
 @2  clk 0 rst 0 preset 0 d 0 q 0
 @3  clk 1 rst 0 preset 0 d 0 q 0
 @4  clk 0 rst 0 preset 0 d 0 q 0
 @5  clk 1 rst 0 preset 0 d 0 q 0
 @6  clk 0 rst 0 preset 0 d 0 q 0
 @7  clk 1 rst 0 preset 0 d 0 q 0
 @8  clk 0 rst 0 preset 0 d 0 q 0
 @9  clk 1 rst 0 preset 0 d 0 q 0
 @10 clk 0 rst 1 preset 0 d 0 q 0
 @11 clk 1 rst 1 preset 0 d 0 q 0
 @12 clk 0 rst 1 preset 0 d 0 q 0
 @13 clk 1 rst 1 preset 0 d 0 q 0
 @14 clk 0 rst 1 preset 0 d 0 q 0
 @15 clk 1 rst 1 preset 0 d 0 q 0
 @16 clk 0 rst 1 preset 0 d 0 q 0
 @17 clk 1 rst 1 preset 0 d 0 q 0
 @18 clk 0 rst 1 preset 0 d 0 q 0
 @19 clk 1 rst 1 preset 0 d 0 q 0
 @20 clk 0 rst 0 preset 0 d 0 q 0
 @21 clk 1 rst 0 preset 0 d 0 q 0
 @22 clk 0 rst 0 preset 1 d 0 q 1
 @23 clk 1 rst 0 preset 1 d 1 q 1
 @24 clk 0 rst 0 preset 0 d 1 q 0
 @25 clk 1 rst 0 preset 0 d 1 q 1
 @26 clk 0 rst 0 preset 1 d 1 q 1
 @27 clk 1 rst 0 preset 1 d 1 q 1
 @28 clk 0 rst 0 preset 0 d 1 q 1
 @29 clk 1 rst 0 preset 0 d 1 q 1
 @30 clk 0 rst 0 preset 1 d 1 q 1
 @31 clk 1 rst 0 preset 1 d 1 q 1
 @32 clk 0 rst 0 preset 0 d 1 q 1
 @33 clk 1 rst 0 preset 0 d 1 q 1
 @34 clk 0 rst 0 preset 1 d 1 q 1
 @35 clk 1 rst 0 preset 1 d 0 q 1
 @36 clk 0 rst 0 preset 0 d 0 q 1
 @37 clk 1 rst 0 preset 0 d 1 q 0
 @38 clk 0 rst 0 preset 1 d 1 q 1
 @39 clk 1 rst 0 preset 1 d 1 q 1
 @40 clk 0 rst 0 preset 0 d 1 q 1
   

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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