In an always block which is used to model combinational logic, forgetting an else leads to an unintended latch. To avoid this mistake, SystemVerilog adds specialized always_comb and always_latch blocks, which indicate design intent to simulation, synthesis and formal verification tools. SystemVerilog also adds an always_ff block to indicate sequential logic.

SystemVerilog has both static processes, introduced by always, initial or fork, and dynamic processes, introduced by built-in fork...join_any and fork...join_none.

New features added SystemVerilog is as below

• always_combo
• always_latch
• always_ff
• join_any
• join_none

For continous assignements SystemVerilog allows to drive other then net type to be driven or assigned using assign statement (Continuous assignments), Like reg or integer.

SystemVerilog provides a special always_comb procedure for modeling combinational logic behavior.

• There is an inferred sensitivity list that includes the expressions defined.
• The variables written on the left-hand side of assignments shall not be written to by any other process.
• The procedure is automatically triggered once at time zero, after all initial and always blocks have been started so that the outputs of the procedure are consistent with the inputs.

  1 module always_comb_process();
  2 
  3 reg [7:0] sum,a,b;
  4 reg       parity;
  5 
  6 initial begin
  7   $monitor ("@%g a = %h b = %h sum = %h parity = %b", 
  8    $time, a, b, sum, parity);
  9    #1  a = 1;
 10    #1  b = 1;
 11    #5  a = 10;
 12    #1  $finish;
 13 end
 14 
 15 always_comb
 16 begin : ADDER
 17   sum = b + a;
 18   parity = ^sum;
 19 end
 20 
 21 endmodule

Simulator Output

 @0 a = xx b = xx sum = xx parity = x
 @1 a = 01 b = xx sum = xx parity = x
 @2 a = 01 b = 01 sum = 02 parity = 1
 @7 a = 0a b = 01 sum = 0b parity = 1

SystemVerilog also provides a special always_latch procedure for modeling latched logic behavior.

• The always_latch procedure determines its sensitivity and executes identically to the always_comb procedure.

  1 module always_latch_process();
  2 
  3 reg [7:0] sum,a,b;
  4 reg       parity;
  5 reg       enable = 0;
  6 
  7 initial begin
  8   $monitor ("@%g a = %h b = %h sum = %h parity = %b", 
  9     $time, a, b, sum, parity);
 10    #2  a = 1;
 11    #2  b = 1;
 12    #2  a = 10;
 13    #2  $finish;
 14 end
 15 
 16 always  #1  enable = ~enable;
 17 
 18 always_latch
 19 begin : ADDER
 20   if (enable) begin
 21     sum    <= b + a;
 22     parity <= ^(b + a);
 23   end
 24 end
 25 
 26 endmodule

Simulator Output

 @0 a = xx b = xx sum = xx parity = x
 @2 a = 01 b = xx sum = xx parity = x
 @4 a = 01 b = 01 sum = xx parity = x
 @5 a = 01 b = 01 sum = 02 parity = 1
 @6 a = 0a b = 01 sum = 02 parity = 1
 @7 a = 0a b = 01 sum = 0b parity = 1

The SystemVerilog always_ff procedure can be used to model synthesizable sequential logic behavior.

• The SystemVerilog always_ff procedure can be used to model synthesizable sequential logic behavior.
• Variables on the left-hand side of assignments within an always_ff procedure, including variables from the contents of a called function, shall not be written to by any other process.

  1 module always_ff_process();
  2 
  3 reg [7:0] sum,a,b;
  4 reg       parity;
  5 logic     clk = 0;
  6 reg       rst = 0;
  7 
  8 initial begin
  9   $monitor ("@%g clk = %b rst = %b a = %h b = %h sum = %h parity = %b", 
 10   $time, clk, rst, a, b, sum, parity);
 11    #1  rst = 1;
 12    #5  rst = 0;
 13    #2  a = 1;
 14    #2  b = 1;
 15    #2  a = 10;
 16    #2  $finish;
 17 end
 18 
 19 always  #1  clk ++;
 20 
 21 // use of iff makes sure that block does not get
 22 // triggered due to posedge of clk when rst == 1
 23 always_ff @(posedge clk iff rst == 0 or posedge rst)
 24 begin : ADDER
 25   if (rst) begin
 26     sum    <= 0;
 27     parity <= 0;
 28     $display ("Reset is asserted BLOCK 1");
 29   end else begin
 30     sum    <= b + a;
 31     parity <= ^(b + a);
 32   end
 33 end
 34 
 35 // To show how iff affected in earlier code
 36 always_ff @(posedge clk  or posedge rst)
 37 begin
 38   if (rst) begin
 39     $display ("Reset is asserted BLOCK 2");
 40   end 
 41 end
 42 
 43 endmodule

Simulator Output

 @0 clk = 0 rst = 0 a = xx b = xx sum = xx parity = x
 Reset is asserted BLOCK 1
 Reset is asserted BLOCK 2
 @1 clk = 1 rst = 1 a = xx b = xx sum = 00 parity = 0
 @2 clk = 0 rst = 1 a = xx b = xx sum = 00 parity = 0
 Reset is asserted BLOCK 2
 @3 clk = 1 rst = 1 a = xx b = xx sum = 00 parity = 0
 @4 clk = 0 rst = 1 a = xx b = xx sum = 00 parity = 0
 Reset is asserted BLOCK 2
 @5 clk = 1 rst = 1 a = xx b = xx sum = 00 parity = 0
 @6 clk = 0 rst = 0 a = xx b = xx sum = 00 parity = 0
 @7 clk = 1 rst = 0 a = xx b = xx sum = xx parity = x
 @8 clk = 0 rst = 0 a = 01 b = xx sum = xx parity = x
 @9 clk = 1 rst = 0 a = 01 b = xx sum = xx parity = x
 @10 clk = 0 rst = 0 a = 01 b = 01 sum = xx parity = x
 @11 clk = 1 rst = 0 a = 01 b = 01 sum = 02 parity = 1
 @12 clk = 0 rst = 0 a = 0a b = 01 sum = 02 parity = 1
 @13 clk = 1 rst = 0 a = 0a b = 01 sum = 0b parity = 1

In verilog only net data types can be driven by continous assignment, In systemVerilog this restriction is removed and any data type can be driven using continuous assignment.

• Nets can be driven by multiple continuous assignments or by a mixture of primitives and continuous assignments.
• Variables can only be driven by one continuous assignment or one primitive output.

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Deepak Kumar Tala - All rights reserved

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