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An always block is one of the procedural blocks in Verilog. Statements inside an always block are executed sequentially.

Syntax

 
always @ (event)
  [statement]
 
always @ (event) begin
  [multiple statements]
end
 

The always block is executed at some particular event. The event is defined by a sensitivity list.

What is the sensitivity list ?

A sensitivity list is the expression that defines when the always block should be executed and is specified after the @ operator within parentheses ( ). This list may contain either one or a group of signals whose value change will execute the always block.

In the code shown below, all statements inside the always block get executed whenever the value of signals a or b change.

 
// Execute always block whenever value of "a" or "b" change
always @ (a or b) begin
  [statements]
end
 

In the following example, all statements within the always block get executed at every positive edge of the signal clk.

 
// Execute always block at positive edge of signal "clk"
always @ (posedge clk) begin
  [statements]
end
 

What happens if there is no sensitivity list ?

The always block repeats continuously throughout the duration of a simulation. The sensitivity list brings along a certain sense of timing i.e. whenever any signal in the sensitivity list changes, the always block is triggered. If there are no timing control statments within an always block, the simulation will hang because of a zero-delay infinite loop !

Example

The example shown below is an always block that attempts to invert the value of the signal clk. The statement is executed after every 0 time units. Hence, it executes forever because of the absence of a delay in the statement.

 
// always block is started at time 0 units
// But when is it supposed to be repeated ?
// There is no time control, and hence it will stay and
// be repeated at 0 time units only. This continues
// in a loop and simulation will hang !
always clk = ~clk;
 

Even if the sensitivity list is empty, there should be some other form of time delay. Simulation time is advanced by a delay statement within the always construct as shown below. Now, the clock inversion is done after every 10 time units.

 
  always #10 clk = ~clk;
 

Note: Explicit delays are not synthesizable into logic gates !

Hence real Verilog design code always require a sensitivity list.

Sequential Element Design Example

The code shown below defines a module called tff that accepts a data input, clock and active-low reset. The output gets inverted whenever d is found to be 1 at the positive edge of clock. Here, the always block is triggered either at the positive edge of clk or the negative edge of rstn.

What happens at the positive edge of clock ?

The following events happen at the positive edge of clock and is repeated for all positive edge of clock.

  1. First if block checks value of active-low reset rstn
    1. If rstn is zero, then output q should be reset to default value of 0
    2. If rstn is one, then it means reset is not applied and should follow default behavior
  2. If the previous step is false:
    1. Check value of d and if it is found to be one, then invert value of q
    2. If d is 0, then maintain value of q

 
module tff (input      d,
            clk,
            rstn,
      output reg   q);
 
  always @ (posedge clk or negedge rstn) begin
    if (!rstn)
      q <= 0;
    else
      if (d)
        q <= ~q;
      else
        q <= q;
  end
endmodule
 

What happens at the negative edge of reset ?

The following events happen at negative edge of rstn and happen at all such occurrences.

  1. First if block checks value of active-low reset rstn. At negative edge of the signal, its value is 0.
    1. If value of rstn is 0, then it means reset is applied and output should be reset to default value of 0
    2. The case where value of rstn is 1 is not considered because the current event is negative edge of the rstn

Combinational Element Design Example

An always block can also be used in the design of combinational blocks. For example the following digital circuit represents a combination of three different logic gates that provide a certain output at signal o.

combinational-gates

The code shown below is a module with four input ports and a single output port called o. The always block is triggered whenever any of the signals in the sensitivity list changes in value. Output signal is declared as type reg in the module port list because it is used in a procedural block. All signals used in a procedural block should be declared as type reg.

 
module combo (  input   a,
            input  b,
                input  c,
                input  d,
                output reg o);
 
  always @ (a or b or c or d) begin
    o <= ~((a & b) | (c^d));
  end
 
endmodule
 

See that the signal o becomes 1 whenever the combinational expression on the RHS becomes true. Similarly o becomes 0 when RHS is false.

Simulation Output combo-gates-wave

Click to try this example in a simulator!   

Step-by-Step simulation

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