SystemVerilog Assertions

Assertions are primarily used to validate the behavior of a design. An assertion is a check embedded in design or bound to a design unit during the simulation. Warnings or errors are generated on the failure of a specific condition or sequence of events.

Assertions are used to,

• Check the occurrence of a specific condition or sequence of events.
• Provide functional coverage.

There are two kinds of assertions:

• Immediate Assertions
• Concurrent Assertions

Immediate Assertions:

Immediate assertions check for a condition at the current simulation time.

An immediate assertion is the same as an if..else statement with assertion control. Immediate assertions have to be placed in a procedural block definition.

Syntax

label: assert(expression) action_block;

• The optional statement label (identifier and colon) creates a named block around the assertion statement
• The action block is executed immediately after the evaluation of the assert expression
• The action_block specifies what actions are taken upon success or failure of the assertion

action_block;

pass_statement; else fail_statement;

• The pass statement is executed if the expression evaluates to true
• The statement associated with else is called a fail statement and is executed if the expression evaluates to false
• Both pass and fail statements are optional

• Since the assertion is a statement that something must be true, the failure of an assertion shall have a severity associated with it. By default, the severity of an assertion failure is an error.
• Other severity levels can be specified by including one of the following severity system tasks in the fail statement:
  • $fatal is a run-time fatal.
  • $error is a run-time error.
  • $warning is a run-time warning, which can be suppressed in a tool-specific manner.
  • $info indicates that the assertion failure carries no specific severity.
• If an assertion fails and no else clause is specified, the tool shall, by default call $error.

Below are the different forms of immediate assertion syntax with and without optional items

//With Pass and Fail statement; Fail verbosity info;
assert(expression) $display(“expression evaluates to true”); else $display(“expression evaluates to false”);

//Only With Pass statement;
assert(expression) $display(“expression evaluates to true”);

//With Pass and Fail statement; Fail verbosity fatal;
assert(expression) $display(“expression evaluates to true”); else $fatal(“expression evaluates to false”);

//Only With Fail statement; Multiple statements in Faile condition and Fail verbosity fatal;
assert(expression)
      else begin
        …….
        …….
        $fatal(“expression evaluates to false”);
      end

//Only With Fail statement; Fail verbosity warning;
assert(expression) else $warning(“expression evaluates to false”);

//With Label and Fail statement; Fail verbosity warning;
label: assert(expression) else $warning(“expression evaluates to false”);

Immediate assertion example

Below is the simple immediate assertion,

always @(posedge clk) assert (a && b);

Below is the wave diagram for the above assertion.
Condition (a && b) will be checked at every posedge of the clock, failure in the condition leads to an assertion failure.

module asertion_ex;
  bit clk,a,b;
  
  //clock generation
  always #5 clk = ~clk; 
  
  //generating 'a'
  initial begin 
    a=1;
    b=1;
    #15 b=0;
    #10 b=1;
        a=0;
    #20 a=1;
    #10;
    $finish;
  end
  
  //Immediate assertion
  always @(posedge clk) assert (a && b);

endmodule

Simulator Output

ncsim: *E,ASRTST (./testbench.sv,23): (time 15 NS) Assertion asertion_ex.__assert_1 has failed
ncsim: *E,ASRTST (./testbench.sv,23): (time 25 NS) Assertion asertion_ex.__assert_1 has failed
ncsim: *E,ASRTST (./testbench.sv,23): (time 35 NS) Assertion asertion_ex.__assert_1 has failed
Simulation complete via $finish(1) at time 55 NS + 0

Click to execute on   

Concurrent Assertions:

Concurrent assertions check the sequence of events spread over multiple clock cycles.

• The concurrent assertion is evaluated only at the occurrence of a clock tick
• The test expression is evaluated at clock edges based on the sampled values of the variables involved
• It can be placed in a procedural block, a module, an interface or a program definition

   c_assert:  assert property(@(posedge clk) not(a && b));

The Keyword differentiates the immediate assertion from the concurrent assertion is “property.”

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