For concurrent assertion to work, it needs clock, there are multiple ways a clock can be resolved for a concurrent assertion as in the list below.

• Sequence instance with a clock : In this clock is specified in sequence declaration itself.
• Property instance with a clock : In this clock is specified in property declaration itself.
• Contextually inferred clock from a procedural block. : In this procedural block trigger resolve to clock.
• Clocking block : A clocking block can be used for resolving the clock.
• default clock : Default clock is another way of clocking block, so same as above.

  1 //+++++++++++++++++++++++++++++++++++++++++++++++++
  2 //   DUT With assertions
  3 //+++++++++++++++++++++++++++++++++++++++++++++++++
  4 module clock_resolve_assertion();
  5 logic clk = 0;
  6 logic req,gnt;
  7 //=================================================
  8 // Clock inside a sequence 
  9 //=================================================
 10 sequence req_gnt_seq;
 11   @ (posedge clk)
 12       req  ##1  gnt;
 13 endsequence
 14 //=================================================
 15 // Clock inside a property
 16 //=================================================
 17 property req_gnt_prop;
 18   @ (posedge clk)
 19       req |=> gnt;
 20 endproperty
 21 //=================================================
 22 // Clock infered from a always block
 23 //=================================================
 24 always @ (posedge clk)
 25 begin
 26    gnt <= req;
 27    //==============================================
 28    // Here clock is infered to be posedge clk
 29    //==============================================
 30    req_gnt_assert : assert property (req  |=> gnt);
 31 end
 32 //=================================================
 33 // Default clocking
 34 //=================================================
 35 default clocking aclk @ (posedge clk);
 36   input req;
 37   input gnt;
 38 endclocking
 39 
 40 property req_gnt_default_prop;
 41    req |->  ##1  gnt;
 42 endproperty
 43 //=================================================
 44 // clocking clocking
 45 //=================================================
 46 clocking reqclk @ (posedge clk);
 47   input req;
 48   input gnt;
 49 endclocking
 50 
 51 property req_gnt_clocking_prop;
 52    reqclk.req |->  ##1  reqclk.gnt;
 53 endproperty
 54 //+++++++++++++++++++++++++++++++++++++++++++++++++
 55 // Now call all the assertion in one go
 56 //+++++++++++++++++++++++++++++++++++++++++++++++++
 57 a1  : assert property (req_gnt_prop);
 58 a2  : assert property (req_gnt_default_prop);
 59 a3  : assert property (req_gnt_clocking_prop);
 60 //+++++++++++++++++++++++++++++++++++++++++++++++++
 61 //  Assertion testing code
 62 //+++++++++++++++++++++++++++++++++++++++++++++++++
 63 always  #1  clk ++;
 64 
 65 initial begin
 66   $monitor("Req %b Gnt %b",req,gnt);
 67   req <= 0; gnt <= 0;
 68   // Make the assertion pass
 69    ##1  req  <= 1;
 70    ##20 ;
 71   req <= 0;
 72    #10  $finish;
 73 end
 74 
 75 endmodule

 Req 0 Gnt 0
 Req 1 Gnt 0
 Req 1 Gnt 1
 Req 0 Gnt 1
 Req 0 Gnt 0

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