Question

The expect statement is a procedural blocking statement that allows waiting on a property evaluation. The syntax of the expect statement accepts a named property or a property declaration, and is given below.

expect_property_statement ::=                    // 引用自附录A.2.10
    expect (property_spec) action_block

Syntax 17-18—expect statement syntax (excerpt from Annex A)

The expect statement accepts the same syntax used to assert a property. An expect statement causes the executing process to block until the given property succeeds or fails. The statement following the expect is scheduled to execute after processing the Observe region in which the property completes its evaluation. When the property succeeds or fails the process unblocks, and the property stops being evaluated (i.e., no property evaluation is started until that expect statement is executed again).

When executed, the expect statement starts a single thread of evaluation for the given property on the subsequent clocking event, that is, the first evaluation shall take place on the next clocking event. If the property fails at its clocking event, the optional else clause of the action block is executed. If the property succeeds the optional pass statement of the action block is executed.

program tst;
    initial begin
        # 200ms;
        expect( @(posedge clk) a ##1 b ##1 c ) else $error( "expect failed" );
        ABC: ...
    end
endprogram

In the above example, the expect statement specifies a property that consists of the sequence a ##1 b ##1 c. The expect statement (second statement in the initial block of program tst) blocks until the sequence a ##1 b ##1 c is matched, or is determined not to match. The property evaluation starts on the clocking event (posedge clk) following the 200ms delay. If the sequence is matched, the process is unblocked and continues to execute on the statement labeled ABC. If the sequence fails to match then the else clause is executed, which in this case generates a run-time error. For the expect above to succeed, the sequence a ##1 b ##1 c must match starting on the clocking event (posedge clk) immediately after time 200ms. The sequence will not match if a, b, or c are evaluated to be false at the first, second or third clocking event respectively.

The expect statement can be incorporated in any procedural code, including tasks or class methods. Because it is a blocking statement, the property can refer to automatic variables as well as static variables. For example, the task below waits between 1 and 10 clock ticks for the variable data to equal a particular value, which is specified by the automatic argument value. The second argument, success, is used to return the result of the expect statement: 1 for success and 0 for failure.

integer data;
...
task automatic wait_for( integer value, output bit success );
    expect( @(posedge clk) ##[1:10] data == value ) success = 1;
    else success = 0;
endtask

initial begin
    bit ok;
    wait_for( 23, ok ); // wait for the value 23
    ...
end