Advanced Sequences - SVA: The Power of Assertions in SystemVerilog - page 254

sequence s1;
s ##1 1;
endsequence
Internally, matched stores the result of its source sequence match until the arrival
of the next destination clock tick after the match. There are no limitations imposed
on the last clocking event of sequence s .
There is not much sense in using matched in singly clocked assertions, but it is
helpful in multiply clocked assertions where triggered might not be directly used.
We explore this topic in depth in Chap. 12 .
Unlike s.triggered , s.matched cannot be used outside sequences.

11.3 Sequence as Events

Sequences may be used as events, both edge sensitive and level sensitive.

11.3.1 Sequence Event Control

The syntax of the sequence event control is @sequence_instance ; where
@sequence_instance is an instance of a named sequence. Arbitrary sequence
expressions cannot be specified with the event control. For example, @(@( posedge
clk)a ##1 b) is illegal, as @( posedge clk)a ##1 b is a sequence expression,
but not an instance.
Sequence event control is especially convenient in programs (see Sect. 2.5 )
to specify starting points of testbench execution. For example, we may wish
to start some testbench activity when the initialization of the subsystem is com-
plete ( ready is asserted), and when the system has entered the normal power
mode ( pmode == normal when the last power mode switch pswitch occurred).
The resulting code is shown in Fig. 11.14 .
When program test starts executing, it immediately blocks until sequence
operational matches. At that point, the program execution is resumed, task
run_test is called, followed by Test started message display.
Note the following:

￿ Since sequence operational is stand-alone, and not part of a property or of
an assertion, it cannot infer its clocking event from an enclosing property or
assertion. Therefore, the sequence clocking event @( posedge clk) must be
explicitly specified unless the sequence belongs to the scope of a clocking block,
or default clocking is specified.
￿ The sequence event occurs if any attempt of the sequence has a match.

This is why the sequence matching starts only from moments when

pswitch && pmode == normal is true, and not from the first clock tick.