SystemVerilog Assertion
Part 3: Sequence Match Operators
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Next: Property Layer
Local Variables in A Sequence
SystemVerilog sequences can have local variables. For each attempt
to match a sequence, a new copy of the local variable is created
and no hierarchical access to these local variables are allowed.
A local variable is assigned a value using a comma separated list
along with other expressions. In the example below, i is
local variable within the sequence seq1 which is assigned
a value of tag_in when data_valid is high. This
value is then checked with the value of tag_out 7 clock
ticks later.
sequence s1;
int i;
(data_valid, (i = tag_in))
##7 (tag_out == i);
endsequence
One important thing to note above is that inputs are often pipelined
in a design. For instance, by the time of the comparison of
tag_out with i, the value of tag_in that was
stored may have been lost. Thus, using a local variable is
very useful here.
A local variable is not accessible from outside of the sequence where
it is instantiated. This applies even to other sequences that will call
this sequence. The only way of accessing a local variable is to pass it
as an argument. Certain rules apply when a local variable is passed as
an argument.
- A local variable can be passed only as an entire actual argument.
- Outside the sequence that instantiate a local variable, the
corresponding argument can not be reference before the local variable is
assigned in the sequence.
System Functions Helpful for Assertion
The following functions in Figure 6 help in assertion formation, but can be used
anywhere in SystemVerilog code.
| Function
| Meaning
|
| $onehot (expression)
| true, if only one of the bits in the expression is high.
|
| $onehot0 (expression)
| true, if at most one of the bit in the expression is high.
|
| $isunknown (expression)
| true, if any bit of the expression is X or Z.
|
| $countones (expression)
| returns the number of 1s in a bit vector expression.
Figure 6. Sampled Value System Functions
|
With this we conclude this part and our discussion on sequences.
Next time, we will look into the next layer of concurrent assertion
- the property layer.
Prev: More Sequence Match Operators |
Next: Property Layer
|
