A statement that checks that a
specified condition is true and reports an error if it is not.
The assertion statement has
three optional fields and usually all three are used.
The condition specified in an assertion
statement must evaluate to a boolean value (true or false).
If it is false, it is said that an assertion violation occurred.
The expression specified in the report
clause must be of predefined type STRING and is a message to
be reported when assertion violation occurred.
If the severity clause is
present, it must specify an expression of predefined type
SEVERITY_LEVEL, which determines the severity level of the assertion
violation. The SEVERITY_LEVEL type is specified in the STANDARD
package and contains following values: NOTE, WARNING, ERROR, and
FAILURE. If the severity
clause is omitted it is implicitly assumed to be ERROR.
When an assertion violation occurs, the report is issued and
displayed on the screen. The supported severity level supplies an
information to the simulator. The severity level defines the degree
to which the violation of the assertion affects operation of the process:
NOTE can be used to pass information messages from simulation
WARNING can be used in unusual situation in which the simulation can
be continued, but the results may be unpredictable (example 2);
ERROR can be used when assertion violation makes continuation of the
simulation not feasible (example 3);
FAILURE can be used when the assertion violation is a fatal error and
the simulation must be stopped at once (example 4).
Assertion statements are not only sequential, but can be used as
concurrent statements as well. A concurrent assertion statement
represents a passive process statement containing the specified
assert Status = OPEN_OK
call to FILE_OPEN was not successful"
Having called the procedure FILE_OPEN, if the status is different
from OPEN_OK, it is indicated by the warning message.
(S= '1' and R= '1')
values of signals S and R are equal to '1'"
When the values of the signals S and R are equal to '1', the message
is displayed and the simulation is stopped because the severity is
set to ERROR.
assert Operation_Code = "0000"
"Illegal Code of Operation"
Event like illegal operation code are severe errors and should cause
immediate termination of the simulation, which is forced by the
severity level FAILURE.
The message is displayed when the condition is NOT met, therefore the
message should be an opposite to the condition.
Concurrent assertion statement is a passive process and as such can
be specified in an entity.
Concurrent assertion statement monitors specified condition continuously.
Synthesis tools generally ignore assertion statements.