How can you check invariants in VHDL? How can you write information to the console? That is what the VHDL assert statement and report statement are for!
The basic syntax of a report statements in VHDL is:
report <message_string> [severity <severity_level>];
The message string obviously has to be a string. The severity level has the datatype
std.standard.severity_level. Possible values are:
note, warning, error, failure
report "this is a message"; -- severity note -- or: report "this is a serious message" severity warning;
severity_level clause is omitted in a report statement it is implicitly assumed to be
Report statements are sequential statements. This means they can only be in sequential regions, like inside the statements part of process or a procedure. They can not be by themselves in an architecture.
entity e0 is end entity e0; architecture demo of e0 is begin process is begin report "message"; wait; end process; report "message"; -- error end architecture demo;
On the other hand, VHDL assertion statements can be either sequential or concurrent statements. You can write them in a process, or in an architecture.
architecture demo of e0 is signal condition : boolean; begin process is begin assert condition; wait; end process; assert condition report "message" severity error; end architecture demo;
These are several ways of writing a VHDL assert statement, where
condition is of
assert <condition>; assert <condition> severity <severity_level>; assert <condition> report <message_string>; assert <condition> report <message_string> severity <severity_level>;
message_string specified then “Assertion violation.” will be used by default. If the
severity_level clause is omitted in an assert statement it is implicitly assumed to be
assert 3 = 2 + 2; assert 3 = 2 + 2 report "Assertion violation."; assert 3 = 2 + 2 report "Assertion violation." severity error;
One last important thing is how to generate strings for the message. You may want to report the value of a signal (or variable) that is not a string. You need to know the datatype and use the
assert i < 5 report "unexpected value. i = " & integer'image(i);