VHDL Programming

the resolved value that a particular driver is driving so that it can be

further used in the model. In the example shown here the second com-

ponent instantiation statement would cause an error because input

port a of U2is trying to read the current value of dout. In VHDL93 the

`driving_valueattribute gets around this problem by reading the driving

value of dout.

ENTITY invert IS

PORT( w: IN STD_LOGIC;

dout, doutb : OUT STD_LOGIC);

END invert;

ARCHITECTURE struct OF invert IS

COMPONENT inv

PORT( a : IN STD_LOGIC;

q : OUT STD_LOGIC);

END COMPONENT;

BEGIN

u1 : inv PORT MAP(a => w, q => dout);

--u2 : inv PORT MAP(a => dout, q => doutb);

-- won’t work because port

-- dout cannot be read

u2 : inv PORT MAP(a => dout`DRIVING_VALUE, q => doutb);

-- In VHDL93 this

-- will work

END struct;

`IMAGE AND `VALUE In VHDL87 it was difficult for an error message

to display the actual error value of a signal or a variable in a string. In

VHDL93 the attributes `IMAGEand `VALUEallow the modeler to convert to

and from type values into string values. Attribute `IMAGEconverts a type

value into a string, and attribute `VALUEconverts a string to a type value.

`PATHNAME, `INSTANCE_NAME, AND `SIMPLE_NAME The

other difficulty in VHDL87 of model error reporting was to uniquely

determine exactly which instance of a model was generating a message.

Most VHDL simulators had some mechanism of reporting the instance

information to the modeler, but this information was simulator-specific

and not standard. In VHDL93 three new attributes allow the modeler

access to all parts of the pathname that describes which instance a partic-

ular message is generated from.

`SIMPLE_NAME—returns a string which is the local name of the

calling entity.

`PATH_NAME—returns a string that describes the path to the entity

starting at the root of the design. The `PATH_NAMEattribute does not

include the names of instantiated entities (`INSTANCE_NAMEdoes)

Appendix D: VHDL93 Updates 451