Difference between revisions of "VHDL"
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Revision as of 15:10, 29 September 2010
there are 3 domains of modeling:
each provides a basic modeling concept.
a module e.g. a register is an entity, it's inputs ans outputs are called ports. an architecture is the internal implementation of an entity. it describes the behavior of an entity. architectures includes only processes, collection of actions which are executed in sequences.
types of action that can be performed:
- evaluating expressions
- assigning variables and values
- conditional and repeated execution
- subprogram calls
behavioral architecture: function of an entity is described in an abstract way. e.g.
entity srlatch is port ( s,r : in std_logic; q,qb : out std_logic); end srlatch; architecture behave of calc is begin foo: process begin Q <= S nand QB; QB <= R nand Q; end process foo; end architecture behave;
structural architecture: only interconnecting subsystems
LIBRARY ieee; USE ieee.std_logic_1164.ALL; entity nand2 is port( a,b : in std_logic; q : out std_logic); end nand2; architecture basic of nand2 is begin q <= a nand b after 2 ns; end basic;
entity srlatch is port( s,r: in std logic; q,qb: out std_logic); end srlatch; architecture struct of srlatch is begin sq: entity work.nand2(basic) port map ( a => s, b => qb, q => q ); rq: entity work.nand2(basic) port map ( a => r, b => q, q => qb ); end architecture struct;
subprograms are procedures, a collection of statements executed for their effect, and functions, a collection of statements to compute a result.
variables etc. defined in the header are local variables.
procedure proc_name is variable total : real :=0.0; --this is a local variable! begin for index in samples loop total = total +sample(index); end loop; answer:=total; end procedure proc_name;
procedures without parameters is called by it's name from any place in architecture. other procedures can also call procedures in their body.
it is possible to use procedures with parameters.
generalization of expressions, combined values with operators and produce new values. the type of result is specified.
function limit (val, min, max: integer) return integer is begin if val > max then return max; elseif val < min then return min; else return val; end if; end function;
a function is called in the architecture like this:
newval := limit(current_val, 10, 100); newval2 := 2 + limit(current_val, 10, 100);
grouping a collection of related declarations to serve common purpose. seperat design units that can be worked on independently, reused in different parts of design. the can be written along with entities and architecture and are analyzed separately. it is also possible to place a package in an other library, in that case the lib "work" has to be replaced by the lib including the package.
the external view is specified in the package declaration, the implementation is defind in the package body.
the package declaration hosts type, constand, subprogram, signal ... declarations which are shared within the model. it defines the interface to a package. in the model then you just need to refer to the package:
package yourpackagename is --declaration constant foo: std_logic_vector(7 downto 0) := X"01"; type foobar is array (foobar2) of somethingelse; end package yourpackagename;
use yourpackagename.all; entity yourentityname is port ( fooport : in work.yourpackagename.foo; foobarport : in work.yourpackagename.foobar; ); end entity yourentityname;
no package body is needed if pkg declaration contains other kinds of declarations like types, signals or fully specified constants. in case of subprograms body needed to fill in missing informations.
items declared in body must include full declaration of subprograms as they are in the pkg declaration. that means that types, modes, default constants... must bee repeated exactly.
to build hierarchical designs, used to describe interconnections in subsystems in a design. component declarations are written in the declarative part of the architecture (before the begin).