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10.6  Packages and Libraries

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10.6  Packages and Libraries

After the VHDL tool has analyzed entities, architectures, and configurations, it stores the resulting design units in a library. Much of the power of VHDL comes from the use of predefined libraries and packages. A VHDL design library [VHDL LRM11.2] is either the current working library (things we are currently analyzing) or a predefined resource library (something we did yesterday, or we bought, or that came with the tool). The working library is named work and is the place where the code currently being analyzed is stored. Architectures must be in the same library (but they do not have to be in the same physical file on disk) as their parent entities.

You can use a VHDL package [VHDL LRM2.5-2.6] to define subprograms (procedures and functions), declare special types, modify the behavior of operators, or to hide complex code. Here is the BNF for a package declaration:

package_declaration ::= 
package identifier is
{subprogram_declaration 	| type_declaration	| subtype_declaration
	| constant_declaration 	| signal_declaration	| file_declaration
	| alias_declaration 	| component_declaration
	| attribute_declaration | attribute_specification
	| disconnection_specification | use_clause
	| shared_variable_declaration | group_declaration
	| group_template_declaration}
	end [package] [package_identifier] ;

You need a package body if you declare any subprograms in the package declaration (a package declaration and its body do not have to be in the same file):

package_body ::=
	package body package_identifier is
{subprogram_declaration 	| subprogram_body
	| type_declaration	| subtype_declaration
	| constant_declaration	| file_declaration	| alias_declaration
	| use_clause 	 
	| shared_variable_declaration | group_declaration
	| group_template_declaration}
	end [package body] [package_identifier] ;

To make a package visible [VHDL LRM10.3] (or accessible, so you can see and use the package and its contents), you must include a library clause before a design unit and a use clause either before a design unit or inside a unit, like this:

library MyLib; -- library clause
use MyLib.MyPackage.all; -- use clause
-- design unit (entity + architecture, etc.) follows:

The STD and WORK libraries and the STANDARD package are always visible. Things that are visible to an entity are visible to its architecture bodies.

10.6.1  Standard Package

The VHDL STANDARD package [VHDL LRM14.2] is defined in the LRM and implicitly declares the following implementation dependent types: TIME , INTEGER , REAL . We shall use uppercase for types defined in an IEEE standard package. Here is part of the STANDARD package showing the explicit type and subtype declarations:

package Part_STANDARD is
 is (FALSE, TRUE); type BIT
 is ('0', '1');
subtype NATURAL
 is INTEGER range 0 to INTEGER'HIGH;
subtype POSITIVE
 is INTEGER range 1 to INTEGER'HIGH;
 is array (NATURAL range <>) of BIT;
 is array (POSITIVE range <>) of CHARACTER;
-- the following declarations are VHDL-93 only:
attribute FOREIGN: STRING; -- for links to other languages
subtype DELAY_LENGTH is TIME range 0 fs to TIME'HIGH;
end Part_STANDARD;

Notice that a STRING array must have a positive index. The type TIME is declared in the STANDARD package as follows:

type TIME is range implementation_defined -- and varies with software
	units fs; ps = 1000 fs; ns = 1000 ps; us = 1000 ns; ms = 1000 us; 
	sec = 1000 ms; min = 60 sec; hr = 60 min; end units;

The STANDARD package also declares the function now that returns the current simulation time (with type TIME in VHDL-87 and subtype DELAY_LENGTH in VHDL-93).

In VHDL-93 the CHARACTER type declaration extends the VHDL-87 declaration (the 128 ASCII characters):

type Part_CHARACTER is ( -- 128 ASCII characters in VHDL-87 
NUL, SOH, STX, ETX, EOT, ENQ, ACK, BEL, -- 33 control characters 
 BS,  HT,  LF,  VT,  FF,  CR,  SO,  SI, -- including:
DLE, DC1, DC2, DC3, DC4, NAK, SYN, ETB, -- format effectors:
CAN,  EM, SUB, ESC, FSP, GSP, RSP, USP, -- horizontal tab = HT
' ', '!', '"', '#', '$', '%', '&', ''', -- line feed = LF
'(', ')', '*', '+', ',', '-', '.', '/', -- vertical tab = VT
'0', '1', '2', '3', '4', '5', '6', '7', -- form feed = FF
'8', '9', ':', ';', '<', '=', '>', '?', -- carriage return = CR
'@', 'A', 'B', 'C', 'D', 'E', 'F', 'G', -- and others:
'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', -- FSP, GSP, RSP, USP use P
'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', -- suffix to avoid conflict
'X', 'Y', 'Z', '[', '\', ']', '^', '_', -- with TIME units
'`', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 
'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 
'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 
'x', 'y', 'z', '{', '|', '}', '~', DEL  -- delete = DEL
-- VHDL-93 includes 96 more Latin-1 characters, like ¥ (Yen) and 
-- 32 more control characters, better not to use any of them.

The VHDL-87 character set is the 7-bit coded ISO 646-1983 standard known as the ASCII character set. Each of the printable ASCII graphic character codes (there are 33 nonprintable control codes, like DEL for delete) is represented by a graphic symbol (the shapes of letters on the keyboard, on the display, and that actually print). VHDL-93 uses the 8-bit coded character set ISO 8859-1:1987(E), known as ISO Latin-1. The first 128 characters of the 256 characters in ISO Latin-1 correspond to the 128-character ASCII code. The graphic symbols for the printable ASCII characters are well defined, but not part of the standard (for example, the shape of the graphic symbol that represents 'lowercase a' is recognizable on every keyboard, display, and font). However, the graphic symbols that represent the printable characters from other 128-character codes of the ISO 8-bit character set are different in various fonts, languages, and computer systems. For example, a pound sterling sign in a U.K. character set looks like this-'£', but in some fonts the same character code prints as '#' (known as number sign, hash, or pound). If you use such characters and want to share your models with people in different countries, this can cause problems (you can see all 256 characters in a character set by using Insert... Symbol in MS Word).

10.6.2  Std_logic_1164 Package

VHDL does not have a built-in logic-value system. The STANDARD package predefines the type BIT with two logic values, '0' and '1' , but we normally need at least two more values: 'X' (unknown) and 'Z' (high-impedance). Unknown is a metalogical value because it does not exist in real hardware but is needed for simulation purposes. We could define our own logic-value system with four logic values:

type MVL4 is ('X', '0', '1', 'Z'); -- a four-value logic system

The proliferation of VHDL logic-value systems prompted the creation of the Std_logic_1164 package (defined in IEEE Std 1164-1993) that includes functions to perform logical, shift, resolution, and conversion functions for types defined in the Std_logic_1164 system. To use this package in a design unit, you must include the following library clause (before each design unit) and a use clause (either before or inside the unit):

library IEEE; use IEEE.std_logic_1164.all;

This Std_Logic_1164 package contains definitions for a nine-value logic system. The following code and comments show the definitions and use of the most important parts of the package 1:

package Part_STD_LOGIC_1164 is
type STD_ULOGIC is
(	'U', -- Uninitialized
	'X', -- Forcing Unknown
	'0', -- Forcing 0
	'1', -- Forcing 1
	'Z', -- High Impedance
	'W', -- Weak Unknown
	'L', -- Weak 0
	'H', -- Weak 1
	'-'  -- Don't Care);
type STD_ULOGIC_VECTOR is array (NATURAL range <>) of STD_ULOGIC;
function resolved (s : STD_ULOGIC_VECTOR) return STD_ULOGIC;
subtype STD_LOGIC is resolved STD_ULOGIC;
type STD_LOGIC_VECTOR is array (NATURAL range <>) of STD_LOGIC;
subtype X01   is resolved STD_ULOGIC range 'X' to '1';
subtype X01Z  is resolved STD_ULOGIC range 'X' to 'Z';
subtype UX01  is resolved STD_ULOGIC range 'U' to '1';
subtype UX01Z is resolved STD_ULOGIC range 'U' to 'Z'; 
-- Vectorized overloaded logical operators:
function "and"  (L : STD_ULOGIC; R : STD_ULOGIC) return UX01;
-- Logical operators not, and, nand, or, nor, xor, xnor (VHDL-93),
-- Strength strippers and type conversion functions:
-- function To_T (X : F) return T; 
-- defined for types, T and F, where 
-- T=types F plus types X01 X01Z UX01 (but not type UX01Z)
-- Exclude _'s in T in name: TO_STDULOGIC not TO_STD_ULOGIC
-- To_XO1 : L->0, H->1 others->X
-- To_XO1Z: Z->Z, others as To_X01
-- To_UX01: U->U, others as To_X01
-- Edge detection functions:
function rising_edge  (signal s: STD_ULOGIC) return BOOLEAN;
function falling_edge (signal s: STD_ULOGIC) return BOOLEAN;
-- Unknown detection (returns true if s = U, X, Z, W):
-- function Is_X (s : T) return BOOLEAN;
end Part_STD_LOGIC_1164;


  • The type STD_ULOGIC has nine logic values. For this reason IEEE Std 1164 is sometimes referred to as MVL9--multivalued logic nine. There are simpler, but nonstandard, MVL4 and MVL7 packages, as well as packages with more than nine logic values, available. Values 'U' , 'X' , and 'W' are all metalogical values.
  • There are weak and forcing logic-value strengths. If more than one logic gate drives a node (there is more than one driver) as in wired-OR logic or a three-state bus, for example, the simulator checks the driver strengths to resolve the actual logic value of the node using the resolution function, resolved , defined in the package.
  • The subtype STD_LOGIC is the resolved version of the unresolved type STD_ULOGIC. Since subtypes are compatible with types (you can assign one to the other) you can use either STD_LOGIC or STD_ULOGIC for a signal with a single driver, but it is generally safer to use STD_LOGIC.
  • The type STD_LOGIC_VECTOR is the resolved version of unresolved type STD_ULOGIC_VECTOR. Since these are two different types and are not compatible, you should use STD_LOGIC_VECTOR. That way you will not run into a problem when you try to connect a STD_LOGIC_VECTOR to a STD_ULOGIC_VECTOR.
  • The don't care logic value '-' (hyphen), is principally for use by synthesis tools. The value '-' is almost always treated the same as 'X'.
  • The 1164 standard defines (or overloads) the logical operators for the STD_LOGIC types but not the arithmetic operators (see Section 10.12).

10.6.3  Textio Package

You can use the textio package, which is part of the library STD , for text input and output [VHDL LRM14.3]. The following code is a part of the TEXTIO package header and, together with the comments, shows the declarations of types, subtypes, and the use of the procedures in the package:

package Part_TEXTIO is -- VHDL-93 version.
type LINE is access STRING; -- LINE is a pointer to a STRING value.
type TEXT is file of STRING; -- File of ASCII records. 
type SIDE is (RIGHT, LEFT); -- for justifying output data. 
subtype WIDTH is NATURAL; -- for specifying widths of output fields. 
file INPUT : TEXT open READ_MODE is "STD_INPUT"; -- Default input file.
file OUTPUT : TEXT open WRITE_MODE is "STD_OUTPUT"; -- Default output.
-- The following procedures are defined for types, T, where 
-- 		procedure READLINE(file F : TEXT; L : out LINE);
-- 		procedure READ(L : inout LINE; VALUE : out T);
-- 		procedure READ(L : inout LINE; VALUE : out T; GOOD: out BOOLEAN);
-- 		procedure WRITELINE(F : out TEXT; L : inout LINE);
-- 		procedure WRITE(
--			L : inout LINE; 
--			VALUE : in T; 
--			FIELD:in WIDTH := 0; 
--			DIGITS:in NATURAL := 0; -- for T = REAL only
--			UNIT:in TIME:= ns); -- for T = TIME only
-- function ENDFILE(F : in TEXT) return BOOLEAN;
end Part_TEXTIO;

Here is an example that illustrates how to write to the screen (STD_OUTPUT ):

library std; use std.textio.all; entity Text is end;
architecture Behave of Text is signal count : INTEGER := 0;
begin count <= 1 after 10 ns, 2 after 20 ns, 3 after 30 ns;
process (count) variable L: LINE; begin 
if (count > 0) then 
	write(L, now); -- Write time.
	write(L, STRING'(" count=")); -- STRING' is a type qualification.
	write(L, count); writeline(output, L);
end if; end process; end;
10 ns count=1
20 ns count=2
30 ns count=3

10.6.4  Other Packages

VHDL does not predefine arithmetic operators on types that hold bits. Many VHDL simulators provide one or more arithmetic packages that allow you to perform arithmetic operations on std_logic_1164 types. Some companies also provide one or more math packages that contain functions for floating-point algebra, trigonometry, complex algebra, queueing, and statistics (see also [IEEE 1076.2, 1996]).

Synthesis tool companies often provide a special version of an arithmetic package, a synthesis package, that allows you to synthesize VHDL that includes arithmetic operators. This type of package may contain special instructions (normally comments that are recognized by the synthesis software) that map common functions (adders, subtracters, multipliers, shift registers, counters, and so on) to ASIC library cells. I shall introduce the IEEE synthesis package in Section 10.12.

Synthesis companies may also provide component packages for such cells as power and ground pads, I/O buffers, clock drivers, three-state pads, and bus keepers. These components may be technology-independent (generic) and are mapped to primitives from technology-dependent libraries after synthesis.

10.6.5  Creating Packages

It is often useful to define constants in one central place rather than using literals wherever you need a specific value in your code. One way to do this is by using VHDL packaged constants [VHDL LRM4.3.1.1] that you define in a package. Packages that you define are initially part of the working library, work . Here are two example packages [VHDL LRM2.5-2.7]:

package Adder_Pkg is -- a package declaration
	constant BUSWIDTH : INTEGER := 16; 
end Adder_Pkg;
use work.Adder_Pkg.all; -- a use clause
entity Adder is end Adder;
architecture Flexible of Adder is -- work.Adder_Pkg is visible here
	begin process begin 
		MyLoop : for j in 0 to BUSWIDTH loop -- adder code goes here
		end loop; wait; -- the wait prevents an endless cycle
	end process;
end Flexible;
package GLOBALS is 
	constant HI : BIT := '1'; constant LO: BIT := '0';

Here is a package that declares a function and thus requires a package body:

package Add_Pkg_Fn is
function add(a, b, c : BIT_VECTOR(3 downto 0)) return BIT_VECTOR;
end Add_Pkg_Fn;
package body Add_Pkg_Fn is
function add(a, b, c : BIT_VECTOR(3 downto 0)) return BIT_VECTOR is
	begin return a xor b xor c; end; 
end Add_Pkg_Fn;

The following example is similar to the VITAL (VHDL Initiative Toward ASIC Libraries) package that provides two alternative methods (procedures or functions) to model primitive gates (I shall describe functions and procedures in more detail in Section 10.9.2):

package And_Pkg is 
	procedure V_And(a, b : BIT; signal c : out BIT); 
	function V_And(a, b : BIT) return BIT;
package body And_Pkg is 
	procedure V_And(a, b : BIT; signal c : out BIT) is 
		begin c <= a and b; end;
	function V_And(a, b : BIT) return BIT is 
		begin return a and b; end;
end And_Pkg;

The software determines where it stores the design units that we analyze. Suppose the package Add_Pkg_Fn is in library MyLib . Then we need a library clause (before each design unit) and use clause with a selected name to use the package:

library MyLib; -- use MyLib.Add_Pkg.all; -- use all the package
use MyLib.Add_Pkg_Fn.add; -- just function 'add' from the package
entity Lib_1 is port (s : out BIT_VECTOR(3 downto 0) := "0000"); end;
architecture Behave of Lib_1 is begin process
begin s <= add ("0001", "0010", "1000"); wait; end process; end;

The VHDL software dictates how you create the library MyLib from the library work and the actual name and directory location for the physical file or directory on the disk that holds the library. The mechanism to create the links between the file and directory names in the computer world and the library names in the VHDL world depends on the software. There are three common methods:

  • Use a UNIX environment variable (SETENV MyLib ~/MyDirectory/
    , for example).
  • Create a separate file that establishes the links between the filename known to the operating system and the library name known to the VHDL software.
  • Include the links in an initialization file (often with an '.ini' suffix).

1. The code in this section is adapted with permission from IEEE Std 1164-1993, © Copyright IEEE. All rights reserved.

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