Basic Language Elements

Basic Language Elements  Identifiers : An identifier in VHDL is composed of a

sequence of one or more characters .  A legal character is  an upper-case letter (A... Z)  a lower-case letter (a. .. Z)  a digit (0 . . . 9)  the underscore ( _ ) character

 The first character in an identifier must be a letter

and the last character may not be an underscore  Examples of identifiers are  DRIVE_BUS

SelectSignal SET_CK_HIGH CONST32_59

Data Objects  A data object holds a value of a specified type.  Every data object belongs to one of the following

three classes:

 1. Constant An object of constant class can hold a

single value of a given type. This value is assigned to the object before simulation starts and the value cannot be changed during the course of the simulation.  2. Variable: An object of variable class can also hold a single value of a given type. However in this case, different values can be assigned to the object at different times using a variable assignment statement.  3. Signal: An object belonging to the signal class has a past history of values, a current value, and a set of future values. Future values can be assigned to a signal object using a signal assignment statement.

Continued.. Signal objects are typically used to model wires

and flip-flops while variable and constant objects are typically used to model the behavior of the circuits Examples of constant declarations are constant RISE_TIME: TIME := 10ns; constant BUS_WIDTH: INTEGER := 8; Examples of variable declarations are 0);

Variable CTRL_STATUS: BIT_VECTOR(10 downto variable SUM: INTEGER range 0 to 100 := 10; variable FOUND, DONE: BOOLEAN;

Continued.. Examples of signal declarations.

signal CLOCK: BIT; signal DATA_BUS: BIT_VECTOR(0 to 7); signal GATE_DELAY: TIME := 10 ns;

Data Types  All the possible types hat can exist in the language

can be categorized into the following four major categories:

 1. Scalar types: Values belonging to these types

appear in a sequential.  2. Composite types: These are composed of elements of a single type (an array type) or elements of different types (a record type).  3. Access types: These provide access to objects of a given type (via pointers).  4. File types: These provides access to objects that contain a sequence of values of a given type. It is possible to derive restricted types, called subtypes, from other predefined or user-defined types.

VHDL Data Types Diagram. Types

File

Composit e

Access

Array Scalar

Physica l

Enumerat ed Real

Intege r

Recor d

Continued.. There are four different kinds of scalar types.

These types are:  1. enumeration  2. integer  3. physical  4. floating point

Every value belonging to an enumeration type

integer type, or a physical type has a position number associated with it. This number is the position of the value in the ordered list of values belonging to that type.

Continued.. Enumeration

Types: An enumeration type declaration defines a type that has a set of user-defined values consisting of identifiers and character literals. Examples are Type MVL is ('U','0','1','Z); type MICRO_OP is (LOAD, STORE, ADD, SUB, MUL, DIV); subtype ARITH_OP is MICRO_OP range ADD to DIV; Examples of objects defined for these types are signal CONTROL_A: MVL; signal CLOCK: MVL range '0' to '1'; -- Implicit subtype declaration. variable IC: MICRO_OP := STORE; -- STORE is the initial value for IC. variable ALU: ARITH_OP;

Continued..  Integer

Types: whose set of values fall within a specified integer range. Examples of integer type declarations are: type INDEX is range 0 to 15; type WORD_LENGTH is range 31 downto 0; subtype DATA_WORD is WORD_LENGTH range 15 downto 0; type MY_WORD is range 4 to 6; Some object declarations using these types are constant MUX_ADDRESS: INDEX := 5; signal DATA_BUS: DATA_WORD;

 INTEGER is the only predefined integer type of the

language. The range of the INTEGER type is implementation dependent but must at least cover the range -(2^ 31 - 1) to +(2^31 - 1).

Continued..  Floating Point Types: floating point type has a set of values

in a given range of real numbers. Examples of floating point type declarations are type TTL_VOLTAGE is range -5.5 to -1.4; type REAL_DATA is range 0.0 to 31.9; An example of an object declaration is variable LENGTH: REAL_DATA range 0.0 to 15.9; variable LI, L2, L3: REAL_DATA range 0.0 to 15.9;  The minimum range of real numbers is also specified by the Standard package in the Standard Library, and is from 1.0E38 to 1.0E38.

Continued..  Physical

Types: physical type contains values that represent measurement of some physical quantity, like time, length, voltage, and current. Values of this type are expressed as integer multiples of a base unit. An example of a physical type declaration is type CURRENT is range 0 to 1 E9 units nA; -- (base unit) nano-ampere uA = 1000 nA; -- micro-ampere mA = 1000 μA; --milli-ampere Amp = 1000 mA; -- ampere end units; subtype FILTER_CURRENT is CURRENT range 10 μA to 5 mA;  The only predefined physical type is TIME and its range of base values, which again is implementation dependent, must at least be -

Composite Types  A composite type represents a collection of values. There are two composite

types:

 An array type: represents a collection of values all belonging to a single type  A record type: represents a collection of values that may belong to same or

different types.

 Array Types

Examples of array type declarations are type type type type

ADDRESS_WORD is array (0 to 63) of BIT; DATA_WORD is array (7 downto 0) of MVL; ROM is array (0 to 125) of DATA_WORD; DECODE_MATRIX is array (POSITIVE range 15 downto 1, NATURAL range 3 downto 0) of MVL; --POSITIVE and NATURAL are predefined subtypes; these are: subtype NATURAL is INTEGER range 0 to INTEGER'HIGH; subtype POSITIVE is INTEGER range 0 to INTEGER'HIGH;

Continued.. Examples of object declarations using these types are variable ROM_ ADDR: ROM; signal ADDRESS.BUS: ADDRESS_WORD; constant DECODER: DECODE_MATRIX; variable DECODE_VALUE: DECODE_MATRIX;  The language also allows array types to be unconstrained  The number of elements in the array is not specified in the type

declaration.  The object declaration for that type declares the number of element of the array.  A subtype declaration may also specife index constraint for an unconstrained array type.

Examples of unconstrained array declarations are type STACK_TYPE is array (INEGER range <>) of ADDRESS_WORD; subtype STACK is STACK_TYPE(0 to 63); type OP_TYPE is (ADD, SUB, MUL, DIV); type TIMING is array (OP_TYPE range<>, OP_TYPE range <>) of TIME;

Examples of object declarations using these types are variable FAST_STK: STACK_TYPE(-127 to 127); constant ALU_TIMING: TIMING := --ADD, SUB, MUL ((10 ns, 20 ns, 45 ns), -- ADD (20 ns, 15 ns, 40 ns), -- SUB (45 ns, 40 ns, 30 ns)); -- MUL

There

are two predefined one-dimensional unconstrained array types in the language  STRING: an array of characters.  BIT_VECTOR: an array of bits.