Data types defined in the standard package

Transcription

1 Data Types Each data object has a type associated with it. The type defines the set of values that the object can have and the set of operation that are allowed on it.

2 Data types defined in the standard package Predefined data type are as follows: Bit Bit_vector Boolean Character File_open_kind* File_open_status* Integer Natural Positive Real Severity_level String Time*

3 User-defined Types The Syntax is TYPE identifier is Type_definition; Example: type small_int is range 0 to 1024; type my_word_length is range 31 downto 0; subtype data_word is my_word_length range 7 downto 0;

4 Data Types A type declaration defines the name of the type and the range of the type. Type declaration are allowed in package declaration sections, entity declaration sections, architecture declaration sections, subprogram declaration section and process declaration sections.

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6 Types available in VHDL Scalar types Integer types Real types Enumerated Physical Composite Types Arrays type Records Access Types (equivalent of pointers in C) File types

7 Scalar Types Scalar types describe objects that can hold, at most, one value at a time. Integer Type It is same as mathematical integers. All of the normal predefined mathematical function apply to integer types. Minimum Range: -2,147,483,647 to 12,147,483,647

8 Example - Integer Architecture test of test is Begin Process (x) Variable a: integer; Begin a := 1; --ok a := -1 ; -- ok a := 1.0; -- error End process; End test;

9 Real Types Real Types are used to declare objects that emulate mathematical real numbers. Real number can be used to represent numbers out of the range of integer value as well as fractional values. Minimum Range: -1.0E+38 to +1.0E+38

10 Example Real Types Architecture test of test is signal a : real; Begin a <= 1.0; -- ok a <= 1; -- error a <= -1.0E10; -- ok a <= 1.5E-20; --ok a <= 5.3 ns; -- error End test;

11 Enumerated types Enumerated type is used to represent exactly the values required for a specific operation. All of the values of an enumerated type are user-defined. This values can be identifiers or single-character literals. A typical example: TYPE fourval IS ( x, 0, 1, z ); TYPE color IS (red,yellow, blue,green,orange);

12 IEEE standard 1164 Type std_logic is ( U, -- uninititalized X, -- forcing unknown 0, -- forcing 0 1, -- forcing 1 Z, --high impedence W, --weak unknown L, --weak 0 H, -- weak 1 - ); -- don t care

13 Physical Types Physical types are used to represent physical quantities such as distance,current, time and so on. A physical type provides for a base unit,and successive units are then defined in terms of this unit. The smallest unit represented is one base unit. The largest unit is determined by the range specified in physical type declaration. TIME is a predefined physical types

14 Example Physical Type Package example is type current is range 0 to units na; ua = 1000 na; -- nano amp ma = 1000 ua; -- micro amp a = 1000 ma; End units; type load_factor is (small, med, big); End example;

15 Architecture delay-cal of delay is Begin delay <= 10 ns when (load = small) else 20 ns when (load = med) else 30 ns when (load = big) else 40 ns; out-current <= 1000 ua when (load= small) else 1 ma when (load = med) else 100 ua; End delay-cal;

16 Predefined physical type - Time Type TIME is Range <implementation defined> units fs; --femtosecond ps = 1000 fs; -- picosecond ns = 1000 ps; -- nansecond us = 1000 ns; -- microsecond ms = 1000 us; -- millisecond sec = 1000 ms; -- second min = 60 sec;-- minute hr = 60 min; -- hour End units;

17 Composite Types Arrays Type are groups of elements of the same type. Arrays are useful for modeling linear structure such as RAMs and ROMs. Record Types allow the grouping of elements of different types. Record are useful for modeling data packets, instructions and so on.

18 Array Types Array types group one or more elements of the same type together as a single object Each element of the array can be accessed by one or more array indices. Format of type declaration: TYPE data_bus IS array (0 to 31) of BIT;

19 Array slices and Ranges Assignment of arrays preserves the left-toright ordering regardless of the direction. Example Signal x : std_logic_vector(0 to 3);--case 1 Signal y : std_logic_vector(3 downto 0); --case2 X <= 1010 ; Y <= 0101 ;

20 Cont.. The assignment x<=y is equivalent to: X(0) <= Y(3); -- the left element X(1) <= y(2); X(2) <=Y(1); X(3) <=Y(0); -- the right elements The array range for case I in 0 to 3 loop, the elements of the array would be accessed from left to right. The array range for caseii in 3 downto 0 loop, the elements of the array would be accessed from right to left.

21 Multidimensional Array Type aggregate is array (range<>) of std_logic; Type identifier_a is array(range<>) of aggregate; Constant identifier_b : identifier_a := ((e1,e2 en), -- E1 (e1,e2, en), --E2. (e1,e2, en)); --En

22 Example-Multidimensional arrays Library ieee; Use ieee.std_logic_1164.all; Package memory is constant width : integer := 3; constant memsize : integer := 7; Type data_out is array (0 to width) of std_logic; Type mem_data is arrary (0 to memsize) of data_out; End memory;

23 Library ieee; Use ieee.std_logic_1164.all; Use work.memory.all; Enity rom is -- ROM declaration port (addr : in integer; data : out data_out; cs : in std_logic; End rom;

24 Architecture basic of rom is constant z_state : data_out := ( z, z, z, z ); constant x_state : data_out := ( x, x, x, x ); constant rom_data : mem_data := ( ( 0, 0, 0, 0 ), ( 0, 0, 0, 1 ), ( 0, 0, 1, 0 ), ( 0, 0, 1, 1 ), ( 0, 1, 0, 0 ), ( 0, 1, 0, 1 ), ( 0, 1, 1, 0 ), ( 0, 1, 1, 1 ), ); Begin data <= rom_data (addr) when cs = 1 else x_state when cs = 0 else z_state; End basic;

25 Record Types Record types group object of may types together as a single object. Each element of the record can be accessed by its field name. Record elements can include elements of any type, including arrays and records. The elements of a record can be of the same type or different types.

26 Features of Records A record is referenced by a single identifier. Records are very useful to collapse multiple related elements into a single type so that record objects can be handle as a objects. They are very useful in Bus Functional Models and test benches to pass a set of information to another process or another component. Values can be assigned to a record type using aggregates in a manner similar to aggregates for arrays.

27 Limitation Many synthesizers can compile records provided the elements of the record are of type Bit, bit_vector, boolean, std_ulogic, Std_ulogic_vector, integer or subtypes of these types. However, most current synthesizers do not accept record aggregate assignments, and thus require individual assignment of the record elements.

28 Example - Records Type optype is (add,sub,mpy,div,jmp); Type instruction is Record -- declaration of record opcode : optype; -- field of record src : integer; dat : integer; End record;

29 Process (x) variable inst : instruction; variable source,dat : integer; variable operator : optype; Begin source := inst.src; -- ok dest := inst.src; -- ok source := inst.opcode; -- error operator := inst.opcode; --ok inst.src := dest; --ok

30 Access Types Access types allow the designer to model objects of a dynamic nature. E.g. Queues, FIFO, Creating and Maintaining of a linked list. Only variables can be declared as access types. Used in Sequential processing Not Synthesizeable statement.

31 Cont.. Access types are used to declare values that access dynamically allocated variables. Dynamically allocated variables are referenced, not by name, but by an access value that acts like a pointer to the variable. Access types are used in the TextIO subprograms to read and write text strings. Access types were efficiently used in VHDL in the functional modeling of large memories where fixed memory allocation is not realistic.

32 File Types A file type allows declaration of objects that have a type FILE. A file object type is actually a subset of the variable object type. A variable object can be assigned with a variable assignment, while a file object cannot be assigned. A file object can be read from, written to, and checked for end of file only with special procedure and functions.

33 Cont.. File types are typically used to access files in the host environment. File types are used to define objects representing files in the host system environment. The value of a file object is the sequence of values contained in the host environment. Package TextIO provides for the definition of file type TEXT as type TEXT is file of string. TextIO package supports human readable IO.

34 Characters, Strings To use character literal in a VHDL Code, one puts it in a single quotation mark. a, B,, A string of character are placed in double quotation marks: aa, bb Any printing character can be included inside a string.

35 Bit Strings A bit-strings represents a sequence of bit values. E.g. Binary : B 1100_1001, b Hexagonal : X C9, X 4b Octal : O 311, o 113