BASIC VHDL LANGUAGE ELEMENTS Digital Design for Instrumentation with VHDL 1

BASIC VHDL LANGUAGE ELEMENTS 2 1. Comments 2. Identifiers 3. Data Objects 4. Data Types 5. VHDL Operators 6. VDHL Design Entity

BASIC VHDL LANGUAGE ELEMENTS 3 1. Comments 2. Identifiers 3. Data Objects 4. Data Types 5. VHDL Operators 6. VDHL Design Entity

1. Comments 4 Comments are preceded by two consecutive hyphens (--) and are terminated at the end of the line. Example: -- This is a comment

BASIC VHDL LANGUAGE ELEMENTS 5 1. Comments 2. Identifiers 3. Data Objects 4. Data Types 5. VHDL Operators 6. VDHL Design Entity

2.Identifiers 6 VHDL identifier syntax: A sequence of one or more uppercase letters, lowercase letters, digits, and the underscore. Upper and lowercase letters are treated the same (i.e., case insensitive). The first character must be a letter. The last character cannot be the underscore Two underscores cannot be together. Identifier values and numbers: 1. Individual logic signals ‘0’, ‘1’ 2. Multiple logic signal “01110”

BASIC VHDL LANGUAGE ELEMENTS 7 1. Comments 2. Identifiers 3. Data Objects 4. Data Types 5. VHDL Operators 6. VDHL Design Entity

3. Data Objects 8 There are three kinds of data objects: SIGNALs, VARIABLEs, and CONSTANTs. 3.1 SIGNAL Data Objects 3.2 VARIABLE Data Objects 3.3 CONSTANT Data Objects

3.1 SIGNAL Data Objects 9 SIGNAL data objects represent logic signals on a wire in the circuit. SIGNALs are used for communication between components. A signal does not have memory; thus, if the source of the signal is removed, the signal will not have a value. There are three places in which SIGNALs can be declared in VHDL code: 1. ENTITY declaration. 2. Declarative part of ARCHITECTURE. 3. Declarative part of PACKAGE. CONT………..

3.1 SIGNAL Data Objects 10 General form of SIGNAL declaration: SIGNAL signal_name, signal_name,.…….. : type name;

3.2 VARIABLE Data Objects 11 A VARIABLE; unlike SIGNAL; does not represent a signal on a wire in the circuit. VARIABLE data objects are sometimes used to hold results of computation and for index variables in the loops. VARIABLES can be declared only inside the declarative part of PROCESS. General form of VARIABLE declaration: VARIABLE variable_name, variable_name, ……. : type_name;

3.3 CONSTANT Data Objects 12 The CONSTANT data objects must be initialized with a value when declared and this value cannot be changed. CONSTANT can be declared only inside the declarative part of ARCHITECTURE. General form of CONSTANT declaration: CONSTANT constant_name: type_name:=constant value;

Example of Data Objects 13 SIGNAL x: BIT; VARIABLE y: INTEGER; CONSTANT one: STD_LOGIC_VECTOR (3 DOWNTO 0):= "0001";

BASIC VHDL LANGUAGE ELEMENTS Comments 2. Identifiers 3. Data Objects 4. Data Types 5. VHDL Operators 6. VDHL Design Entity

4. Data Types 15 The VHDL data types are: 1. BIT and BIT_VECTOR Data Type 2. STD_LOGIC and STD_LOGIC_VECTOR Data Type 3. SIGNED and UNSIGNED Data Type 4. INTEGER Data Type 5. BOOLEAN Data Type 6. Enumeration Data Type 7. ARRAY Data Type

4.1 BIT and BIT_VECTOR Data Type 16 The BIT and BIT_VECTOR types are predefined in VHDL standards IEEE1076 and IEEE1164, hence no need for LIBRARY statement. Objects of these types can only have the values ‘0’ or ‘1’. The BIT_VECTOR type is simply a vector of type BIT. CONT…………

4.1 BIT and BIT_VECTOR Data Type 17 Example: SIGNAL x: BIT; SIGNAL Y: BIT_VECTOR (5 DOWNTO 0); SIGNAL z: BIT_VECTOR (0 TO 4);. x <= '1'; y <= "000010"; z '0'); -- same as "00000" z(4)Z(3)Z(z)z(1)z(0) Y(5)Y(4)Y(3)Y(2)Y(1)Y(0) Yz CONT…………

18 Notes: 1. The syntax “lower_index TO higher index” is useful for a multi bit signal that is simply an array of bits. 2. The syntax “higher_index DOWNTO lower_index” is useful if the signal represents a binary number. 4.1 BIT and BIT_VECTOR Data Type

4.2 STD_LOGIC and STD_LOGIC_VECTOR Data Type 19 The STD_LOGIC and STD_LOGIC_VECTOR types are not predefined. Therefore, the following two library statements must be included in order to use these types: LIBRARY IEEE; USE IEEE.STD_LOGIC_1164.ALL; CONT…………

20 If objects of type STD_LOGIC_VECTOR are to be used as binary numbers in arithmetic manipulations, then either one of the following two USE statements must also be included: For signed number arithmetic For unsigned number arithmetic 4.2 STD_LOGIC and STD_LOGIC_VECTOR Data Type CONT………… USE IEEE.STD_LOGIC_SIGNED.ALL; USE IEEE.STD_LOGIC_UNSIGNED.ALL;

21 The STD_LOGIC and STD_LOGIC_VECTOR types provide more values than the BIT type for modelling a real circuit more accurately. Objects of these types can have the following values: '0' = normal 0 Useful‘L’ =weak 0 '1' = normal 1 for'H' =weak 1 'Z' =high impedance Logic'U' =uninitialized '_' = don’t-care Circuits ‘X’ = unknown 'W'=weak unknown 4.2 STD_LOGIC and STD_LOGIC_VECTOR Data Type CONT…………

22 Example: 4.2 STD_LOGIC and STD_LOGIC_VECTOR Data Type LIBRARY IEEE; USE IEEE.STD_LOGIC_1164.ALL; SIGNAL x: STD_LOGIC; SIGNAL y: STD_LOGIC_VECTOR (7 DOWNTO 0); x <= 'Z'; y <= " Z"; y '0'); -- same as " "

4.3 SIGNED and UNSIGNED Data Type 23 These types are used for arithmetic operation; they represent an array of STD_LOGIC signals. The purpose of SIGNED and UNSIGNED data types is to allow the user to indicate in the VHDL code what kind of number representation is being used. CONT…………

24 To use these types, the code must include the following statement: The SIGNED is used with 2’s complement representation. 4.3 SIGNED and UNSIGNED Data Type CONT………… LIBRARY IEEE; USE IEEE.STD_LOGIC_ARITH.ALL;

4.4 INTEGER Data Type 25 The predefined INTEGER type defines binary number objects for use with arithmetic operators. By default, an INTEGER signal uses 32 bits to represent a signed number. Integers using fewer bits can also be declared with the RANGE keyword. CONT…………

26 Example: This defines y as 7-bit binary number. 4.4 INTEGER Data Type SIGNAL x: INTEGER; SIGNAL y: INTEGER RANGE –64 to 63;

4.5 BOOLEAN Data Type 27 The predefined BOOLEAN type defines objects having the two values TRUE and FALSE. Example: SIGNAL x: BOOLEAN;

BASIC VHDL LANGUAGE ELEMENTS Comments 2. Identifiers 3. Data Objects 4. Data Types 5. VHDL Operators 6. VDHL Design Entity

5. VHDL Operators Logical Operator 2. Arithmetic Operators 3. Assignment Operators 4. Relational Operators 5. Shift and Rotate Operators

5.1 Logical Operators 30

5.2 Arithmetic Operators 31

5.2 Arithmetic Operators 32 Used with STD_LOGIC_VECTOR, SIGNED, UNSIGNED, INTEGER c <= -a; (c equals to the 2’s complement of a). There are no synthesis restrictions regarding (Addition, Subtraction, and Multiplication). For Division, only power of two dividers is allowed. CONT…….

5.2 Arithmetic Operators 33 For Exponentiation, only static values of base and exponent are accepted. (y MOD x) returns the reminder of y/x. with the signal of x. (y REM x) returns the reminder of y/x with the signal y. (MOD, REM, ABS) operators are generally little or no synthesis support. CONT…….

5.3 Assignment Operators 34 <=Used to assign a value to a SIGNAL := Used to assign Value to VARIABLES, CONSTANTS, or GENERIC => Used to assign values to individual vector elements, or with OTHERS

5.3 Assignment Operators cont…. 35

5.4 Relational Operators 36 Used to compare expressions Result of comparison TRUE or FALSE Compared expressions must be of the same type

Relational Operators cont…..

5.5 Shift and Rotate Operators 38

Shift and Rotate Operators - Example