INTRO TO VHDL Appendix A: page page 831 1

VHDL is an IEEE and ANSI standard. VHDL stands for Very High Speed IC hardware description language. VHDL supports –A wide range of abstraction levels ranging from abstract behavioral descriptions to very precise gate-level descriptions. –Synchronous and asynchronous timing models. –Three different description styles: structural, data-flow, and behavioral. 2

Documentation in VHDL Code –Two characters – denote the beginning of the comments –VHDL ignore any text on the line after – Data Objects –Names, values and numbers, Signal –SINGAL signal_name : type_name; Operators : Precedence Design entity Package Using sub-circuits Current assignments Sequential assignments 3

Three Basic Data objects in VHDL SIGNAL –the most important data objects, which represent the logic signals (wires) in the circuit. CONSTANT VARIABLE 4

A.2 SIGNAL is the most important data object in VHDL A signal must be declared with a type as SIGNAL signal_name : type_name; 10 signal types: BIT, BIT_VECTOR, STD_LOGIC, STD_LOGIC_VECTOR, STD_ULOGIC, SIGNED, UNSIGNED, INTEGER, ENUMERATION, BOOLEAN. 5

BIT and BIT_VECTOR Objects of BIT type can only have the values ‘0’ or ‘1’. An object of BIT_VECTOR type is a linear array of BIT objects. SIGNAL x1 : BIT; SIGNAL C : BIT_VECTOR (1 TO 4); SIGNAL Byte: BIT_VECTOR (7 DOWNTO 0); C <= “1010”; -- results in C(1) = 1, C(2) = 0, C(3) = 1, C(4) = 0 6

STD_LOGIC and STD_LOGIC_VECTOR STD_LOGIC provides more flexibility than the BIT type. To use this type, must include LIBRARY ieee; USE ieee.std_logic_1164.all; Legal values for a STD_LOGIC data objects: 0, 1, Z, - Z – high impedance ‘-’ – ‘don’t care’ 7

A.3 VHDL Operators 8

Table A.1. The VHDL operators.

Figure A.1. The general structure of a VHDL design entity. A.4 VHDL Design entity 10

Figure A simple logic function. f x 3 x 1 x 2 11

Figure VHDL entity declaration for the circuit in Figure ENTITY example1 IS PORT ( x1, x2, x3 : IN BIT ; f : OUT BIT ) ; END example1; /* declare signals In an entity declaration */ A4.1 Entity declaration describes the external view of the entity. 12

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A4.2 Architecture body contains the internal description of the entity. 14

Figure VHDL architecture for the entity in Figure ARCHITECTURE LogicFunc OF example1 IS BEGIN f <= (x1 AND x2) OR (NOT x2 AND x3) ; END LogicFunc ; /* A set of concurrent or sequential statements that represents the behavior of the entity. */ 15

Figure Complete VHDL code for the circuit in Figure

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ENTITY example2 IS PORT(x1, x2, x3, x4 : IN BIT; f, g : OUT BIT); ENDexample2; ARCHITECTURE LogicFunc OF example2 IS BEGIN f <= (x1 AND x2) OR (x2 AND x4); g <= (x1 OR (NOT x3)) AND ((NOT x2) OR x4); END LogicFunc; 18

ENTITY example2 IS PORT(x1, x2, x3, x4 : IN BIT; f, g : OUT BIT); ENDexample2; ARCHITECTURE LogicFunc OF example2 IS BEGIN f <= (x1 AND x2) OR (x2 AND x4); g <= (x1 OR (NOT x3)) AND ((NOT x2) OR x4); END LogicFunc; 19

ENTITY example2 IS PORT(x1, x2, x3, x4 : IN BIT; f, g : OUT BIT); ENDexample2; ARCHITECTURE LogicFunc OF example2 IS BEGIN f <= (x1 AND x2) OR (x2 AND x4); g <= (x1 OR (NOT x3)) AND ((NOT x2) OR x4); END LogicFunc; 20

A.6 Using sub-circuits An entity X, when used in another entity Y, is called component X. - A component is also an entity. 21

22 w1 w2 w3 w4 g h B x1 x2 x3 f A in1 in2 in3 in4 in5 out

ENTITY part3 IS PORT(in1, in2, in3, in4, in5 : IN BIT; out: OUT BIT); END part3; ARCHITECTURE Structure OF part3 IS COMPONENT A PORT(x1, x2, x3 : IN BIT; f : OUT BIT); END COMPONENT; COMPONENT B PORT(w1, w2, w3, w4 : IN BIT; g, h : OUT BIT); END COMPONENT; SINGAL g, h : BIT; BEGIN instB : B PORT MAP (in1, in2, in3, in4, g, h); instA: A PORT MAP (g, in5, h, out); END Structure; 23 Architecture body consists of a set of interconnected components that represents the structure of the entity.