CSET 4650 Field Programmable Logic Devices

CSET 4650 CSET 4650 Field Programmable Logic DevicesField Programmable Logic Devices

Dan SolarekDan SolarekDan SolarekDan Solarek

VHDLVHDLAdditional Details & ExamplesAdditional Details & Examples

2

VHDL: Modeling StylesVHDL: Modeling Styles

DataflowDataflowMost are like Most are like assigningassigning expressions to signals expressions to signals

StructuralStructuralDefine explicit Define explicit componentscomponents and the connections between and the connections between them.them.

BehavioralBehavioralWrite an Write an algorithmalgorithm that describes the circuit’s output that describes the circuit’s output

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VHDL: Dataflow StyleVHDL: Dataflow Style

Dataflow descriptionDataflow descriptionThe detail is less when compared with the structural descriptionThe detail is less when compared with the structural description

““Concurrent” statements include assignment and select statementsConcurrent” statements include assignment and select statements

Concurrency is needed to model the behavior of parallel, Concurrency is needed to model the behavior of parallel, interconnected hardware elementsinterconnected hardware elements

Data dependencies are described, not the components and Data dependencies are described, not the components and connectionsconnections

Includes “Includes “when-else” and “” and “with-select” statements” statements

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VHDL: Structural StyleVHDL: Structural Style

A structural description is just like the schematicA structural description is just like the schematic

Includes concurrent statements Includes concurrent statements A A componentcomponent statement is a concurrent statement statement is a concurrent statement

All interconnections are precisely described

2-to4DCD

V2to4dec

I1

I0

EN

Y0

Y1Y2Y3

5

VHDL: Behavioral StyleVHDL: Behavioral Style

Behavioral descriptionBehavioral descriptionMay not be synthesizable and may lead to a very large May not be synthesizable and may lead to a very large circuitcircuit

Primarily used for simulationPrimarily used for simulation

Normally uses VHDL “Normally uses VHDL “processesprocesses””Each VHDL process executes in parallel with other VHDL Each VHDL process executes in parallel with other VHDL processes and concurrent statementsprocesses and concurrent statementsBut “But “sequentialsequential” statements can be used within a process” statements can be used within a process

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• A process can be given a unique name using an optional LABEL

• This is followed by the keyword process

• The keyword BEGIN is used to indicate the start of the process

• All statements within the process are executed SEQUENTIALLY. Hence, order of statements is important.

• A process must end with the keywords end process.

TESTING: process begin

TEST_VECTOR<=“00”;wait for 10 ns;




end process;

A process is a sequence of instructions, referred to A process is a sequence of instructions, referred to as sequential statements.as sequential statements.

What is a PROCESS?What is a PROCESS?

the keyword process

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Process Sensitivity ListProcess Sensitivity ListList of signals to which List of signals to which the process is sensitive.the process is sensitive.Whenever there is an Whenever there is an event on any of the signals event on any of the signals in the sensitivity list, the in the sensitivity list, the process starts.process starts.Every time the process Every time the process starts, it will run in its starts, it will run in its entirety.entirety.WAIT statements not WAIT statements not allowed in a processes allowed in a processes with a sensitivity list.with a sensitivity list.

label: process(sensitivity list) declaration part begin

statement part end process;

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VHDL ProcessesVHDL Processes

• Processes Describe Sequential BehaviorProcesses Describe Sequential Behavior• Processes in VHDL Are Very Powerful StatementsProcesses in VHDL Are Very Powerful Statements

• Allow to define an arbitrary behavior that may be difficult to Allow to define an arbitrary behavior that may be difficult to represent by a real circuitrepresent by a real circuit

• Not every process can be synthesizedNot every process can be synthesized

• Use Processes with Caution in the Code to Be Use Processes with Caution in the Code to Be SynthesizedSynthesized

• Use Processes Freely in TestbenchesUse Processes Freely in Testbenches

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VHDL Design StylesVHDL Design Styles

Components andinterconnects

structural

VHDL Design Styles

dataflow

Concurrent statements

behavioral

• Registers• State machines• Test benches

Sequential statements

Subset most suitable for synthesis

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VHDL Details: Design FlowVHDL Details: Design Flow

VHDL VHDL compilercompiler analyzes code for syntax errors and analyzes code for syntax errors and checks for compatibility with other moduleschecks for compatibility with other modulesSynthesizerSynthesizer converts VHDL program to a circuit with converts VHDL program to a circuit with components components Place and routePlace and route fits the circuit to a device fits the circuit to a device

Elements of VHDLElements of VHDL

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Logic OperatorsLogic Operators

VHDL provides the following predefined basic logic VHDL provides the following predefined basic logic operators:operators:

Keywordandorxorxnor*

nandnornot

Definitionconjunctioninclusive orexclusive orcomplement exclusive orcomplement conjunctioncomplement inclusive orcomplement

* only predefined in VHDL-93

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Logic Operators (cont.)Logic Operators (cont.)

Predefined operators are all binary except for Predefined operators are all binary except for ‘not’‘not’

Multi-input operators formed from series of binary Multi-input operators formed from series of binary operatorsoperators

NAND-3:NAND-3: A nA nandand B n B nandand C C

Expression evaluation differs from switching algebraExpression evaluation differs from switching algebraand, or, nand, norand, or, nand, nor are ‘short-circuit’ operators are ‘short-circuit’ operators

right operand not evaluated if left operand determines right operand not evaluated if left operand determines resultresult

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Operator PrecedenceOperator Precedence

Unary ‘Unary ‘notnot’ has a higher precedence than any ’ has a higher precedence than any binary operatorbinary operator

ALL binary operators have the SAME precedenceALL binary operators have the SAME precedence

Operators with the same precedence are evaluated Operators with the same precedence are evaluated left-to-rightleft-to-right

Operators in parentheses are evaluated first; Operators in parentheses are evaluated first; innermost to outermost orderinnermost to outermost order

Must be used for proper AND - OR evaluationMust be used for proper AND - OR evaluation

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ConcurrencyConcurrency

Software source code statements execute in page/line Software source code statements execute in page/line order (i.e., sequential order)order (i.e., sequential order)

VHDL concurrent signal assignments execute only VHDL concurrent signal assignments execute only when associated signal change value (i.e., concurrent when associated signal change value (i.e., concurrent order)order)

page/line sequence has nothing to do with executionpage/line sequence has nothing to do with execution

assignments are on a nonprocedural stimulus/ response assignments are on a nonprocedural stimulus/ response basis (event driven)basis (event driven)

signal assignments may trigger other concurrent signal assignments may trigger other concurrent assignmentsassignments

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Concurrent Operation ExampleConcurrent Operation Example

entity XOR2_OP is port (A, B : in std_logic; Z : out std_logic);end XOR2_OP;

architecture AND_OR_CONC of XOR2_OP is signal INT1, INT2: std_logic;begin Z <= INT1 or INT2; INT2 <= not A and B; INT1 <= A and not B;end AND_OR_CONC ;

assignments are dependent upon each other

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Design Units and LibrariesDesign Units and Libraries

VHDL is defined such that more complex pieces VHDL is defined such that more complex pieces are built from simpler piecesare built from simpler pieces

Libraries

Design Units

Statements

Expressions

Objects

Types

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Design Units and Libraries (cont.)Design Units and Libraries (cont.)

Part of a VHDL model that can be independently Part of a VHDL model that can be independently analyzed (error checked) is a design unitanalyzed (error checked) is a design unit

Primary Design UnitsPrimary Design UnitsEntity DeclarationEntity Declaration

Package DeclarationPackage Declaration

Configuration DeclarationConfiguration Declaration

Secondary Design UnitsSecondary Design UnitsArchitectural BodyArchitectural Body

Package BodyPackage Body

Primary units analyzed before secondary unitsPrimary units analyzed before secondary units

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Design Units and Libraries (cont.)Design Units and Libraries (cont.)

Two predefined libraries in VHDLTwo predefined libraries in VHDLSTD - contains predefined VHDL constructs such as types, STD - contains predefined VHDL constructs such as types, objects, etc.objects, etc.

WORK - the working libraryWORK - the working library

Many other libraries may exist as part of development Many other libraries may exist as part of development environmentenvironment

IEEE library - standard types and operators needed for IEEE library - standard types and operators needed for simulation and implementation simulation and implementation

User-defined libraries - designs for reuseUser-defined libraries - designs for reuse

Implementation specific libraries - logic familiesImplementation specific libraries - logic families

Modeling Latches and Flip-FlopsModeling Latches and Flip-Flops

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SR Latch: StructuralSR Latch: StructuralNAND LatchR S Qt+1

0 0 U0 1 11 0 01 1 Qt

R

S

Q

Q

ENTITY latch ISPORT( R, S: IN std_logic; Q, Qbar: OUT std_logic);

END ENTITY latch;

ARCHITECTURE latch_arch OF latch ISBEGIN

Q <= R NAND Qbar;Qbar <= S NAND Q;

END ARCHITECTURE;

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SR Latch: Asynchronous SR Latch: Asynchronous

ARCHITECTURE latch2_arch OF latch ISBEGIN

PROCESS (R, S) BEGINIF R = ‘0’ THEN

Q <= ‘1’; Qbar <= ‘0’;ELSIF S =‘0’ THEN

Q <= ‘0’; Qbar <= ‘1’;END IF;

END PROCESS;END ARCHITECTURE;

NANDR S Qt+1

0 0 U0 1 11 0 01 1 Qt

R

S

Q

Q

SequentialStatements

Sensitivity list of signals:Every time a change of state or event occurs on these signals this process will be called

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SR Flip-Flop: Gated-ClockSR Flip-Flop: Gated-Clock

S

R

Q

Q

LE

ARCHITECTURE Latch_arch OF GC_Latch IS BEGINPROCESS (R, S, LE) BEGIN

IF LE =‘1’ THENIF R = ‘0’ THEN

Q <= ‘1’; Qbar <= ‘0’;ELSIF S =‘0’ THEN

Q <= ‘0’; Qbar <= ‘1’;END IF;

END IF;END PROCESS;

END ARCHITECTURE;

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Data-Flip Flops: SynchronousData-Flip Flops: Synchronous

ARCHITECTURE Dff_arch OF Dff ISBEGIN

PROCESS (Clock) BEGINIF Clock’EVENT AND Clock=‘1’ THEN

Q <= D;END IF;

END PROCESS;

END ARCHITECTURE;Clock’EVENT is what distinguishes a D-FlipFlip from a Latch

Notice the Process does not contain D:PROCESS(Clock, D)

Sensitivity lists contain signals used in conditionals (i.e., IF)

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D-Flip Flops: rising_edgeD-Flip Flops: rising_edge

ARCHITECTURE Dff_arch OF Dff IS BEGINPROCESS (Clock) BEGIN

IF Clock’EVENT AND Clock=‘1’ THENQ <= D;

END IF;END PROCESS;

END ARCHITECTURE;

ARCHITECTURE dff_arch OF dff IS BEGINPROCESS (Clock) BEGIN

IF rising_edge(Clock) THENQ <= D;

END IF;END PROCESS;

END ARCHITECTURE;

Alternate andmore readable way is to use the rising_edge function

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D-Flip Flop: Asynchronous ResetD-Flip Flop: Asynchronous Reset

ARCHITECTURE dff_reset_arch OF dff_reset IS BEGIN

PROCESS (Clock, Reset) BEGIN

IF Reset= ‘1’ THEN -- Asynchronous ResetQ <= ‘0’

ELSIF rising_edge(Clock) THEN --Synchronous Q <= D;

END IF;END PROCESS;

END ARCHITECTURE;

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D-Flip Flops: Synchronous ResetD-Flip Flops: Synchronous Reset

PROCESS (Clock, Reset) BEGINIF rising_edge(Clock) THEN

IF Reset=‘1’ THENQ <= ‘0’

ELSEQ <= D;

END IF;END IF;

END PROCESS;

Synchronous Reset

Synchronous FF

PROCESS (Clock, Reset) BEGINIF Reset=‘1’ THEN

Q <= ‘0’ELSIF rising_edge(Clock) THEN

Q <= D;END IF;

END PROCESS;

Asynchronous Reset

Synchronous FF

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D-Flip Flop: Async. Reset & PresetD-Flip Flop: Async. Reset & Preset

PROCESS (Clock, Reset, Preset) BEGINIF Reset=‘1’ THEN --highest priority

Q <= ‘0’;ELSIF Preset=‘1’ THEN

Q <= ‘0’;ELSIF rising_edge(Clock) THEN

Q <= D;END IF;

END PROCESS;

Vector operations and conditional Vector operations and conditional concurrent signal assignmentsconcurrent signal assignments

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Bit VectorsBit Vectors

Signals can be more than one bit (a vector)Signals can be more than one bit (a vector)Represent Represent P address and data, function selection, etc.P address and data, function selection, etc.

Declaration is similar to single bit signalsDeclaration is similar to single bit signalstype is type is bit_vectorbit_vector or or std_logic_vectorstd_logic_vector

We also must specify vector index range and We also must specify vector index range and directiondirection

big endian: big endian: (low (low toto high) high)

little endian:little endian: (high (high downtodownto low) low)

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VectorVector Declarations Declarations

port (A, B: in std_logic_vector(7 downto 0);Z: out std_logic_vector(1 to 16)

);

A and B:

Z: 1 2 3 4 5 6 7 8 9

7 6 5 4 3 2 1 0

10 11 12 13 14 15 16

Note! The first bit and last bit index numbers define the number of bits in the vector (i.e., max - min + 1)

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Vector LiteralsVector Literals

Single bit binary literals are Single bit binary literals are ‘‘00’’ and and ‘‘11’’

Vector binary literals are Vector binary literals are ““01010101”” , , ““10_0110_01””

literal values may have an underscore embedded literal values may have an underscore embedded to improve readabilityto improve readability

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Vector Logical OperationsVector Logical Operations

Single bit logical operations also apply to Single bit logical operations also apply to vectorsvectors

Operands Operands MUSTMUST be the same size (generally be the same size (generally applies to all vector operations)applies to all vector operations)

Assignment target must also have the same Assignment target must also have the same number of bits as the resultnumber of bits as the result

Operations are applied bitwise to operands to Operations are applied bitwise to operands to produce the vector resultproduce the vector result

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Vector OperationsVector Operations

Given:

Signal A, B, Z: std_logic_vector(7 downto 0);

Then the following logical operation and assignment

Z <= A and B;

Is equivalent to:

for i = 0 to 7Zi = Ai and Bi;

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Vector Arithmetic OperationsVector Arithmetic Operations

Vector arithmetic operations are basically the same as Vector arithmetic operations are basically the same as vector logical operationsvector logical operations

Operands Operands MUSTMUST be the same size be the same size

Assignment target must also have the same number of bits as Assignment target must also have the same number of bits as the resultthe result

Operations are applied bitwise to operands to produce the Operations are applied bitwise to operands to produce the vector resultvector result

The only difference is the carry or borrowThe only difference is the carry or borrowCarry in/out must be specially handledCarry in/out must be specially handled

Result can be 1 bit larger than operands (CO)Result can be 1 bit larger than operands (CO)

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4-bit Adder (Dataflow VHDL)4-bit Adder (Dataflow VHDL)

entity add4 isport (a, b: in std_logic_vector (3 downto 0);

cin: in std_logic; cout: out std_logic;s: out std_logic_vector(3 downto 0)

);end add4;

architecture df of add4 issignal tmpsum std_logic_vector(4 downto 0);

begintmpsum <= (‘0’ & a) + (‘0’ & b) + (“0000” & cin);s <= tmpsum(3 downto 0);cout <= tmpsum(4);

end df;

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Add4 ExampleAdd4 Example

In the previous example note:In the previous example note:The “&” symbol is the concatenation operatorThe “&” symbol is the concatenation operator

joins operands together so that result length is sum of lengths of joins operands together so that result length is sum of lengths of operands.operands.

In order to be able to access the MSB carry out we had to add In order to be able to access the MSB carry out we had to add 5-bit values (used & operator to add leading zeros to 5-bit values (used & operator to add leading zeros to operands)operands)

To assign result to S, we had to access only the least To assign result to S, we had to access only the least significant 4 bits of S; this is a significant 4 bits of S; this is a SLICESLICE

The carry out is a single bit assignment of the LSB of the The carry out is a single bit assignment of the LSB of the resultresult

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Vectors and ConcatenationVectors and Concatenation

signal A: std_logic_vector(3 downto 0);signal B: std_logic_vector(3 downto 0);signal C, D, E: std_logic_vector(7 downto 0);

A <= ”0000”; B <= ”1111”; C <= A & B; -- C = ”00001111”

D <= ‘0’ & ”0001111”; -- D <= ”00001111”

E <= ‘0’ & ‘0’ & ‘0’ & ‘0’ & ‘1’ & ‘1’ & ‘1’ & ‘1’; -- E <= ”00001111”

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Slice Reference & AssignmentSlice Reference & Assignment

A slice is a part of a vectorA slice is a part of a vector

accessed by a range clauseaccessed by a range clause (hi downto lo)(hi downto lo) or or (lo to hi)(lo to hi)

indexes cannot go out of bounds of original declarationindexes cannot go out of bounds of original declaration

range direction must be the same as the original vectorrange direction must be the same as the original vector

a single index is use to access a single bita single index is use to access a single bite.g., e.g., tmpsum(4);tmpsum(4);

assignee must be the same size as the sliceassignee must be the same size as the slicecout <= tmpsum(4);cout <= tmpsum(4);

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Conditional Concurrent AssignmentConditional Concurrent Assignment

Up to now, signal assignment has been only based Up to now, signal assignment has been only based on evaluation of operand changeson evaluation of operand changes

expressions are boolean algebra onlyexpressions are boolean algebra only

hard to understand what is being implementedhard to understand what is being implemented

e.g., a 4-to-1 MUX:

Z <= (a and not s(1) and not s(0)) or (b and not s(1) and s(0)) or (c and s(1) and not s(0)) or (d and s(1) or s(0));

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Conditional Concurrent AssignmentConditional Concurrent Assignment

General Form:

target_signal <= value1 when cond1 else value2 when cond2 else

* valuem when condm else valuen;

Note that the condition clauses must evaluate to a logical expression.

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4-to-1 Mux (Cond. Concurrent Form)4-to-1 Mux (Cond. Concurrent Form)

Z <= A when s = “00” else B when s = “01” else C when s = “10” else D;

Note that in the last case, we did not specify a condition; this is the “when no other condition is met” case.Note also that we can conditionalize the last case by if so, we must ensure that all possible condition combinations are addressed.

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Relational OperatorsRelational Operators

In the previous example we introduced a new In the previous example we introduced a new operator, the relational “equals”operator, the relational “equals”

The relational operators areThe relational operators are

= (equals)= (equals) /= (not equals)/= (not equals)

> (greater than)> (greater than) < (less than)< (less than)

>= (greater or equal)>= (greater or equal) <= (less or equal)<= (less or equal)

Note that <= (less or equal) is same operator as <= Note that <= (less or equal) is same operator as <= (signal assignment); i.e., context dependent(signal assignment); i.e., context dependent

Precedence of relational operators is between “not” and Precedence of relational operators is between “not” and the other logical operators.the other logical operators.

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Selected Signal Assignment Selected Signal Assignment

Another form of concurrent signal assignment is Another form of concurrent signal assignment is the Select assignmentthe Select assignment

Similar to a software CASE statementSimilar to a software CASE statementwe first identify the “discriminator” signal or expression we we first identify the “discriminator” signal or expression we will testwill test

values and associated conditions are then identifiedvalues and associated conditions are then identified

Like conditional signal assignment we must ensure that Like conditional signal assignment we must ensure that all cases of discriminator are coveredall cases of discriminator are covered

““others” condition makes this easyothers” condition makes this easy

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Selected Signal AssignmentSelected Signal Assignment

General Form:

WITH discriminator SELECT target_signal <= value1 WHEN choices1, value2 WHEN choices2,

* valuem WHEN choicesm, valuen WHEN others;

The “choices” are values of the discriminator; either single, multiple or a range.

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All possible values of the discriminator must be coveredAll possible values of the discriminator must be coveredsingle value:single value: when “0001”,when “0001”,

multiple values:multiple values: when “0100” | “0110” | when “0100” | “0110” | “1000”,“1000”,

value range:value range: when“1010” to “1111”,when“1010” to “1111”,

everything else:everything else: when others;when others;

The last case “The last case “when otherswhen others” must be the last clause if ” must be the last clause if usedused

Comma separates clauses, semicolon ends the statementComma separates clauses, semicolon ends the statement

OR

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WITH digit SELECTsegs <= “1110111” when “0000”,

“0010010” when “0001”, “1011101” when “0010”, “1011011” when “0011”, “0111010” when “0100”, “1101011” when “0101”, “0101111” when “0110”, “1010010” when “0111”, “1111111” when “1000”, “1111010” when “1001”, “1101101” when others;

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Vector AttributesVector Attributes

Attributes allow access to signal definition Attributes allow access to signal definition informationinformation

useful when designing generic VHDL useful when designing generic VHDL

tells use range, index, length of a signaltells use range, index, length of a signal

General form isGeneral form issignal_name’attr_name

Some attributes are pre-definedSome attributes are pre-defined

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Pre-defined AttributesPre-defined Attributes

Name‘left‘right‘high‘low‘range‘reverse_range‘length

Definitionindex value on left of rangeindex value on right of rangegreatest index value of rangeleast index value of rangerange expression if signalreversed signal range expressionnumber of bits in range

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Pre-DefinedPre-Defined Attributes Attributes

Attributeex‘leftex‘rightex‘highex‘lowex‘rangeex‘reverse_rangeex‘length

Value118118(11 downto 8)(8 to 11)4

signal ex std_logic_vector(11 downto 8);

Latches, Flip-Flops and RegistersLatches, Flip-Flops and Registers

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Clock D 0 1 1

– 0 1

0 1

Truth Table Graphical Symbol

t 1 t 2 t 3 t 4

Time

Clock

D

Q

Timing Diagram

Q(t+1)

Q(t)

Data LatchData Latch

D Q

Clock

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LIBRARY ieee ; USE ieee.std_logic_1164.all ;

ENTITY latch IS PORT ( D, Clock : IN STD_LOGIC;

Q : OUT STD_LOGIC); END latch;

ARCHITECTURE Behavior OF latch IS BEGIN

PROCESS ( D, Clock ) BEGIN

IF Clock = '1' THEN Q <= D;

END IF; END PROCESS;

END Behavior;

Data LatchData Latch

D Q

Clock

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Clk D

0 1

0 1

Truth Table

t 1 t 2 t 3 t 4

Time

Clock

D

Q

Timing Diagram

Q(t+1)

Q(t)

Data Flip-FlopData Flip-Flop

D Q

Clock

Graphical Symbol

0 – Q(t)1 –

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LIBRARY ieee; USE ieee.std_logic_1164.all;

ENTITY flipflop IS PORT ( D, Clock : IN STD_LOGIC;

Q : OUT STD_LOGIC); END flipflop;

ARCHITECTURE Behavior_1 OF flipflop IS BEGIN

PROCESS ( Clock ) BEGIN

IF Clock'EVENT AND Clock = '1' THEN Q <= D;

END IF; END PROCESS;

END Behavior_1;


D Q

Clock

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ENTITY flipflop IS PORT ( D, Clock : IN STD_LOGIC ;

Q : OUT STD_LOGIC) ; END flipflop ;

ARCHITECTURE Behavior_2 OF flipflop IS BEGIN

PROCESSBEGIN

WAIT UNTIL Clock'EVENT AND Clock = '1' ; Q <= D ;

END PROCESS ; END Behavior_2 ;


D Q

Clock

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ENTITY flipflop IS PORT ( D, Resetn, Clock : IN STD_LOGIC ;

Q : OUT STD_LOGIC) ; END flipflop ;

ARCHITECTURE Behavior OF flipflop IS BEGIN

PROCESS ( Resetn, Clock ) BEGIN

IF Resetn = '0' THEN Q <= '0' ;

ELSIF Clock'EVENT AND Clock = '1' THEN Q <= D ;

END IF ; END PROCESS ;

END Behavior ;

Data Flip-Flop: Asynchronous ResetData Flip-Flop: Asynchronous Reset

D Q

Clock

Resetn

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LIBRARY ieee ; USE ieee.std_logic_1164.all ; ENTITY flipflop IS

PORT ( D, Resetn, Clock : IN STD_LOGIC; Q : OUT STD_LOGIC);

END flipflop ;

ARCHITECTURE Behavior OF flipflop IS BEGIN

PROCESS BEGIN

WAIT UNTIL Clock'EVENT AND Clock = '1' ; IF Resetn = '0' THEN

Q <= '0' ; ELSE

Q <= D ; END IF ;

END PROCESS ;END Behavior ;

Data Flip-Flop: Synchronous ResetData Flip-Flop: Synchronous Reset

D Q

Clock

Resetn

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8-Bit Register: Asynchronous Reset8-Bit Register: Asynchronous ResetLIBRARY ieee ;USE ieee.std_logic_1164.all ;

ENTITY reg8 ISPORT ( D : IN STD_LOGIC_VECTOR(7 DOWNTO 0) ;

Resetn, Clock : IN STD_LOGIC ;Q : OUT STD_LOGIC_VECTOR(7 DOWNTO 0) ) ;

END reg8 ;

ARCHITECTURE Behavior OF reg8 ISBEGIN

PROCESS ( Resetn, Clock )BEGIN

IF Resetn = '0' THENQ <= "00000000" ;

ELSIF Clock'EVENT AND Clock = '1' THENQ <= D ;

END IF ;END PROCESS ;

END Behavior ;`

Resetn

Clock

reg8

8 8

D Q

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N-Bit Register: Asynchronous ResetN-Bit Register: Asynchronous ResetLIBRARY ieee ;USE ieee.std_logic_1164.all ;

ENTITY regn ISGENERIC ( N : INTEGER := 16 ) ;PORT ( D : IN STD_LOGIC_VECTOR(N-1 DOWNTO 0) ;

Resetn, Clock : IN STD_LOGIC ;Q : OUT STD_LOGIC_VECTOR(N-1 DOWNTO 0) ) ;

END regn ;

ARCHITECTURE Behavior OF regn ISBEGIN

PROCESS ( Resetn, Clock )BEGIN

IF Resetn = '0' THENQ <= (OTHERS => '0') ;

ELSIF Clock'EVENT AND Clock = '1' THENQ <= D ;

END IF ;END PROCESS ;

END Behavior ;

Resetn

Clock

regn

N N

D Q

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LIBRARY ieee ;USE ieee.std_logic_1164.all ;

ENTITY regn ISGENERIC ( N : INTEGER := 8 ) ;PORT ( D : IN STD_LOGIC_VECTOR(N-1 DOWNTO 0) ;

Enable, Clock : IN STD_LOGIC ;Q : OUT STD_LOGIC_VECTOR(N-1 DOWNTO 0) ) ;

END regn ;

ARCHITECTURE Behavior OF regn ISBEGIN

PROCESS (Clock)BEGIN

IF (Clock'EVENT AND Clock = '1' ) THENIF Enable = '1' THEN

Q <= D ;END IF ;

END IF;END PROCESS ;

END Behavior ;

N-Bit Register With EnableN-Bit Register With Enable

QD

Enable

Clock

regn

N N

CSET 4650 Field Programmable Logic Devices

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