1

Lecture 2

Introduction to VHDLAim of HDLs (documentation/verification/synthesis)Four main parts of VHDL description

Library declarationsBuilt-in type: bitThe most popular type: std_logic

Entity (definition of interface)Ports (the most popular: in, out, inout)Parameters (generics)

Architecture (definition of functionality)Declarations (constants, signals, types, functions, components)Body (concurrent statements as pieces of hardware)

ConfigurationTwo types of digital elements (combinational and sequential)Basic concurrent statements

Concurrent assignmentComponent instantiation

SimulationsTestbenchDelayed assignmentsInertial vs transport assignment

Authors: Rafał Kiełbik, Grzegorz Jabłoński

2

Aim of HDLs (documentation/verification/synthesis)

3

About VHDL● VHDL is a hardware description language

– Describes behaviour of electronic system

– Based on this description, the system will be implemented

● Very High Speed Integrated Circuits Hardware Description Language

– Created by order of USA Department of Defense

● VHDL 87● VHDL 93● VHDL 2008

● The first HDL to become the IEEE standard

– IEEE 1076

– IEEE 1164 describes multivalue logic

● Originally developed to document the behavior of the ASICs

● Later used also for simulations

● Today VHDL can be used for synthesis, but not all VHDL constructs are synthesizable

VHDL

SynthesizableVHDL

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Four main parts of VHDL description

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Basic VHDL structures

● LIBRARY DECLARATIONS – contains the list of libraries used in the project

● ENTITY – defines system pins

● ARCHITECTURE – describes how the circuit should behave (circuit functionality)

● CONFIGURATION – selects which components should be used in architecture

LIBRARYdeclarations

ENTITY

ARCHITECTURE

BasicVHDL code

CONFIGURATION

AdvancedVHDL code

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Library declaration

LIBRARY library_name;USE library_name.package_name.package_parts;

● To see the content of the library in the project, two lines of code are needed:– the first containing the library name, and

– the second – the use clause

7

Most frequently used libraries● Three different libraries, are usually needed in the project:

– ieee

– std

– work

–

–

● std and work libraries are included by default, there is no need to do it explicitly

LIBRARY ieee; -- A semi-colon (;) indicates the end of a statement or a declarationUSE ieee.std_logic_1164.all; -- Double dash (--) indicates a comment

LIBRARY std; -- Included by default USE std.standard.all;USE std.textio.all

LIBRARY work; -- Library containing ll the modules already compiledUSE work.all;

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Package STANDARD is provided with compiler

package STANDARD is

-- predefined enumeration types:type BOOLEAN is (FALSE, TRUE);type BIT is ('0', '1');type SEVERITY_LEVEL is (NOTE, WARNING, ERROR, FAILURE);

-- predefined numeric types:type INTEGER is range -2147483647 to 2147483647;

-- predefined array types: type STRING is array (POSITIVE range <>) of CHARACTER; type BIT_VECTOR is array (NATURAL range <>) of BIT;...

Why BIT is not good enough?

‘1’

‘0’

‘?’‘0’

‘1’ ‘?’

Tri-state logic

Error - short circuit

9

std_ulogic from package STD_LOGIC_1164

PACKAGE std_logic_1164 IS

TYPE std_ulogic IS ('U', -- Uninitialized 'X', -- Forcing Unknown '0', -- Forcing 0 '1', -- Forcing 1 'Z', -- High Impedance 'W', -- Weak Unknown 'L', -- Weak 0 'H', -- Weak 1 '-' -- Don't care );

Now every thing is clear!

‘1’

‘0’

‘X’

Error - short circuit

‘0’

‘1’ ‘Z’

Tri-state logic

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From std_ulogic to std_logic

PACKAGE std_logic_1164 IS

... CONSTANT resolution_table : stdlogic_table := ( -- --------------------------------------------------------- -- | U X 0 1 Z W L H - | | -- --------------------------------------------------------- ( 'U', 'U', 'U', 'U', 'U', 'U', 'U', 'U', 'U' ), -- | U | ( 'U', 'X', 'X', 'X', 'X', 'X', 'X', 'X', 'X' ), -- | X | ( 'U', 'X', '0', 'X', '0', '0', '0', '0', 'X' ), -- | 0 | ( 'U', 'X', 'X', '1', '1', '1', '1', '1', 'X' ), -- | 1 | ( 'U', 'X', '0', '1', 'Z', 'W', 'L', 'H', 'X' ), -- | Z | ( 'U', 'X', '0', '1', 'W', 'W', 'W', 'W', 'X' ), -- | W | ( 'U', 'X', '0', '1', 'L', 'W', 'L', 'W', 'X' ), -- | L | ( 'U', 'X', '0', '1', 'H', 'W', 'W', 'H', 'X' ), -- | H | ( 'U', 'X', 'X', 'X', 'X', 'X', 'X', 'X', 'X' ) -- | - | );

FUNCTION resolved ( s : std_ulogic_vector ) RETURN std_ulogic;

SUBTYPE std_logic IS resolved std_ulogic;...

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Resolution function – example 1

‘1’

‘0’

‘X’

Error - short circuit

...'X', -- Forcing Unknown'0', -- Forcing 0'1', -- Forcing 1...

PACKAGE std_logic_1164 IS CONSTANT resolution_table : stdlogic_table := ( -- --------------------------------------------------------- -- | U X 0 1 Z W L H - | | -- --------------------------------------------------------- ( 'U', 'U', 'U', 'U', 'U', 'U', 'U', 'U', 'U' ), -- | U | ( 'U', 'X', 'X', 'X', 'X', 'X', 'X', 'X', 'X' ), -- | X | ( 'U', 'X', '0', 'X', '0', '0', '0', '0', 'X' ), -- | 0 | ( 'U', 'X', 'X', '1', '1', '1', '1', '1', 'X' ), -- | 1 | ( 'U', 'X', '0', '1', 'Z', 'W', 'L', 'H', 'X' ), -- | Z | ( 'U', 'X', '0', '1', 'W', 'W', 'W', 'W', 'X' ), -- | W | ( 'U', 'X', '0', '1', 'L', 'W', 'L', 'W', 'X' ), -- | L | ( 'U', 'X', '0', '1', 'H', 'W', 'W', 'H', 'X' ), -- | H | ( 'U', 'X', 'X', 'X', 'X', 'X', 'X', 'X', 'X' ) -- | - | );

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PACKAGE std_logic_1164 IS CONSTANT resolution_table : stdlogic_table := ( -- --------------------------------------------------------- -- | U X 0 1 Z W L H - | | -- --------------------------------------------------------- ( 'U', 'U', 'U', 'U', 'U', 'U', 'U', 'U', 'U' ), -- | U | ( 'U', 'X', 'X', 'X', 'X', 'X', 'X', 'X', 'X' ), -- | X | ( 'U', 'X', '0', 'X', '0', '0', '0', '0', 'X' ), -- | 0 | ( 'U', 'X', 'X', '1', '1', '1', '1', '1', 'X' ), -- | 1 | ( 'U', 'X', '0', '1', 'Z', 'W', 'L', 'H', 'X' ), -- | Z | ( 'U', 'X', '0', '1', 'W', 'W', 'W', 'W', 'X' ), -- | W | ( 'U', 'X', '0', '1', 'L', 'W', 'L', 'W', 'X' ), -- | L | ( 'U', 'X', '0', '1', 'H', 'W', 'W', 'H', 'X' ), -- | H | ( 'U', 'X', 'X', 'X', 'X', 'X', 'X', 'X', 'X' ) -- | - | );

Resolution function – example 2

R

‘1’

‘L’‘1’

Pull-down resistor

...'X', -- Forcing Unknown'0', -- Forcing 0'1', -- Forcing 1... 'L', -- Weak 0...

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ENTITY● The ENTITY declaration contains the list of input and output pins

of the circuit.

● The following port modes are possible:

– IN

– OUT

– INOUT

– BUFFER

– LINKAGE

ENTITY entity_name IS PORT ( port_name : signal_mode signal_type; port_name : signal_mode signal_type; ...);END entity_name;

INOUT

INOUT

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ENTITY Example

ENTITY nand_gate IS PORT (a, b : IN BIT; x : OUT BIT);END nand_gate;

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ENTITY with GENERIC● The ENTITY can be parametrized:

ENTITY entity_name IS GENERIC (

parameter_name : parameter type;parameter_name : parameter type;

...) PORT ( port_name : signal_mode signal_type; port_name : signal_mode signal_type; ...);END entity_name;

INOUT

INOUT

n

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Parametrized ENTITY Example

ENTITY nand_gate IS GENERIC(width : NATURAL := 8) PORT (a, b : IN BIT_VECTOR(width-1 downto 0); x : OUT BIT_VECTOR(width-1 downto 0));END nand_gate;

88

8

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ARCHITECTURE

● ARCHITECTURE* describes, how the circuit should behave.

ARCHITECTURE architecture_name OF entity_name IS

[declarations]

BEGIN

(code)

END architecture_name;

*VHDL is case INSENSITIVE == vhdl IS CASE insensitive

Practical hint:- use small letters for keywords- use small letters or CamelStyle for names of variables, signals, functions- use capital letters for constants- be consistent

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ARCHITECTURE - declarationsarchitecture architecture_name of entity_name is

-- CONSTANTS, e.g.: constant MASK : std_logic_vector(7 downto 0) := x”FF”;

-- SIGNALS, e.g.: signal internal_sum : integer;

-- TYPES, e.g.: type short_vector is array(3 downto 0) of std_logic;

-- FUNCTIONS / PROCEDURES, e.g.: function convert_vector_to_number(vec: short_vector) return natural is

… end function;

-- COMPONENTS, e.g.: component nand_gate is generic(width : natural := 8) port (a, b : in bit_vector(width-1 downto 0); x : out bit_vector(width-1 downto 0)); end component nand_gate;

begin

(code)

end architecture_name;

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ARCHITECTURE - code

architecture architecture_name of entity_name is

[declarations]

begin

z <= x and y;

S: entity work.adderport map

(in1 => a;in1 => b;result => sum

);

y_sum <= y & sum;

end architecture_name;

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Concurrent statements: pieces of hardware

architecture architecture_name of entity_name is

[declarations]

begin

z <= x and y;

S: entity work.adderport map

(in1 => a;in1 => b;result => sum

);

z_sum <= z & sum;

end architecture_name;

andx

zy

addera

sumb

z_sum

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Concurrent means: “order is not important”

architecture architecture_name of entity_name is

[declarations]

begin

z_sum <= z & sum;

S: entity work.adderport map

(in1 => a;in1 => b;result => sum

);

z <= x and y;

end architecture_name;

andx

zy

addera

sumb

z_sum

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CONFIGURATION - exampleentity my_entity is

port( a : in std_logic; b : out std_logic);

end entity;

-- =================================================architecture my_arch of my_entity is

component submodule isport( a : in std_logic; b : out std_logic);

end component;

begin

L1: submoduleport map (a => a, b => b);

end my_arch;

-- =================================================configuration my_config of my_entity is for my_arch for L1:submodule use entity work.similar_submodule(arch_1) end for; end for;end configuration;

“submodule” vs. “similar_submodule” - the names can be different, also the names of the ports can be different, types must allow mapping

“arch_1” - different architectures of the same entity can be defined and used when required

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Two types of digital elements (combinational and sequential)

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Digital Logic:Combinational vs Sequential

ab

q

clk

clk

a

b

q

a

b

f

a

b

f

asynchronous (“immediate”) changes

clock-synchronized changes

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Combinational delaysab

q

clk

clk

a

b

q

a

b

f

a

b

f

clock-synchronized changes

Dt

asynchronous (“immediate”) changes

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LUT – Look-up Table

00000001

a

b

c

f

LUT

000

001

010

011

100

101

110

111

f

00000001

a

b

c

LUTa b c f

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 1

Truth table Symbol Internal structure

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8x1 Multiplexer Using Pass Gate Transistor Logic

7

6

5

4

3

2

1

0

c b a

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8x1 Multiplexer Using Pass Gate Transistor Logic

7

6

5

4

3

2

1

0

c b a‘0’ ‘1’ ‘0’

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8x1 Multiplexer Using Pass Gate Transistor Logic

7

6

5

4

3

2

1

0

c b a‘0’ ‘1’ ‘0’

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asynchronous (“immediate”) changes

Setup & Hold Time – No Input Changes Allowed

ab

q

clk

clk

a

b

q

a

b

f

a

b

f

clock-synchronized changes

th

ts hold time

setup time

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Latch in CMOS

D

clk = 1 Q = D

D

clk = 0 Q = const

t1

t2

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Flip-Flop in CMOS

t1

t2

D

clk = 0

Q = const

D

clk = 1

Q = const

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Latch vs Flip-Flop in Timings

clk

D

QLatch

QFlip-Flop

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Latch vs Flip-Flop in Timings

clk

D

QLatch

QFlip-Flop

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Basic concurrent statements

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Concurrent Signal Assignmententity my_entity is

port( a : in std_logic; b : out std_logic);end entity;

architecture my_arch of my_entity is signal c : std_logic; signal d, e: std_logic_vector(7 downto 0); signal i, j, k: integer; signal t: boolean;begin

-- redirections b <= a; -- Ports can be used as signals c <= a; a <= c; -- ERROR: cannot write input port c <= b; -- ERROR: cannot read output port c <= d(3);

-- operations(1)

b <= a and c; -- logical, binary (and, or, nand, nor, xor, xnor) b <= not a; -- logical, unary

k <= i + j; -- arithmetical, binary (+, -, *, /, **, mod(2), rem(3)) k <= - j; -- arithmetical, unary (+, -, abs)

t <= (j <= i); -- relational (=, /=, <, <=, >, >=)

e <= d sll 3; -- shifts (sll, srl, sla, sra, rol, ror)

e <= d(4 downto 0) & “000”; -- concatenation

end my_arch;

(2) mod - rounds down, has sign of divisor(3) rem - rounds towards 0, has sign of dividend

(1) http://www.vhdl.renerta.com/mobile/source/vhd00047.htm

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Component Instantiationentity my_entity is

port( a : in std_logic;b : out std_logic);

end entity;

-- =================================================architecture my_arch of my_entity is

component submodule_1 isport( a : in std_logic; b : out std_logic);

end component;

begin

L1: submodule_1port map (a => a, b => b); -- Named mapping

L2: submodule_1port map (a, b); -- Positional mapping (dangerous!)

L3: entity work.submodule_2 -- No component declaration requiredport map (a => a, b => b);

end my_arch;

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Component Instantiationentity my_entity is

port( a : in std_logic;b : out std_logic);

end entity;

-- =================================================architecture my_arch of my_entity is

component submodule_1 isport( a : in std_logic; b : out std_logic);

end component;

begin

L1: submodule_1port map (a => a, b => b); -- Named mapping

L2: submodule_1port map (a, b); -- Positional mapping (dangerous!)

L3: entity work.submodule_2 -- No component declaration requiredport map (a => a, b => b);

end my_arch;

ERROR:XST:528 - Multi-source in Unit <my_entity> on signal <b>; this signal is connected to multiple drivers

ERROR:XST:528 - Multi-source in Unit <my_entity> on signal <b>; this signal is connected to multiple drivers

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Simulations

40

Testing Using Graphical Tools

Specification of inputs

Analysis of waveforms

41

Testbench

Testing in VHDL

UUT

andx zy

addera sumb

z_sum

STIMULI (in VHDL)

x

y

a

b

VERIFICATION (in VHDL)

...wait 17 ns;assert z_sum = “0110”

report “Not Correct”severity ERROR

...

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Testbench Has No Ports

entity my_entity_testbench isend entity;

-- =================================================architecture my_arch_testbench of my_entity_testbench is

signal a : std_logic;signal b : std_logic;

begin

UUT: entity work.my_entity(my_arch) port map (a => a, b => b);

-- STIMULI:a <= ...

-- VERIFICATION:...wait 17 ns;assert b = ‘1’ report “b is not 1” -- report printed when assertion fails severity WARNING -- type SEVERITY_LEVEL is (NOTE, WARNING, ERROR, FAILURE);...

end my_arch_testbench;

-- Unit Under Test (UUT)entity my_entity is

port( a : in std_logic; b : out std_logic);

end entity;

-- =================================================architecture my_arch of my_entity is...end my_arch;

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Concurrent assignments with delays - NOT for SYNTHESIS

d <= ‘1’ after 5 ns, ‘0’ after 17 ns, ‘1’ after 32 ns, ‘Z’ after 51 ns;

clk <= not clk after 5 ns; -- PERIOD: 10 ns

x <= x1 after t1, x2 after t2, x3 after t3, … -- t1 < t2 < t3 ...

package STANDARD is(1)

...type time is range -2147483647 to 2147483647 units fs; ps = 1000 fs; ns = 1000 ps; us = 1000 ns; ms = 1000 us; sec = 1000 ms; min = 60 sec; hr = 60 min; end units; subtype delay_length is time range 0 fs to time'high;

(1) https://www.hdlworks.com/hdl_corner/vhdl_ref/VHDLContents/StandardPackage.htm

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Inertial vs Transport Delay

10 ns

a

20 ns 30 ns 40 ns

b

a <= ‘1’ after 13 ns, ‘0’ after 15 ns;b <= a after 10 ns;

a <= ‘1’ after 13 ns, ‘0’ after 15 ns;b <= transport a after 10 ns;

10 ns

a

20 ns 30 ns 40 ns

b