Introduction to VHDL (Continued)

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Introduction to VHDL (Continued)

• EE19D

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• Basic elements of a VHDL Model

Package Declaration
ENTITY (interface description)
ARCHITECTURE (functionality)
PACKAGE BODY (often used functions, constants,
components, .)
CONFIGURATION (connection entity ?? architecture)
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• Two concepts are often used in modeling digital
  circuits with VHDL
• The external view reflected in the entity
  declaration which represents an interface
  description. The important part of this interface
  description consists of signals over which
  different modules communicate with one another.
• The internal view is described in the
  architecture body. The architecture can be
  expressed according to two major approaches
• structural description which serves as a base for
  the hierarchical design,
• behavioral description (algorithm, sequential and
  concurrent).
• Being able to investigate different architectural
  alternatives permits the development of systems
  to be done in an efficient top-down manner.
• If the architecture body consists of a structural
  description, the binding of architectures and
  entities of the instantiated submodules, the
  so-called components is done using configuration
  statements.
• The package contains declarations of frequently
  used data types, components, functions, etc. It
  consists of a package declaration and a package
  body.

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• Entity declaration

This correspond to the information given by the
symbols in traditional methods based on drawing
schematics
Signals which are used for communication with the
surrounding modules are called ports.
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• Example of entity

• Entity fulladder
• -- (after a double minus sign (-) the rest of the
  line is treated as a comment)
• --
• -- Interface description of FULLADDER
• Port (A, B, Cin in bit
• Sum, Carry out bit)
• End fulladder
• This module has five ports. A port is used for
  interface purpose. It is characterized by its
  direction (mode) and the type of data it carries.
• We can identify three different modes in (read
  only), out (write only), and buffer (read and
  write)
• The type can be a bit, bit-vector, integer,
  etc
• Syntax entity entity_name is
• generics
• ports
• declarations (types, constants, signals)
• definitions (functions, procedures)
• begin -- normally not used
• statements
• End entity_name

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Architecture

• The internal body of digital system is described
  by its architecture.
• Syntax architecture architecture_name of
  entity_name is
• arch_declarative_part
• begin
• arch_statement_part
• end architecture_name
• Models of description
• Structural description (connection of different
  components)
• Behavioral description (algorithmic or testbench,
  concurrent, and sequential)

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Fig. Hierarchical Circuit Design
All the modeling styles share the same
organization of the architecture. Syntax
architecture architecture_name of
entity_name arch_declarative_part begin archite
cture_part end architecture_name
keywords
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Architecture Concurrent Behavioral Description

• This kind of description specifies a dataflow
  through the entity based on concurrent signal
  assignment statements.
• Example 1
• architecture Concurrent of fulladder is
• begin
• sum lt A xor B xor Cin after 5 ns
• Carry lt (A and B) or (B and Cin) or (A and
  Cin) after 3 ns
• end concurrent
• The symbol lt indicates the signal assignment.
• A concurrent signal assignment is executed
  whenever the value of a signal in the expression
  on the right side changes.
• Some concurrent statements are listed below
• a) concurrent signal assignment
• Syntax label
• signal_name lt transport expression
  after time_expr ,
• expression after time_expr
• The keyword transport affects the handling of
  multiple signal events coming in short time one
  after another.
• b) conditional signal assignment statement
  different assignments are possible to one target
  signal. The choice of a particular assignment is
  done using if elsif else structure.
• - Syntax label
• signal_name lt expression when condition else
• expression when condition else

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• c) selected signal assignment
• - Syntax label
• with select-expression select
• signal_name lt expression when value ,
• expression when value
• d) assertion statement This statement serves to
  generate warning or error message during
  simulation after testing a certain condition.
• - Syntax assert_label
• Assert condition
• report string_expr
• severity failureerrorwarningnote
• If the test condition results in false the the
  message string_expr is displayed. Different
  severity levels of the generated message provide
  control over the VHDL simulator.
• e) process statement it defines a region of code
  within all statements are executed sequentially.
  Every process statement as a whole is treated as
  a concurrent statement which is executed in
  parallel with all other concurrent statements.

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Architecture Sequential Behavioral Description

• Sequential behavioral descriptions are based on
  processes.
• A process is constantly switching between the two
  states the execution phase in which the process
  is active and the statements within this process
  is executed and the suspended state.
• A process becomes active by an event on at least
  one signal belonging to the sensitivity list.
• Syntax proc_label
• process (sensitivity list)
• process_declarativ_part
• begin
• sequential-statement_part
• end process proce_label
• With wait statements (the process is executed
  until it reaches a wait statement)
• Syntax proc_label
• Process
• proc_declaratiV_part
• Begin
• seqential_statements
• Wait -- at least one wait statement
• sequential_statements

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Example architecture SEQUENTIAL of FULLADDER is
begin   process (A, B, C)     variable TEMP
integer     variable SUM_CODE bit_vector(0 to
3) "0101"     variable CARRY_CODE
bit_vector(0 to 3) "0011"   begin     if A
'1' then TEMP 1                 else TEMP
0     end if     if B '1' then TEMP
TEMP 1     end if     if C '1' then TEMP
TEMP 1     end if    -- variable TEMP now
holds the number of ones     SUM lt
SUM_CODE(TEMP)     CARRY lt CARRY_CODE(TEMP)
  end process end SEQUENTIAL
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Example architecture SEQUENTIAL of DFF is
begin   process (CLK, NR)   begin     if (NR
'0') then       -- Reset assigning "000...00"
to the       -- parameterized output signal Q
      Q lt (others gt '0')     elsif
(CLK'event and CLK '1') then       Q lt D
    end if   end process end SEQUENTIAL
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STRUCTURAL DESCRIPTION Case of the fulladder
1-bit Full Adder
OR
Half Adder
(2)
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Sum
Cin
S
I1
I2
C2
B
C
X
S1
I1
S
o
Carry
A
C1
C
Y
I2
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Design
Library
Structural Model
Half Adder Model
Or Model
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Strutural description 1 use of components --
Declarations use work.all architecture STRUCTUAL
of fulladder is signal S1, C1, C2
BIT component HA port (I1, I2 in bit S, C
out bit) end component component Ora port
(X, Y in bit O out bit) end component
-- component instantiations begin INST_HA1
HA port map(I1gtA, I2gtB, SgtS1,
CgtC1) INST_HA2 HA port map(I1gtCin,
I2gtS1, SgtSum, Cgt C2) INST_OR ORa port
map(XgtC1, YgtC2, 0gtCarry) end STRUCTURAL
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Strutural description 1 use of entity --
Declarations use work.all architecture STRUCTUAL
of fulladder is signal S1, C1, C2 BIT
0 -- pointer to library models for all HA use
entity HA(BEHAVIOR) for all ORa use entity ORa
(BEHAVIOR) -- component instantiations begin C1
HA port map(I1gtA, I2gtB, SgtS1,
CgtC1) C2 HA port map(I1gtCin, I2gtS1,
SgtSum, Cgt C2) C3 ORa port map(XgtC1,
YgtC2, 0gtCarry) end STRUCTURAL
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Design Library Components entity HA is port(I1,
I2 in BIT S, C out BIT) end
HA architecture BEHAVIOR of HA is Begin S lt
I1 exor I2 C lt I1 and I2 end BEHAVIOR entity
ORa is port(X, Y in BIT O out BIT) end
ORa architecture BEHAVIOR of ORa is begin O lt
X or Y end BEHAVIOR
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Configuration

• The concept of configuration in VHDL allows an
  entity to have multiple associated architectures.
• The role of the configuration is to define a
  unique system description from the various design
  units.
• Configuration Specifications - specify the
  bindings between a component instance in a
  structural architecture and a library model.
• Configuration specifications can be in the
  structural architecture itself or in a
  configuration declaration.
• Configuration Declaration (Body) - A separate
  analyzable entity which holds all the component
  bindings for a structural architecture.

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Model Bindings
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Steps in VHDL Modeling
A Design Entity
Interface Description
Arch 1
Arch 2
Arch 3
ONES COUNTER
1
1
0
A
C
0
1
This circuit counts the number of 1s in an input
vector of length 3
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entity ONES_CNT is port (A in BIT_VECTOR(2
downto 0) C out BIT_VECTOR(1
downto 0)) ------ Truth Table -----------------
------------------ ---A2 A1 A0 C1
C0 -------------------------------- -- 0 0
0 0 0 -- 0 0 1
0 1 -- 0 1 0 0 1 --
0 1 1 1 0 -- 1 0 0
0 1 -- 1 0 1 1
0 -- 1 1 0 1 0 -- 1
1 1 1 1 -----------------------
------------ end ONES_CNT
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architecture ALGORITHMIC of ONES_CNT is begin
process(A) variable NUM INTEGER range 0 to
3 begin NUM 0 for I in 0 to 2 loop
if A(I) '1' then NUM NUM 1
end if end loop case NUM is
when 0 gt C lt "00" when 1 gt C lt
"01" when 2 gt C lt "10" when 3
gt C lt "11" end case end process end
ALGORITHMIC
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Kmap Design
Truth Table -----------------------------------
A2 A1 A0 C1 C0 -----------------
--------------- 0 0 0 0 0
0 0 1 0 1 0 1 0
0 1 0 1 1 1 0
1 0 0 0 1 1 0 1
1 0 1 1 0 1 0
1 1 1 1 1
-----------------------------------
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K - Maps for the Ones Counter
A1 A0 A2 00 01 11 10 0
1 1 1 1 1
C1
A1 A0 A2 00 01 11 10 0
1 1 1 1 1
C0
C1 A1A0 A2A0 A2A1 C0 A2A1A0 A2A1A0
A2A1A0 A2A1A0
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architecture DATA_FLOW of ONES_CNT is begin
C(1) lt (A(1) and A(0)) or (A(2) and A(0))
or (A(2) and A(1)) C(0) lt (A(2) and not
A(1) and not A(0)) or (not A(2) and not
A(1)and A(0)) or (A(2) and A(1) and
A(0)) or (not A(2) and A(1) and not
A(0)) end DATA_FLOW
1
1
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architecture MACRO of ONES_CNT is begin C(1) lt
MAJ3(A) C(0) lt OPAR3(A) end MACRO
Must be previously declared
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Structural Decomposition For Ones Counter
Structural design hierarchy for the ones counter.
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Design
Library
Structural Model
AND2 MODEL
OR2 MODEL
AND3 MODEL
OR4 MODEL
INV MODEL
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structural model
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AND2 Description
entity AND2 is port (I1,I2 in BIT O out
BIT) end AND2 architecture BEHAVIOR of AND2
is begin O lt I1 and I2 end BEHAVIOR

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OR3 Description
entity OR3 is port (I1,I2,I3in BIT O out
BIT) end OR3 architecture BEHAVIOR of OR3
is begin O lt I1 or I2 or I3 end BEHAVIOR

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A properly labeled schematic
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entity MAJ3 is port (X in BIT_VECTOR(2 downto
0) Z out BIT) end MAJ3 architecture AND_OR of
MAJ3 is component AND2 -- unbound port
(I1,I2 in BIT O out BIT) end component
component OR3 -- unbound port (I1,I2,I3 in
BIT O out BIT) end component signal
A1,A2,A3 BIT begin G1 AND2 port map
(X(0),X(1),A1) G2 AND2 port map
(X(0),X(2),A2) G3 AND2 port map
(X(1),X(2),A3) G4 OR3 port map
(A1,A2,A3,Z) end AND_OR
1
1
MAJ3 Structural Model (Unbound)
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Configuration case of behavioral descriptions

• The only information which the configuration has
  to include is the choice of one architecture for
  the given entity.
• Syntax configuration configuration_name of
  entity_name is
• for architecture_name
• End for
• End configuration_name
• Example confiCFG_ONE of fulladder is
• For CONCURRENT
• End for
• End CFG_ONE
• Configuration CFG_TWO of fulladder is
• For SEQUENTIAL
• End for
• End CFG_ONE

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Configuration case of structural descriptions
If the configuration binds a structural
description to an entity then further information
about the instantiated components is required.
Due to the fact that the name of a component in
the component declaration needs not be the same
as the entity name of the instantiated component,
their binding must be done by the configuration.
Furthermore, the binding of the component's
entity and architecture must be established by
the configuration
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Example configuration THREE of FULLADDER is
  for STRUCTURAL     for INST_HA1, INST_HA2 HA
      use entity WORK.HALFADDER(CONCURRENT)
    end for     for INST_XOR XOR       use
entity WORK.XOR2D1(CONCURRENT)     end for
  end for end THREE
In general, a configuration declaration belonging
to an architecture with instantiated components
is of the form Syntax configuration
configuration_name of entity_name is   for
architecture_name     for labelothersall
comp_name       use entity lib_name.comp_entity
_name(comp_arch_name)       use configuration
lib_name.comp_configuration_name
        generic map (...)         port map
(...)     end for     ...   end for end
configuration_name