Lecture 16 Introduction to VHDL

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Lecture 16Introduction to VHDL

• Prith Banerjee
• ECE C03
• Advanced Digital Design
• Spring 1998

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Outline

• VHDL Language Basics
• Interface
• Architecture Body
• Behavioral VHDL
• Various Sequential Statements
• Process Statements
• Delay Models
• READING Dewey 11.2, 11.3, 11.4, 11.5. 11.6,
  15.1, 15.2, 18.2, 18.3, 18.4, 18.5

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Modeling Digital Systems

• Digital system Any digital circuit that
  processes or stores information in digital form
  gates to functional units
• Model represents only relevant information and
  abstracts away irrelevant detail
• Model needed to
• Develop and specify requirements
• Communicate understanding of a system to a user
• Allow testing of design through simulation
• Allow formal verification
• Allow automated synthesis

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What is VHDL

• (Very High Speed Integrated Circuits) VHSIC
  Hardware Description Language
• Used for two things
• (1) Used to model digital systems, designs can
  then be SIMULATED
• A simulator runs a VHDL description computing the
  outputs of a modeled system
• (2) Used as a language to enter designs into CAD
  tools, designs can then be SYNTHESIZED
• VHDL also provides a blackboard for designing
  digital systems
• An initial design is progressively expanded and
  refined
• Another popular hardware language is Verilog

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Relationship between VHDL and hardware
VHDL description
Simulator
Hardware
Simulated / actual outputs
Model
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Domains and Level of Modeling
Structural
Functional
Register transfer
Algorithm
Gates
Register Transfer Level
Transistor
Boolean Equation
Polygons
Standard Cells
Floorplan
Geometric
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VHDL Modeling Concepts

• VHDL Includes facilities for describing the
  FUNCTION and STRUCTURE
• At Various levels from abstract to the gate level
• Intended as a modeling language for specification
  and simulation
• Also used for synthesis

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Example of VHDL Description
VHDL Model of a 2input exclusive OR gate
entity XOR2_OP is -- input output ports port
(A, B in BIT Z out BIT) --
Body architecture EX_DISJUNCTION of XOR_OP2
is begin Z lt A xor B end EX_DISJUNCTION

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VHDL Entity Definitions

• A VHDL entity consists of two parts
• interface denoted by keyword entity
• body denoted by keyword architecture
• Interface describes aspects visible outside
• Body describes how black box operates inside
• FORMAT
• entity identifier is
• port (name in / out / inout BIT/type)
• end identifier
• -- lines beginning with two dashes are
  comments

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VHDL Architecture Body

• Architecture body describes how entity operates
• Allows for different implementations
• Can have behavioral or structural or mixed
  representations
• FORMAT
• architecture EX_DISJUNCTION of XOR_OP2 is
• begin
• Z lt A xor B
• end EX_DISJUNCTION

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Architecture Body

• Body is divided into two parts
• Declarative part
• Statement part
• architecture EX_DISJUNTION of XOR_OP2 is
• -- declarative part
• -- objects must be declared before they are used
  
• begin
• -- statement part
• Z lt A xor B
• end EX_DISJUNCTION

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Data Types in VHDL

• The type of a data object defines the set of
  values that object can assume and set of
  operations on those values
• VHDL is a strongly typed language
• Four classes of objects
• constants
• variables
• signals
• files

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Constant Declaration

• The value of a constant cannot be changed
• FORMAT
• constant identifier , subtype
  expression
• EXAMPLES
• constant number_of_bytes integer 4
• constant prop_delay time 3nsec
• constant e real 2.2172

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Variable Declaration

• The value of a variable can be changed
• FORMAT
• variable identifier , .. subtype
  expression
• EXAMPLES
• variable index integer 0
• variable sum, average, largest real
• variable start, finish time 0 nsec

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Variable Assignment Statement

• Once a variable is declared, its value can be
  modified by an assignment statement
• FORMAT
• label name expression
• EXAMPLES
• program_counter 0
• index index 1
• Variable assignment different from signal
  assignment
• A variable assignment immediately overviews
  variable with new value
• A signal assignment schedules new value at later
  time

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Scalar Types

• Variable can only assign values of nominated type
• Default types
• integer , real, character, boolean, bit
• User defined types
• FORMAT
• type small_int is range 0 to 255
• Enumerated type
• FORMAT
• type logiclevel is (unknown, low, driven,
  high)

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Sub Types

• A type defines a set of values
• We can define a sub-type as a restricted set of
  values from a base type
• FORMAT
• subtype identifier is name range
  simple_expression to/downto simple_expression
• EXAMPLE
• subtype small_int is integer range -128 to 127
• subtype bit_index is integer range 31 downto 0

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Attributes of Types

• A type defines a set of values and set of
  applicable operations
• A predefined set of attributes are used to give
  information about the values included in the type
• Tleft first (leftmost) value in T
• Tright last (righmost) value in T
• Tvalue the value in T that is represented by s
• EXAMPLES
• type set_index_range is range 21 downto 11
• set_index_rangeleft 21
• set_index_rangeright 11
• set_index_rangevalue(20) 20

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Expressions and Operators
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Sequential Statements

• Sequential statements of various types are
  executed in sequence within each VHDL process
• Variable statement
• variable expression
• If statement
• Case statement
• Loop statement

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If Statement

• FORMAT
• if boolean_expression then
• sequential statement
• elsif boolean_expression then
• sequential statement
• else sequential statement
• endif
• EXAMPLE
• if sel0 then
• result lt input_0 -- executed if sel 0
• else result lt input_1 -- executed if sel
  1
• endif

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Case Statement

• EXAMPLE of an ALU operation
• case func is
• when pass1 gt
• result operand1
• when pass2 gt
• result operand2
• when add gt
• result operand1 operand2
• when subtract gt
• result operand1 - operand2
• end case

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Loop Statements

• While condition loop
• sequential statements
• end loop
• for identifier in range loop
• sequential statements
• end loop

• while index gt 0 loop
• index index -1
• end loop
• for count in 0 to 127 loop
• count_out lt count
• wait for 5 ns
• end loop
• for i in 1 to 10 loop
• count count 1
• end loop

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Concurrent Statements

• The concurrent statement in an architecture body
  describe the modules operation.
• Concurrency is useful in modeling the way real
  circuits behave
• architecture abstract of adder is
• begin
• add_a_b process(a,b) is
• begin
• sum lt a b
• end process add_a_b
• end architecture abstract

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Signal Declaration and Assignment

• Signal declaration describes internal signal
• signal identifier subtype
  expression
• EXAMPLE
• signal and_a, and_b bit
• Signal Assignment
• name lt value_expression after
  time_expression
• EXAMPLE
• y lt not or_a_b after 5 ns
• This specifies that signal y is to take on a new
  value at a time 5 ns later statement execution.
• Difference from variable assignment
• which only assigns some values to a variable

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Concepts of Delays and Timing

• The time dimension in the signal assignment
  refers to simulation time in a discrete event
  simulation
• There is a simulation time clock
• When a signal assignment is executed, the delay
  specified is added to current simulation time to
  determine when new value is applied to signal
• Schedules a transaction for the signal at that
  time

output
input
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Specifying Technology Information

• One predefined physical type in VHDL TIME
• Units fs (10 -15 seconds), ps (1000 fs), ns,
  us, ms, sec, min ( 60 sec), hr (60 min)
• User-defined physical types
• type CAPACITANCE is range 0 to INTEGERHIGH
• units
• fF -- Femtofarads
• pF 1000 fF -- Picofarads
• nF 1000 pF -- Nanofarads
• end units
• type VOLTAGE is range 0 to 2 32 -1
• units
• uV -- Microvolt
• mV 1000 uV
• V 1000 mV
• end units

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Specifying Delays

• Inertial Delay Model
• reflects physical inertia of physical systems
• glitches of very small duration not reflected in
  outputs
• SIG_OUT lt not SIG_IN after 7 nsec --implicit
• SIG_OUT lt inertial ( not SIG_IN after 7 nsec )
• Logic gates exhibit lowpass filtering

3 ns
10ns
SIG_IN
2ns
SIG_OUT
9 ns
19 ns
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Transport Delays

• Under this model, ALL input signal changes are
  reflected at the output
• SIG_OUT lt transport not SIG_IN after 7 ns

3 ns
10ns
SIG_IN
2ns
SIG_OUT
9 ns
19 ns
30 ns
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Example of Architecture Body(AND_OR_INVERT)
or_gate process (and_a, and_b) is begin
or_a_b lt and_a or and_b end process
or_gate inv process (or_a_b) is begin y
lt not or_a_b end process inv end architecture
primitive
architecture primitive of and_or_inv is signal
and_a, and_b, or_a_b bit begin and_gate_a
process (a1,a2) is begin and_a lt a1 and
a2 end process and_gate_a and_gate_b process
(b1,b2) is begin and_b lt b1 and b2 end
process and_gate_b
a1 a2
y
b1 b2
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Process Declaration of Clock Generator
Clock_gen process (clk) is begin if clk
0 then clk lt 1 after T_pw, 0
after 2T_pw endif end process clock_gen
2T_pw
T_pw
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Process Generator for Multiplexer
mux process (a, b, sel) is begin case sel
is when 0 gt z lt a after
prop_delay when 1 gt z lt b
after prop_delay end process mux
a
z
b
sel
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Wait Statement

• A wait statement specifies how a process responds
  to changes in signal values.
• wait on signal_name
• wait until boolean_expression
• wait for time_expression
• Example on right shows process sensitivity list
• EXAMPLE SAME AS

half_add process is begin sum lt a xor b
after T_pd carry lt a and b after T_pd
wait on a, b end process
half_add process (a,b) is begin sum lt a
xor b after T_pd carry lt a and b after
T_pd end process
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Process Statement

• A process statement is a concurrent statement
  that can be included in an architecture body to
  describe all or part of the behavior of a module.
• FORMAT
• process (signal_name, ) is
• process_declarative item
• begin
• sequential statement
• end process

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Summary

• VHDL Language Basics
• Interface
• Architecture Body
• Behavioral VHDL
• Various Sequential Statements
• Process Statements
• Delay Models
• NEXT LECTURE VHDL Structural and Mixed
  Description
• READING Dewey 12.1, 12.2, 12.3, 12.4, 13.1,
  13.2, 13.3. 13.4, 13.6, 13.7. 13.8