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ECE C03 Lecture 12 1
Lecture 12Introduction to VHDL
Hai Zhou
ECE 303
Advanced Digital Design
Spring 2002
ECE C03 Lecture 12 2
Outline
• VHDL Language Basics• Interface• Architecture Body• Process• Signal Assignment and Delay Models• Various Sequential Statements• READING: Dewey 11.2, 11.3, 11.4, 11.5. 11.6,
15.1, 15.2, 18.2, 18.3, 18.4, 18.5
ECE C03 Lecture 12 3
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
ECE C03 Lecture 12 4
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
ECE C03 Lecture 12 5
Relationship between VHDL and hardware
Model
VHDL description
Simulator
Simulated / actual outputs
Hardware
ECE C03 Lecture 12 6
Example of VHDL DescriptionVHDL 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 <= A xor B;
end EX_DISJUNCTION;
ECE C03 Lecture 12 7
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
ECE C03 Lecture 12 8
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 <= A xor B;
end EX_DISJUNCTION;
ECE C03 Lecture 12 9
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 <= A xor B;
end EX_DISJUNCTION;
ECE C03 Lecture 12 10
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
ECE C03 Lecture 12 11
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;
ECE C03 Lecture 12 12
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;
ECE C03 Lecture 12 13
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
ECE C03 Lecture 12 14
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);
ECE C03 Lecture 12 15
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;
ECE C03 Lecture 12 16
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• T’left = first (leftmost) value in T• T’right = last (righmost) value in T• T’value(s) = the value in T that is represented by s• EXAMPLES:
type set_index_range is range 21 downto 11;set_index_range’left = 21set_index_range’right = 11set_index_range’value(“20”) = 20
ECE C03 Lecture 12 17
Expressions and Operators
Operator Operation Operand Types** Exponentiation Integer, realAbs Absoluet value numeric*, / , mod, rem Mul, div Integer, realAnd, nand, or, nor, xor, xnor, not Logical ops Bit, boolean, or 1-D arraySll, srl, sla,sra Shift left/right 1-D array of bit/boolean+, - Add, subtract Integer, real=, /=, <, <=, >, >= Equal, greater Scalar
ECE C03 Lecture 12 18
VHDL Modeling Concepts
• Semantics (meaning) is heavily based on SIMULATION
• A design is described as a set of interconnected modules
• A module could be another design (component) or could be described as a sequential program (process)
ECE C03 Lecture 12 19
A general VHDL design
I1I2
O1IO1
Entity … is…End entity;
component
process 1 process 2
concurrentassignment
concurrentassignment
I1
I2
O1
IO1
s1
s2
s3 s4
s5
s6
s7
s8 s9
architecture … of … is...begin…end;
ECE C03 Lecture 12 20
VHDL Simulatorstart
stop
Initt = 0
more event
get earliestevent
delta delay
updatesignals
advancetime
execute triggeredprocesses
during process execution,new events may be added
ECE C03 Lecture 12 21
Process Statements• FORMAT
PROCESS_LABEL: process-- declarative part declares functions, procedures, types, constants,
variables, etcbegin-- Statement partsequential statement;sequential statement;wait statement; -- eg. Wait for 1 ms; or wait on ALARM_A;sequential statement;…wait statement;end process;
Flow of
control
ECE C03 Lecture 12 22
Sequential Statements
• Sequential statements of various types are executed in sequence within each VHDL process
• Variable statementvariable := expression;
• Signal Assignment• If statement• Case statement• Loop statement• Wait statement
ECE C03 Lecture 12 23
Variable and Sequential Signal Assignment
• Variable assignment– new values take effect immediately after execution
variable LOGIC_A, LOGIC_B : BIT;
LOGIC_A := ‘1’;
LOGIC_B := LOGIC_A;
• Signal assignment– new values take effect after some delay (delta if not specified)
signal LOGIC_A : BIT;
LOGIC_A <= ‘0’;
LOGIC_A <= ‘0’ after 1 sec;
LOGIC_A <= ‘0’ after 1 sec, ‘1’ after 3.5 sec;
ECE C03 Lecture 12 24
Signal Declaration and Assignment• Signal declaration: describes internal signal signal identifier {…} : subtype [ := expression]• EXAMPLE: signal and_a, and_b : bit;
• Signal Assignment name <= value_expression [ after time_expression];• EXAMPLE y <= 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
ECE C03 Lecture 12 25
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
input
output
ECE C03 Lecture 12 26
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 INTEGER’HIGHunitsfF; -- FemtofaradspF = 1000 fF; -- PicofaradsnF = 1000 pF; -- Nanofaradsend units
type VOLTAGE is range 0 to 2 ** 32 -1unitsuV; -- Microvolt;mV = 1000 uV;V = 1000 mV;end units;
ECE C03 Lecture 12 27
Specifying Delays
• Inertial Delay Model– reflects physical inertia of physical systems
– glitches of very small duration not reflected in outputs
• SIG_OUT <= not SIG_IN after 7 nsec --implicit• SIG_OUT <= inertial ( not SIG_IN after 7 nsec )• Logic gates exhibit lowpass filtering
SIG_IN
SIG_OUT2ns
9 ns 19 ns
3 ns10ns
ECE C03 Lecture 12 28
Transport Delays
• Under this model, ALL input signal changes are reflected at the output
• SIG_OUT <= transport not SIG_IN after 7 ns;
SIG_IN
SIG_OUT2ns
9 ns 19 ns
3 ns10ns
30 ns
ECE C03 Lecture 12 29
If Statement• FORMAT if boolean_expression then {sequential statement} elsif boolean_expression then {sequential statement} else {sequential statement} endif;• EXAMPLE if sel=0 then result <= input_0; -- executed if sel = 0 else result <= input_1; -- executed if sel = 1
endif;
ECE C03 Lecture 12 30
Case Statement• EXAMPLE of an ALU operation:case func is when pass1 => result := operand1; when pass2 => result := operand2; when add => result := operand1 + operand2; when subtract => result := operand1 - operand2; end case;
ECE C03 Lecture 12 31
Loop Statements
While condition loop
{sequential statements}
end loop;
for identifier in range loop
{sequential statements}
end loop;
while index > 0 loop index := index -1;end loop;
for count in 0 to 127 loop count_out <= count;wait for 5 ns;end loop;
for i in 1 to 10 loop count := count + 1;end loop;
ECE C03 Lecture 12 32
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 isbegin sum <= a xor b after T_pd; carry <= a and b after T_pd; wait on a, b;end process;
half_add: process (a,b) isbegin sum <= a xor b after T_pd; carry <= a and b after T_pd;end process;
ECE C03 Lecture 12 33
Example of Architecture Body(AND_OR_INVERT)
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 <= a1 and a2;
end process and_gate_a;
and_gate_b : process (b1,b2) is
begin
and_b <= b1 and b2;
end process and_gate_b;
or_gate: process (and_a, and_b) is
begin
or_a_b <= and_a or and_b;
end process or_gate;
inv : process (or_a_b) is
begin
y <= not or_a_b;
end process inv;
end architecture primitive;a1a2
b1b2
y
ECE C03 Lecture 12 34
Process Declaration of Clock Generator
Clock_gen: process (clk) is
begin
if clk = ‘0’ then
clk <= ‘1’ after T_pw, ‘0’ after 2*T_pw;
endif;
end process clock_gen;T_pw
2*T_pw
ECE C03 Lecture 12 35
Process Generator for Multiplexer
a
b
sel
z
mux: process (a, b, sel) is
begin
case sel is
when ‘0’ =>
z <= a after prop_delay;
when ‘1’ =>
z <= b after prop_delay;
end process mux;
ECE C03 Lecture 12 36
Summary
• VHDL Language Basics
• Interface
• Architecture Body
• Process
• Signal Assignment and Delay Models
• Various Sequential Statements
• NEXT LECTURE: VHDL Structural 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
Recommended