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Introduction To VHDL for Combinational Logic
? VHDL is a language used for simulation and
synthesis of digital logic.
? A VHDL description of a digital system can be
transformed into a gate level implementation. This
process is know as synthesis.
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library ieee;
use ieee.std_logic_1164.all;
entity majconc is
port ( A, B, C : in std_logic;
Y: out std_logic
);
end majconc;
ARCHITECTURE a of majconc is
begin
Y <= (A and B) or (A and C) or (B and C);
end a;
A Sample Model
Description Implementation
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VHDL Statements
? VHDL has a reputation as a complex language (it is!)
? We will use a small subset of the language for our
purposes
? Some VHDL constructs:
– Signal Assignment: A <= B;
– Comparisons = (equal), > (greater than), < (less than), etc.
– Boolean operations AND, OR, NOT, XOR
– Sequential statements (CASE, IF, FOR)
– Concurrent statements (when-else)
? READ YOUR BOOK. We will cover VHDL by
'example'; will explain VHDL constructs as we get to
them. The book has many examples.
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VHDL Combinational Template
? Every VHDL model is composed of an entity and
at least one architecture .
? Entity describes the interface to the model (inputs,
outputs)
? Architecture describes the behavior of the model
? Can have multiple architectures for one entity (we
will only use one in this class).
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A VHDL Template for Combinational Logic
entity model_name is
port (
list of inputs and outputs );
end model_name;
architecture arch_name of model_name is
begin
concurrent statement 1
concurrent statement 2
... concurrent statement N;
end arch_name;
All of the text not in italics are VHDL keywords.
VHDL is NOT case sensitive. (ENTITY is same as entity
is same as EnTiTy).
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Majority Gate Example
The following is an example of a three input XOR gate (majority
gate) implemented in VHDL
library ieee;
use ieee.std_logic_1164.all;
entity majority is
port ( A, B, C : in std_logic; -- two dashes is a COMMENT in VHDL
Y: out std_logic
);
end majority;
-- this is the architecture declaration, uses only one concurrent statement.
ARCHITECTURE concurrent of majority is
begin
Y <= (A and B) or (A and C) or (B and C);
end concurrent;
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Majority Gate with Temporary Signals
The following version of the majority gate uses some temporary
signals (entity has been left out, is same).
-- the architecture now uses 4 concurrent statements
ARCHITECTURE newconc of majority is
signal t1, t2, t3 : std_logic ;
begin
t1 <= A and B;
t2 <= A and C;
t3 <= B and C;
Y <= t1 or t2 or t3;
end newconc;
Note that temporary signals are declared between architecture
statement and begin statement.
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Majority Gate with when-else statement
The following version of the majority gate uses a 'when-else'
statement:
-- the architecture now uses a when-else statement.
ARCHITECTURE whenelse of majority is
begin
Y <= '1' when ( (A and B) or (A and C) or (B and C))
else '0';
end whenelse;
You will find that there are many different ways to accomplish the
same result in VHDL. There is usually no best way; just use one
that you feel most comfortable with.
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Concurrent Versus Sequential Statements
? The statements we have looked at so far are called
concurrent statements.
– Each concurrent statement will synthesize to a block of
logic.
? Another class of VHDL statements are called
sequential statements.
– Sequential statements can ONLY appear inside of a process
block.
– A process block is considered to be a single concurrent
statement.
– Can have multiple process blocks in an architecture.
– Usually use process blocks to describe complex
combinational or sequential logic.
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Majority Gate using process block and if statement
The entity declaration has been left out (same as before).
ARCHITECTURE ifstate of majority is
begin
main: process (A, B, C)
begin
Y <= '0'; -- default output assignment.
if ((A = '1') and (B = '1')) then
Y <= '1';
end if;
if ((A = '1') and (C = '1') ) then
Y <= '1';
end if;
if ((B = '1') and (C = '1') ) then
Y <= '1';
end if;
end process main;
end ifstate;
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Comments on process block model
? The first line in the process "main: process (A, B, C)" has the
name of the process (main) and the sensitivity list of the process.
– The process name is user defined, can also be left out (unnamed process).
– The sensitivity list should contain any signals that appear on the right hand
side of an assignment (inputs) or in any boolean for a sequential control
statement.
? The if statement condition must return a boolean value (TRUE or
FALSE) so that is why the conditional is written as:
( (A='1') and (B= '1') )
Cannot write it as:
( A and B)
because this will return a 'std_logic' type (more on types later).
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Use of if-else
ARCHITECTURE ifelse of majority is
begin
process (A, B, C)
begin
if (((A = '1') and (B = '1')) or
((A = '1') and (C = '1')) or
((B = '1') and (C = '1')) ) then
Y <= '1';
else
Y <= '0';
end if;
end process;
end ifelse;
Comments:
Process is anonymous (no
name)
Used an 'else' clause to specify
what the output should be if
the if condition test was not
true.
CAREFUL! The boolean
operators (OR, AND) do not
have any precedence so must
use parenthesis to define
precedence order
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Unassigned outputs in Process blocks
A common mistake in writing a combinational process is to leave an
output unassigned. If there is a path through the process in which an
output is NOT assigned a value, then that value is unassigned.
ARCHITECTURE bad of majority is
begin
process (A, B, C)
begin
if (((A = '1') and (B = '1')) or
((A = '1') and (C = '1')) or
((B = '1') and (C = '1')) ) then
Y <= '1';
end if;
end process;
end bad;
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Comments on ‘bad’ architecture
? In the above process, the ELSE clause was left out. If the 'if'
statement condition is false, then the output Y is not assigned a
value.
– In synthesis terms, this means the output Y should have a
LATCH placed on it!
– The synthesized logic will have a latch placed on the Y output;
once Y goes to a '1', it can NEVER return to a '0'!!!!!
? This is probably the #1 student mistake in writing processes. To
avoid this problem do one of the following things:
– ALL signal outputs of the process should have DEFAULT
assignments right at the beginning of the process (this is my
preferred method, is easiest).
– OR, all 'if' statements that affect a signal must have ELSE
clauses that assign the signal a value if the 'if' test is false.
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library ieee;
use ieee.std_logic_1164.all;
entity priority is
port ( y1, y2, y3, y4, y5, y6, y7 : in std_logic;
dout: out std_logic_vector(2 downto 0)
);
end priority;
architecture ifels of priority is
begin
-- priority circuit, Y7 highest priority input
-- Y1 is lowest priority input
process (y1, y2,y3, y4, y5, y6, y7)
begin
if (y7 = '1') then dout <= "111";
elsif (y6 = '1') then dout <= "110";
elsif (y5 = '1') then dout <= "101";
elsif (y4 = '1') then dout <= "100";
elsif (y3 = '1') then dout <= "011";
elsif (y2 = '1') then dout <= "010";
elsif (y1 = '1') then dout <= "001";
else dout <= "000";
end process;
end ifels;
This priority circuit has 7 inputs;
Y7 is highest priority, Y0 is
lowest priority.
Three bit output should indicate
the highest priority input that is a
'1' (ie. if Y6 ='1' , Y4 = '1', then
output should be "101"). If no
input is asserted, output should
be "000".
Priority circuit example
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Comments on Priority Example
? This is the first example that used a bus. The DOUT signal is a
3 bit output bus.
– std_logic_vector(2 downto 0) describes a 3 bit bus where dout(2) is most
significant bit, dout(0) is least significant bit.
– std_logic_vector (0 to 2) is also a 3 bit bus, but dout(0) is MSB, dout(2) is
LSB. We will always use 'downto' in this class.
? A bus assignment can be done in many ways:
– dout <= "110"; assigns all three bits
– dout(2) <= '1'; assigns only bit #2
– dout(1 downto 0) <= "10"; assigns two bits of the bus.
? This architecture used the 'elsif' form of the 'if' statement
– Note that it is 'elsif', NOT 'elseif'.
– This called an elsif chain.
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Priority Circuit with just IF statements.
architecture plainif of priority is
begin
-- priority circuit, Y7 highest priority input
-- Y1 is lowest priority input
process (y1, y2,y3, y4, y5, y6, y7)
begin
dout <= "000;
if (y1 = '1') then dout <= "001"; end if;
if (y2 = '1') then dout <= "010"; end if;
if (y3 = '1') then dout <= "011"; end if;
if (y4 = '1') then dout <= "100"; end if;
if (y5 = '1') then dout <= "101"; end if;
if (y6 = '1') then dout <= "110"; end if;
if (y7 = '1') then dout <= "111"; end if;
end process;
end plainif;
By reversing the order of the
assignments, we can
accomplish the same as the
elsif priority chain.
In a process, the LAST
assignment to the output is
what counts.
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Priority Circuit with when-else statements.
architecture whenelse of priority is
begin
-- priority circuit, Y7 highest priority input
-- Y1 is lowest priority input
-- uses just one when-else concurrent statement.
dout <= "111" when (y7 = '1') else
"110" when (y6 = '1') else
"101" when (y5 = '1') else
"100" when (y4 = '1') else
"011" when (y3 = '1') else
"010" when (y2 = '1') else
"001" when (y1 = '1') else
"000";
end process;
end whenelse;
No process; just one
concurrent when-else
statement.
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A Bad attempt at a Priority Circuit
architecture bad of priority is
begin
-- priority circuit, Y7 highest priority input
-- Y1 is lowest priority input
-- uses just one when-else concurrent statement.
dout <= "111" when (y7 = '1') else "000";
dout <= "110" when (y6 = '1') else "000";
dout <= "101" when (y5 = '1') else "000";
dout <= "100" when (y4 = '1') else "000";
dout <= "011" when (y3 = '1') else "000";
dout <= "010" when (y2 = '1') else "000";
dout <= "001" when (y1 = '1') else "000";
dout <= "000" ;
end process;
end bad;
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Comments on “bad” Priority Circuit
? This is a bad attempt by a neophyte VHDL writer at a priority
circuit. There are multiple things wrong with this description.
? There are multiple concurrent statments driving the DOUT
signal. This means MULTIPLE GATE output are tied to dout
signal! Physically, this will create an unknown logic condition
on the bus.
? The writer seems to think that the order of the concurrent
statements makes a difference (ie, the last concurrent statement
just assigns a '000'). The order in which you arrange concurrent
statements MAKES NO DIFFERENCE. The synthesized logic
will be the same.
– Ordering of statements only makes a difference within a process. This is why
statements within a process are called 'sequential' statements; the logic
synthesized reflects the statement ordering (only for assignments to the same
output).
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4-to-1 mux with 8 bit Datapaths
library ieee;
use ieee.std_logic_1164.all;
entity mux4to1_8 is
port ( a,b,c,d : in std_logic_vector(7 downto 0);
sel: in std_logic_vector (1 downto 0);
dout: out std_logic_vector(7 downto 0)
);
end mux4to1_8;
architecture whenelse of mux4to1_8 is
begin
dout <= b when (sel = "01") else
c when (sel = "10") else
d when (sel = "11") else
a; -- default
end process;
end whenelse;
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Comments on Mux example
? This is one way to write a mux, but is not the best way. The
when-else structure is actually a priority structure.
– A mux has no priority between inputs, just a simple
selection.
– The synthesis tool has to work harder than necessary to
understand that all possible choices for sel are specified
and that no priority is necessary.
? Just want a simple selection mechanism.
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4-to-1 Mux using Select Concurrent Statement
architecture select_statement of mux4to1_8 is
begin
with sel select
dout <= b when "01",
c when "10",
d when "11",
a when others;
end select_statement;
Some synthesis tools will automatically recognize this structure as a
mux and will find a more efficient implementation than using a whenelse
or if statement structure (when-else and if structures define a
priority structure). The others case must be specified.
This is a concurrent statement; the sequential version of the select
statement is the case statement.
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4-to-1 Mux using Case Sequential Statement
architecture select_statement of mux4to1_8 is
begin
process (a, b, c, d, sel)
begin
case sel is
when "01" => dout <= b ;
when "10" => dout <= c;
when "11" => dout <= d;
when others => dout <= a;
end case;
end process;
end select_statement;
There can be multiple
statements for each case;
only one statement is needed
for each case in this
example.
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Logical Shift Left by 1
library ieee;
use ieee.std_logic_1164.all;
entity lshift is
port ( din : in std_logic_vector(7 downto 0);
shift_en: in std_logic;
dout: out std_logic_vector(7 downto 0)
);
end lshift;
architecture brute_force of lshift is
begin
process (din, shift_en)
begin
dout <= din; -- default case
if (shift_en = '1') then
dout(0) <= '0'; -- shift a zero into LSB
dout (1) <= din(0);
dout (2) <= din(1);
dout (3) <= din(2);
dout (4) <= din(3);
dout (5) <= din(4);
dout (6) <= din(5);
dout (7) <= din(6);
end if;
end process;
end brute_force;
end lshift;
This is one way to do it; surely there is a better way?
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Logical Shift Left by 1 (better way)
architecture better of lshift is
begin
process (din, shift_en)
begin
dout <= din; -- default case
if (shift_en = '1') then
dout(0) <= '0'; -- shift a zero into LSB
dout (7 downto 1) <= din(6 downto 0);
end if;
end process;
end better;
end lshift;
This illustrates the assignment of a segment of one bus to another
bus segment. The bus ranges on each side of the assignment
statement must be the name number of bits (each 6 bits in this case).
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4 Bit Ripple Carry Adder
A B
S
Co Ci
A B
S
Co Ci
A B
S
Co Ci
A B
S
Co Ci Cin
A(0)
Cout
A(3) B(3) A(2) B(2) A(1) B(1) B(0)
C(4) C(3) C(2) C(1) C(0)
Sum(3) Sum(2) Sum(1) Sum(0)
Want to write a VHDL model for a 4 bit ripple carry adder.
Logic equation for each full adder is:
sum <= a xor b xor ci;
co <= (a and b) or (ci and (a or b));
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4 Bit Ripple Carry Model
library ieee;
use ieee.std_logic_1164.all;
entity adder4bit is
port ( a,b: in std_logic_vector(3 downto 0);
cin : in std_logic;
cout: out std_logic;
sum: out std_logic_vector(3 downto 0)
);
end adder4bit;
architecture bruteforce of adder4bit is
-- temporary signals for internal carries
signal c : std_logic_vector(4 downto 0); .
begin
process (a, b, cin, c)
begin
c(0) <= cin;
-- full adder 0
sum(0) <= a(0) xor b(0) xor c(0);
c(1) <= (a(0) and b(0)) or (c(0) and (a(0) or b(0)));
-- full adder 1
sum(1) <= a(1) xor b(1) xor c(1);
c(2) <= (a(1) and b(1)) or (c(1) and (a(1) or b(1)));
-- full adder 2
sum(2) <= a(2) xor b(2) xor c(2);
c(3) <= (a(2) and b(2)) or (c(2) and
(a(2) or b(2)));
-- full adder 3
sum(3) <= a(3) xor b(3) xor c(3);
c(4) <= (a(3) and b(3)) or (c(3) and
(a(3) or b(3)));
cout <= c(4);
end process;
end bruteforce;
Straight forward
implementation. Nothing
wrong with this.
However, is there an
easier way?
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4 Bit Ripple Carry Model using For Statement
architecture forloop of adder4bit is
signal c : std_logic_vector(4 downto 0); -- temporary signals for internal carries.
begin
process (a, b, cin, c)
begin
c(0) <= cin;
for i in 0 to 3 loop
-- all four full adders
sum(i) <= a(i) xor b(i) xor c(i);
c(i+1) <= (a(i) and b(i)) or (c(i) and (a(i) or b(i)));
end loop;
cout <= c(4);
end process;
end forloop;
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Comments on for-loop statement
? The for-loop can be used to repeat blocks of logic
? The loop variable i is implicity declared for this
loop; does not have to be declared anywhere else.
? To visualize what logic is created, 'unroll' the loop
by writing down each loop iteration with loop
indices replaced hard numbers.
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Summary
? There are many different ways to write VHDL synthesizable
models for combinational logic.
? There is no 'best' way to write a model; for now, just use
the statements/style that you feel most comfortable with and
can get to work (of course!)
? READ THE BOOK!!!!!!!!
– There is NO WAY that we can cover all possible examples in class.
The book has many other VHDL examples.
– I have intentionally left out MANY, MANY language details. You
can get by with what I have shown you, but feel free to experiment
with other language features that you see discussed in the book or
elsewhere.
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Summary (cont.)
? SEARCH THE WWW!!!!!
– The WWW is full of VHDL examples, tutorials, etc.
? TRY IT OUT!!!!
– If you have a question about a statement or example, try it out in the
Altera Maxplus package and see what happens!
? This course is about Digital System DESIGN, not VHDL.
As such, we will only have 3-4 lectures about VHDL, the
rest will be on design topics.
– VHDL is only a means for efficiently implementing your design - it
is not interesting by itself.
– You will probably learn multiple synthesis languages in your design
career - it is the digital design techniques that you use that will be
common to your designs, not the synthesis language.