This document discusses behavioral modeling in VHDL. It covers different VHDL design styles including behavioral, dataflow, and structural. Behavioral modeling uses sequential statements inside processes to model functionality. Key concepts covered include processes with and without sensitivity lists, concurrent vs sequential execution, if/case statements, loops, and wait statements. An example of a behavioral model for a full adder is presented using two processes.

1. EECL 309B
VHDL
Behavioral Modeling
Spring 2014 Semester

2. VHDL Design Styles
VHDL Design
Styles
structural
Components and
interconnects
dataflow
Concurrent
statements
behavioral
Sequential statements
• Registers
• Shift registers
• Counters
• State machines
synthesizable

3. Behavioral Design/Modelling
• Functional performance is the goal of behavioral modeling
• Timing optionally included in the model
• Software engineering practices should be used to develop
behavioral models
• Sequential, inside a process
• Just like a sequential program
• The main character is ‘process(sensitivity list)’
3

4. Anatomy of a Process
• The process statement is a concurrent statement , which
delineates a part of an architecture where sequential statements
are executed.
• Syntax
[label:] process [(sensitivity list )]
declarations
begin
sequential statements
end process [label];

5. PROCESS with a SENSITIVITY LIST
• List of signals to which the
process is sensitive.
• Whenever there is an event on
any of the signals in the
sensitivity list, the process fires.
• Every time the process fires, it
will run in its entirety.
• WAIT statements are NOT
ALLOWED in a processes
with SENSITIVITY LIST.
label: process (sensitivity list)
declaration part
begin
statement part
end process;

6. Concurrent VS sequential
• Every statement inside the architecture body is
executed concurrently, except statements
enclosed by a process.
• Process
– Statements within a process are executed sequentially.
Result is known when the whole process is complete.
– You may treat a process as one concurrent statement in
the architecture body.
– Process(sensitivity list): when one or more signals in the
sensitivity list change state, the process executes once.
– Process should either have sensitivity list or an explicit
wait statement. Both should not be present in the same
process statement.
6

7. Process contd..
• The order of execution of statements is
the order in which the statements appear
in the process
• All the statements in the process are
executed continuously in a loop .
• The simulator runs a process when any
one of the signals in the sensitivity list
changes.
• For a wait statement, the simulator
executes the process after the wait is over.

8. Example of Process with/without wait
process (clk,reset)
begin
if (reset = ‘1’) then
A <= ‘0’;
elsif (clk’event and clk = ‘1’) then
A <= ‘B’;
end if;
end process;
process
begin
if (reset = ‘1’) then
A <= ‘0’ ;
elsif (clk’event and clk = ‘1’) then
A <= ‘B’;
end if;
wait on reset, clk;
end process;

9. Let’s Write a VHDL Model of Full Adder
using Behavioral Modeling
ENTITY full_adder IS
PORT ( A, B, Cin : IN BIT;
Sum, Cout : OUT BIT
);
END full_adder;
A
B
Cin
Sum
Cout

10. Full Adder Architecture
A B Cin Sum Cout
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1
for Sum:
Cin (I.e. Carry In):
A B 0 1
0 0 0 1
0 1 1 0
1 1 0 1
1 0 1 0
for Cout (I.e. Carry Out):
Cin (I.e. Carry In)
A B 0 1
0 0 0 0
0 1 0 1
1 1 1 1
1 0 0 1

11. Two Full Adder Processes
A
B
Cin
Sum
Cout
Summation:
PROCESS( A, B, Cin)
BEGIN
Sum <= A XOR B XOR Cin;
END PROCESS Summation;
Carry:
PROCESS( A, B, Cin)
BEGIN
Cout <= (A AND B) OR
(A AND Cin) OR
(B AND Cin);
END PROCESS Carry;

12. Complete Architecture
ARCHITECTURE example OF full_adder IS
-- Nothing needed in declarative block...
BEGIN
Summation: PROCESS( A, B, Cin)
BEGIN
Sum <= A XOR B XOR Cin;
END PROCESS Summation;
Carry: PROCESS( A, B, Cin)
BEGIN
Cout <= (A AND B) OR
(A AND Cin) OR
(B AND Cin);
END PROCESS Carry;
END example;

13. VHDL Sequential Statements
• Assignments executed sequentially in processes
• Sequential statements
– {Signal, variable} assignments
– Flow control
• IF <condition> THEN <statements> [ELSIF <statements] [ELSE
<statements>] END IF;
• FOR <range> LOOP <statements> END LOOP;
• WHILE <condition> LOOP <statements> END LOOP;
• CASE <condition> IS WHEN <value> => <statements>
{WHEN <value> => <statements>}
[WHEN others => <statements>]
END CASE;
– WAIT [ON <signal>] [UNTIL <expression>] [FOR <time>] ;
– ASSERT <condition> [REPORT <string>] [SEVERITY <level>] ;

14. The if statement
• Syntax
if condition1 then
statements
[elsif condition2 then
statements]
[else
statements]
end if;
Priority
• An if statement selects one or none of a sequence of
events to execute . The choice depends on one or more
conditions.

15. The if statement contd.
if sel = ‘1’ then
c <= a;
else
c <= b;
end if;
if (sel = “00”) then
• If statements can be nested.
o <= a;
elsif sel = “01” then
x <= b;
elsif (color = red) then
y <= c;
else
o <= d;
end if;
• If statement generates a priority structure
• If corresponds to when else concurrent statement.

16. Alternate Carry Process
Carry: PROCESS( A, B, Cin)
BEGIN
IF ( A = ‘1’ AND B = ‘1’ ) THEN
Cout <= ‘1’;
ELSIF ( A = ‘1’ AND Cin = ‘1’ ) THEN
Cout < = ‘1’;
ELSIF ( B = ‘1’ AND Cin = ‘1’ ) THEN
Cout <= ‘1’;
ELSE
Cout <= ‘0’;
END IF;
END PROCESS Carry;

17. Priority Encoder
clk
• All signals which appear on the left of
signal assignment statement (<=) are
outputs e.g. y, z
• All signals which appear on the right of
signal assignment statement (<=) or in
logic expressions are inputs e.g. w, a,
b, c
• All signals which appear in the
sensitivity list are inputs e.g. clk
• Note that not all inputs need to be
included in the sensitivity list
priority: PROCESS (clk)
BEGIN
IF w(3) = '1' THEN
y <= "11" ;
ELSIF w(2) = '1' THEN
y <= "10" ;
ELSIF w(1) = c THEN
y <= a and b;
ELSE
z <= "00" ;
END IF ;
END PROCESS ;
w
a
y
z
priorit
y
b
c

18. BEHAVIORAL ( Processes using signals)
Sig1 = 2 + 3 = 5
Sig2 = 1
Sig3 = 2
Sum = 1 + 2 + 3 = 6

19. BEHAVIORAL ( Processes using Variables)
var1 = 2 + 3 = 5
var2 = 5
var3 = 5
Sum = 5 + 5 + 5 = 15

20. Loop, While & For Statement
 Syntax
[label:] loop
{sequential_statement}
end loop [label];
 Syntax
[label:] while condition loop
{sequential_statement}
end loop [label];
 Syntax
[label:] for identifier in discrete_range
loop
{sequential_statement}
end loop [label];

21. For Loops

22. Add Function

23. WHILE LOOP :
 Syntax :
loop_label: while condition loop
<sequence of statements>
end loop loop_label
 Statements are executed continuously as long as
condition is true.
 Has a Boolean Iteration Scheme.
 Condition is evaluated before execution.

24. Example: A square wave generation

25. The case statement - syntax
case expression is
when choice 1 =>
statements
when choice 3 to 5 =>
statements
when choice 8 downto 6 =>
statements
when choice 9 | 13 | 17 =>
statements
when others =>
statements
end case;

26. The case statement
• The case statement selects, for execution one of a number
of alternative sequences of statements .
• Corresponds to with select in concurrent statements .
• Case statement does not result in prioritized logic structure
unlike the if statement.

27. The case statement contd.
process(sel, a, b, c, d)
begin
case sel is
when “00” =>
dout <= a;
when “01” =>
dout <= b;
when “10” =>
dout <= c;
when “11” =>
dout <= d;
when others =>
null;
end case;
end process;
process (count)
begin
case count is
when 0 =>
dout <= “00”;
when 1 to 15 =>
dout <= “01”;
when 16 to 255 =>
dout <= “10”;
when others =>
null;
end case;
end process;

28. Think Hardware! (Mutually exclusive conditions)
myif_pro: process (s, c, d, e, f)
begin
if s = "00" then
pout <= c;
elsif s = "01" then
pout <= d;
elsif s = "10" then
pout <= e;
else
pout <= f;
end if;
end process myif_pro;
This priority is useful for timings.

29. Think Hardware! Use a case for mutually
exclusive things
mycase_pro: process (s, c, d, e, f)
begin
case s is
when "00" =>
pout <= c;
when "01" =>
pout <= d;
when "10" =>
pout <= e;
when others =>
pout <= f;
end if;
C
D
E
F
S
end process mycase_pro;
POUT
There is no priority with case.

30. Use of others
 All the choices in case statement must be enumerated
 The choices must not overlap
 If the case expression contains many values, the others is usable
entity case_ex1 is
port (a: in integer range 0 to 30; q: out integer range 0 to 6);
end;
architecture bhv of case_ex1 is
begin
p1: process(a)
begin
case a is
when 0 => q<=3;
when 1 | 2 => q<=2;
when others => q<=0;
end case;
end process;
end;

31. Multiple assignment - examples
 architecture rtl of case_ex8 is
begin
p1: process (a)
begin
case a is
when “00” => q1<=“1”;
q2<=‘0’;
q3<=‘0’;
when “10” => q1<=“0”;
q2<=‘1’;
q3<=‘1’;
when others => q1<=“0”;
q2<=‘0’;
q3<=‘1’;
end case;
end process;
end;
 architecture rtl of case_ex9 is
begin
p1: process (a)
begin
q1<=“0”;
q2<=‘0’;
q3<=‘0’;
case a is
when “00” => q1<=“1”;
when “10” => q2<=‘1’;
q3<=‘1’;
when others => q3<=‘1’;
end case;
end process;
end;

32. Null statement
null_statement::=
[ label : ] null ;
 The null - statement explicitly prevents any action from being carried out.
 This statement means “do nothing”.
 This command can, for example, be used if default signal assignments have
been used in a process and an alternative in the case statement must not change
that value.

33. Null statement - example
 architecture rtl of ex is
begin
p1: process (a)
begin
q1<=“0”;
q2<=‘0’;
q3<=‘0’;
case a is
when “00” => q1<=“1”;
when “10” => q2<=‘1’;
q3<=‘1’;
when others => null;
end case;
end process;
end;

34. Wait statement
 wait_statement::=
[ label : ] wait [ sensitivity_clause ]
[ condition_clause ]
[ timeout_clause ] ;
 Examples:
 wait ;
 The process is permanently interrupted.
 wait for 5 ns ;
 The process is interrupted for 5 ns.
 wait on sig_1, sig_2 ;
 The process is interrupted until the value of one of the
two signals changes.
 wait until clock = '1' ;
 The process is interrupted until the value of clock is 1.

35. Wait statement in a process
 There are three ways of describing a wait statement in a process:
process (a,b)
wait until a=1
wait on a,b;
wait for 10 ns;
 The first and the third are identical, if wait on a,b; is placed at the end of the
process:
p0: process (a, b)
begin
if a>b then
q<=‘1’;
else
q<=‘0’;
end if;
end process;
p1: process
begin
if a>b then
q<=‘1’;
else
q<=‘0’;
end if;
wait on a,b;
end process;

36. Features of the wait statement
 In the first example the process will be triggered each time that signal a or b
changes value (a’event or b’event)
 Wait on a,b; has to be placed at the end of the second example to be identical
with the first example because all processes are executed at stat-up until they
reach their first wait statement.
 That process also executed at least once, which has sensitivity list and there
is no changes in the values of the list members
 If a wait on is placed anywhere else, the output signal’s value will be different
when simulation is started.
 If a sensitivity list is used in a process, it is not permissible to use a wait
command in the process.
 It is permissible, however, to have several wait commands in the same process.

37. Details of the wait’s types
 Wait until a=‘1’; means that, for the wait condition to be satisfied and
execution of the code to continue, it is necessary for signal a to have an event,
i.e. change value, and the new value to be ‘1’, i.e. a rising edge for signal a.
 Wait on a,b; is satisfied when either signal a or b has an event (changes value).
 Wait for 10 ns; means that the simulator will wait for 10 ns before continuing
execution of the process.
 The starting time point of the waiting is important and not the actual changes of any
signal value.
 It is also permissible to use the wait for command as follows:
constant period:time:=10 ns;
wait for 2*period;
 The wait alternatives can be combined into: wait on a until b=‘1’ for 10 ns;,
but the process sensitivity list must never be combined with the wait
alternatives
 Example: wait until a=‘1’ for 10 ns;
The wait condition is satisfied when a changes value or after a wait of 10 ns
(regarded as an or condition).

38. Examples of wait statement
 type a: in bit; c1, c2, c3, c4, c5, c6, c7: out bit;
Example 1
process (a)
begin
c1<= not a;
end process;
Example 2
process
begin
c2<= not a;
wait on a;
end process;
Example 3
process
begin
wait on a;
c3<= not a;
end process;
Example 4
process
begin
wait until a=‘1’;
c4<= not a;
end process;
Example 5
process
begin
c5<= not a;
wait until a=‘1’;
end process;
Example 6
process
begin
c5<= not a;
wait for 10 ns;
end process;
Example 7
process
begin
c5<= not a;
wait until a=‘1’ for 10 ns;
end process;

39. Simulation results
10 20 30 40 50 60 time [ns]
A
C1
C2
C3
C4
C5
C6
10 ns 20 ns
C7

40. Wait statement in synthesis tools
 Synthesis tools do not support use of wait for 10 ns;.
 This description method produces an error in the synthesis.
 The majority of synthesis tools only accept a sensitivity list being used for
combinational processes, i.e. example 1, but not example 2, can be synthesized.
 But some advanced systems accept example 2, while example 3 is not
permitted in synthesis.
 Example 4 is a clocked process and results in a D-type flip-flop after synthesis.
 Examples 3 and 5 can only be used for simulation, and not for design.
 The wait command is a sequential command, it is not permissible to use wait in
functions, but it can be used in procedures and processes.

41. Behavioral Description of a 3-to-8 Decoder
Except for different
syntax, approach is
not all that different
from the dataflow
version

42. Attributes
• Value kind—A simple value is returned.
• Function kind—A function call is performed to
return a value.
• Signal kind—A new signal is created whose value is
derived from another signal.
• Type kind—A type mark is returned.
• Range kind—A range value is returned.

43. Array Attributes
A can be either an array name or an array type.
Array attributes work with signals, variables, and constants.

44. Signal Attributes
Attributes associated with signals
that return a value
A’event – true if a change in S has just occurred
A’active – true if A has just been reevaluated, even if A does not change

45. Review: Signal Attributes (cont’d)
Attributes that create a signal

46. Example of D flip flop using s’EVENT
1. LIBRARY IEEE;
2. USE IEEE.std_logic_1164.ALL;
3. ENTITY dff IS
4. PORT( d, clk : IN std_logic;
5. PORT( q : OUT std_logic);
6. END dff;
7. ARCHITECTURE dff OF dff IS
8. BEGIN
9. PROCESS(clk)
10. BEGIN
11. IF ( clk = ’1’) AND ( clk’EVENT ) THEN
12. q <= d;
13. END IF;
14. END PROCESS;
15. END dff;

47. Timing Model
• VHDL uses the following simulation cycle to
model the stimulus and response nature of
digital hardware
Delay
Start Simulation
Update Signals Execute Processes
End Simulation

48. Delay Types
• All VHDL signal assignment statements
prescribe an amount of time that must transpire
before the signal assumes its new value
• This prescribed delay can be in one of three
forms:
 Transport -- prescribes propagation delay only
 Inertial -- prescribes propagation delay and minimum input pulse width
 Delta -- the default if no delay time is explicitly specified
Input
delay
Output

49. Transport Delay
• Transport delay must be explicitly specified
 I.e. keyword “TRANSPORT” must be used
• Signal will assume its new value
after specified delay
-- TRANSPORT delay example
Output <= TRANSPORT NOT Input AFTER 10 ns;
Input Output
0 5 10 15 20 25 30 35
Input
Output

50. Inertial Delay
• Provides for specification propagation delay and input
pulse width, i.e. ‘inertia’ of output:
target <= [REJECT time_expression] INERTIAL waveform;
• Inertial delay is default and REJECT is optional:
Output <= NOT Input AFTER 10 ns;
-- Propagation delay and minimum pulse width are 10ns
Input
Output
0 5 10 15 20 25 30 35
Input Output

51. Inertial Delay (cont.)
• Example of gate with ‘inertia’ smaller than propagation
delay
 e.g. Inverter with propagation delay of 10ns which suppresses
pulses shorter than 5ns
Output <= REJECT 5ns INERTIAL NOT Input AFTER 10ns;
Input
Output
0 5 10 15 20 25 30 35
• Note: the REJECT feature is new
to VHDL 1076-1993

52. Delta Delay
• Default signal assignment propagation delay if no delay
is explicitly prescribed
 VHDL signal assignments do not take place immediately
 Delta is an infinitesimal VHDL time unit so that all signal
assignments can result in signals assuming their values at a
future time
 E.g.
Output <= NOT Input;
-- Output assumes new value in one delta cycle
• Supports a model of concurrent VHDL process
execution
 Order in which processes are executed by simulator does not
affect simulation output

53. Transport and Inertial Delay

54. Simulation Example

55. D f/f WITH PRESET AND CLEAR
1. ENTITY dtype IS
2. PORT(clk, d, clr, pre : IN std_logic;
3. q, n_q : OUT std_logic);
4. END dtype;
5. ARCHITECTURE behav OF dtype IS
6. SIGNAL temp_q : std_logic; -- internal signal
7. BEGIN
8. PROCESS (clk, clr, pre)
9. BEGIN
10. IF clr = ‘1’ THEN -- clear operation
11. temp_q <= ‘0’;
12. ELSIF pre = ‘1’ THEN -- preset operation
13. temp_q <= ‘1’;
14. ELSIF clk’EVENT AND clk = ‘1’ THEN -- clock
15. temp_q <= d;
16. END IF;
17. END PROCESS;
18. q <= temp_q;
19. n_q <= NOT temp_q;
20. END behav;