This presentation discusses the sequential statements in VHDL and other aspects of VHDL behavioral description. All codes in simulated in Xilinx ISE.

1. VHDL BEHAVIORAL DESCRIPTION
Sudhanshu Janwadkar, TA, SVNIT,
21st-24th Aug 2017

2. Features of Behavioural Description
 The behavioral description describes the system by
showing how the outputs behave according to changes in
the inputs.
 In this description, we need not know the logic diagram
of the system or its components, what must be known is
how the outputs behaves in response to changes in input
 In VHDL, the major behavioral description statement is
'process'. The statements inside the process statement
are sequential, however process statement itself is
concurrent.

3. Process statement
A process statement contains sequential statements that describe
the functionality of a portion of an entity in sequential terms
Syntax:
[ process-label: ] process [ ( sensitivity-list ) ]
[process-item-declarations]
begin
.
.
sequential-statements;
.
.
.
end process [ process-label];
variable-assignment-statement,
signal-assignment-statement,
wait-statement,
if-statement,
case-statement,
loop-statement,
null-statement,
exit-statement,
next-statement,
assertion-statement,
procedure-call-statement,
return-statement.

4. Process Statement
 A set of signals that the process is sensitive to is defined
by the sensitivity list. In other words, each time an
event occurs on any of the signals in the sensitivity list,
the sequential statements within the process are
executed in a sequential order, that is, in the order in
which they appear.
 The process then suspends after executing the last
sequential statement and waits for another event to
occur on a signal in the sensitivity list.
 Items declared in the item declarations part are
available for use only within the process.

5. Process Statement
Summarising,
 A process is activated when a signal in the
sensitivity list changes its value
 Its statements will be executed sequentially
until the end of the process

6. Process Statement
Example:
P1: process(a,b,c)
begin
y1 <= a and b and c;
y2 <= a or b;
end process;
The process statement has a label P1(optional) and its sensitivity list has signals
a, b and c. As soon as there is an event on a, b or c, the process statement will be
executed. It must be noted that the two signal assignment statements inside
process will be executed sequentially.

7. Process Statement
For a combinational circuit, it is advisable that the all
inputs should be included in the sensitivity list.
Example:
P1: process(a)
begin
y1 <= a and b and c;
y2 <= a or b;
end process;
The process statement will be executed only when there is an event on Signal a. The
events on signals b and c will not be captured.

8. Process Statement
Write a VHDL process statement which counts the number of
transitions on a signal.
process (A)
variable count: INTEGER := 0;
begin
count := count+1;
end process;
This example is to count number of signal transition on signal ‘A’. At start of
simulation, the process is executed once. The variable count gets initialized
to 0 and then incremented by 1. After that, each time an event occurs on
signal A, the process is activated and the single variable assignment
statement is executed and the value of count is incremented.
Note the position of the
Variable declaration
statement.
Any variable that is created
in one process cannot be
used in another process

9. Process Statement
Write a VHDL behavioural code to generate a clock
signal of period 10ns.

10. How are sequential statements
executed inside a Process statement?
 All statements inside the body of the process are
executed sequentially
 The execution of a signal-assignment statement has
two phases: calculation and assignment.
 Sequential execution, here, refers to sequential
calculation
 i. e The calculation of a subsequent statement will
not wait until the preceding statement is assigned, it
will wait only till the calculation of preceding
statement is completed.

11. How are sequential statements
executed inside a Process statement?
Example:
process (I1,I2)
begin
O1 <= I1 xor I2 after 10ns; -- Statement 1
O2 <= I1 and I2 after 10ns; -- Statement 2
end process;
To understand sequential execution, assume that at T = T0, I1 changes
from 0 to 1, while I2 stays at 1. This change constitutes an event on I1 and
it activates the process. Statement 1 is calculated as O1 =(I1 xor I2) = ( 1
xor 1)=0. This new value 0 is not assigned to O1 at T0, but rather at T0
+10ns. However, as soon as calculation of O1 = (I1 xor I2) is completed, at
same time instant T0, statement 2 is calculated by using values of I1 and
I2 at T0, so that O2 = (1 and 1) = 1. The value of 0 is assigned to O2 at T0
+ 10ns

12. How are sequential statements
executed inside a Process statement?

13. How are sequential statements
executed inside a Process statement?
 For the above example, both data-flow and
behavioural description result in the same output
for the signal assignments
 Is this always true? No
 This is not true when a signal appears on the right
side of a signal assignment statement and left side
of another signal assignment.
 We prefer to use variable assignment statements
inside the process statement. -> Why?

14. Variable Assignment Statement
Variables can be declared and used inside a process statement.
Syntax for variable declaration:
variable [variable_Identifier] : [type] := [optional_initial_value]
Example: variable count : integer := 0;
Syntax for variable assignment:
variable-object := expression;
Example: Count := count + 1;
 The expression is evaluated when the statement is executed and
the computed value is assigned to the variable object
instantaneously, that is, at the current simulation time

15. Signals versus Variable
-Differences in Syntax
 Variables need to be defined after the keyword process but
before the keyword begin. Signals are defined in the
architecture before the begin statement.
 Variables are assigned using the := assignment symbol.
Signals are assigned using the <= assignment symbol.
 Variables can only be used inside processes, signals can be
used inside or outside processes.
 Any variable that is created in one process cannot be used
in another process, signals can be used in multiple
processes though they can only be assigned in a single
process.

16. Why are variable assignments suited
inside process statement?
 All statements inside the process statement are executed
sequentially.
 To understand the trouble of using signal assignments inside
process statement, consider this example:
...
signal x,y,z : bit;
...
process (y)
begin
x<=y; -- st1
z<=not x; --st2
end process;
If the signal y changes then an event will be
scheduled on x to make it the same as y. Also, an
event is scheduled on z to make it the opposite of x.
It is now expected that the value of z must be
opposite to that of y.
The question is, will the value of z be the opposite of
y? NO!!

17. Why are variable assignments suited
inside process statement?
...
signal x,y,z : bit;
...
process (y)
begin
x<=y; -- st1
z<=not x; --st2
end process;
• If there is an event on y at T0, the value of x is
calculated as x=y at T0, but the value is assigned to
X at T0 + D.
• Next, at T0 itself, next statement is executed. At
this moment, x is not yet updated. This results in
calculation of z with previous value of x itself.
• Thus at T0+ D, the value of Z will be same as that
of y, instead being opposite of y.

18. Why are variable assignments suited
inside process statement?

19. Why are variable assignments suited
inside process statement?
Next, consider the example using variable assignment:
Example:
…
process (y)
variable x, z : bit;
begin
x:=y;
z:=not x;
end process;
........
The value of the variable z would be the opposite of
the value of y because the value of the variable x is
changed immediately.

20. Why are variable assignments suited
inside process statement?

21. Concurrent v/s sequential signal
assignment
 Signal assignment statements which occur in the body
of a process statements are sequential signal
assignment statements,
while
 Signal assignment statements that appear outside the
process are concurrent signal assignment statements.
1

22. Concurrent v/s sequential signal
assignment
 Concurrent signal assignment statements are event-
triggered, that is they are executed whenever there is
an event on a signal that appears in its expression,
while
 Sequential signal assignment statements are not event
triggered and are executed in sequence in relation to
other sequential statements that appear within the
process.
2

23. Concurrent v/s sequential signal
assignment
y1 <= not a; -- st1
y2 <= not b; -- st2
p1: process(b)
y1 <= not a; -- st1
y2 <= not b; -- st2
end process p1;
At time instant T1, lets say signal a undergoes a 0-> 1 transition, while b retains its value.
At some other time instant T2 (T2> T1), signal b undergoes 0->1 transition and signal a
undergoes 1-> 0 transition
How is this executed by Concurrent and sequential statements?
Example:
Concurrent Sequential I Sequential II
p1: process(a)
y1 <= not a; -- st1
y2 <= not b; -- st2
end process p1;

24. Concurrent v/s sequential signal
assignment
 Concurrent Execution
At time instant T1, all
signal assignment
statements which have
signal a as their driver
will be executed
concurrently. No other
signal assignment
takes place.

25. Concurrent v/s sequential signal
assignment
 Sequential Execution-I
In sequential
statements, whenever
there is an event on
sensitivity list, all the
statements in the
process are executed,
irrespective of the
events on their drivers.

26. Concurrent v/s sequential signal
assignment
 Sequential Execution-II

27. Concurrent v/s sequential signal
assignment
Q
 How will the following sets of VHDL statements be executed?

28. Concurrent v/s sequential signal
assignment
A
Assume 0->1 transition on B and draw waveforms!!

29. Sequential Statements in VHDL
 Wait statement- wait on, wait for, wait until
 If statement – if, if-else, if-elsif-else
 Case statement
 Loop statement – for, while
 Null statement
 Exit statement
 Next statement
 Assert & report statement
 Procedure and Return statement

30. Wait Statement
There are three basic forms of the wait statement:
 wait on sensitivity-list;
 wait until boolean-expression ;
 wait for time-expression ;

31. Wait Statement
wait on (sensitivity-list) statement
The wait on statement suspends the execution of a process
till an event on the sensitivity list occurs.
Example:
wait on a, b, c;
In above statement, the execution of the wait statement
causes the process to suspend and then it waits for an
event to occur on signals a, b, or c. Once that happens, the
process resumes execution from the next statement
onwards.

32. Wait Statement
wait until (boolean_expression) statement
The wait until statement suspends the execution of a process
until the boolean express evaluates to be true.
Example:
wait until a= b;
In above statement, the process is suspended until the
specified condition becomes true. When an event occurs
on signal a or b, the condition a= b is evaluated and if it is
true, the process resumes execution from the next
statement onwards, otherwise, it suspends again

33. Wait Statement
wait for (time expression) statement
The wait until statement suspends the execution of a
process until the boolean express evaluates to be true.
Example:
wait for 10ns;
When the wait statement is executed, say at time T, the
process suspends for 10 ns and when simulation time
advances to T+10 ns, the process resumes execution
from the statement following the wait statement

34.  Write a VHDL behavioural code to generate a
clock signal of period 20ns using wait statement
Note that it is
legitimate to use
signal inside the
process in this
example, as the
signal clock
appears only on
one side of the
signal assignment
statement
Also, note
that a
process
cannot have
both
sensitivity
list and wait
statement.

35.  Write a VHDL behavioural code to generate a
clock signal of period 20ns using wait statement
• Using variable
assignment
statements.
• Note the
changes in
syntax

36. What if a process doesn’t have a
sensitivity list?
 It is possible for a process not to have an explicit
sensitivity list. In such a case, the process may have one
or more wait statements.
 It must have at least one wait statement, otherwise, the
process will never get suspended and would remain in
an infinite loop during the initialization phase of
simulation.
 It is an error if both the sensitivity list and a wait
statement are present within a process.
 The presence of a sensitivity list in a process implies the
presence of an implicit "wait on sensitivity-list"
statement as the last statement in the process.

37.  Write the below process statement without
using sensitivity list

38. If statement
An if statement selects a sequence of statements for execution based on
the value of a condition. The condition can be any expression that
evaluates to a boolean value.
Syntax:
if boolean-expression then
--sequential-statements
elsif boolean-expression then
--sequential-statements
-- elsif clause; if stmt can have 0 or more elsif clauses
else
-- else clause.
-- sequential-statements
end if;

39. If statement
 Semantics:
The if statement is executed by checking each
condition sequentially until the first true condition is
found; then, the set of sequential statements
associated with this condition is executed. The default
action is listed under else clause.
 The if statement can have zero or more elsif clauses
and an optional else clause.
 if statement is a sequential statement
 nesting of if statements is allowed

40. Example:
4 X 1 Multiplexer using if-else

41. case statement
 case statement selects one of the branches for execution
based on the value of the expression.
 The expression value must be of a discrete type or of a
one-dimensional array type.
 Choices may be expressed as single values, as a range of
values, by using I (vertical bar: represents an "or"), or by
using the others clause.
 All possible values of the expression must be covered in
the case statement. “
 The others clause can be used as a choice to cover the
"catch-all" values and, if present, must be the last branch in
the case statement.

42. case statement
Syntax:
case expression is
when choices => sequential-statements -- branch #1
when choices => sequential-statements -- branch #2
-- Can have any number of branches.
[ when others => sequential-statements ] -- last branch
end case;

43. Example:
4 X 1 Multiplexer using case statement

44. Null statement
 The statement
null;
is a sequential statement that does not cause any
action to take place and execution continues
with the next statement.

45. Example : D-Flip flop with
asynchronous reset

47. Example: Leading Zeros
Refer Class notes for meaning of
code and alternate approach
If loop runs from 8
to 0, one can find
number of trailing
zeros
Instead of exit, if
next is used, it can
be used to find total
number of 0’s. This
can also be used to
check parity.

48. Example: Parity Detector
Refer Class notes for meaning of
code and alternate approach
How will you find
both odd parity
and even parity
from same code?

49. Example – 4-bit Ripple Carry Adder
This code is easy to understand, but is not very efficient when
comes to hardware implementation. Alternate versions of code
are available online. Can you try?

50. Example: Factorial of Positive Integers
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
entity factorial is
port(N : in natural; z : out natural); -- consider positive integers only
end factorial;
architecture behavioral of factorial is
begin
process (N)
variable y, i : natural;
begin
y := 1;
i := 0;
while (i < N) loop
i := i + 1;
y := y * i;
end loop;
z <= y;
end process;
end factorial;
Run the code in Xilinx and find the RTL
description of this circuit