Advanced FPGA Based System Design
Lecture-9 & 10
VHDL
Sequential Code
By: Dr Imtiaz Hussain
imtiaz.hussain@faculty.muet.edu.pk
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Contents
• Introduction
• Process
• Signals & Variables
• IF
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Introduction
• VHDL Code is Inherently Concurrent.
• PROCESSES, FUNCTIONS, and PROCEDURES are the only
sections of code that are executed sequentially.
• One important aspect of sequential code is that it is not
limited to sequential logic.
• With it we can build sequential circuits as well as
combinational circuits.
• Sequential code is also called behavioral code.
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Introduction
• The statements discussed in this lecture are all
sequential, that is, allowed only inside
ROCESSES, FUNCTIONS, or PROCEDURES.
• They are: IF, WAIT, CASE, and LOOP.
4
Process
• The statements discussed in this lecture are all
sequential, that is, allowed only inside ROCESSES,
FUNCTIONS, or PROCEDURES.
• They are: IF, WAIT, CASE, and LOOP.
• A PROCESS is a sequential section of VHDL code.
• It is characterized by the presence of IF, WAIT, CASE, or
LOOP, and by a sensitivity list (except when WAIT is used).
• A PROCESS must be installed in the main code, and is
executed every time a signal in the sensitivity list changes
(or the condition related to WAIT is fulfilled).
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Process
• Its syntax is shown below.
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Process Example
• DFF with Asynchronous Reset
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Signals and Variables
• VHDL has two ways of passing non-static values
around: by means of a SIGNAL or by means of a
VARIABLE.
• A SIGNAL can be declared in a PACKAGE, ENTITY or
ARCHITECTURE (in its declarative part).
• While a VARIABLE can only be declared inside a piece
of sequential code (in a PROCESS, for example).
• Therefore the value of the SIGNAL can be global, the
VARIABLE is always local.
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Signals and Variables
• The value of a VARIABLE can never be passed out of the
PROCESS directly.
• If necessary, then it must be assigned to a SIGNAL.
• On the other hand, the update of a VARIABLE is immediate,
that is, we can promptly count on its new value in the next
line of code.
• That is not the case with a SIGNAL (when used in a
PROCESS), for its new value is generally only guaranteed to
be available after the conclusion of the present run of the
PROCESS.
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Signals and Variables
• The assignment operator for a SIGNAL is ‘‘<=’’ (ex:
sig <= 5).
• And for a VARIABLE it is ‘‘:=’’ (ex: var := 5).
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IF
• As mentioned earlier, IF, WAIT, CASE, and LOOP are the
statements intended for sequential code.
• Therefore, they can only be used inside a PROCESS,
FUNCTION, or PROCEDURE.
• The natural tendency is for people to use IF more than
any other statement.
• The syntax of IF is shown below.
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IF (Example)
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Example: One Digit Counter
• 1-digit decimal counter (0 to 9 to 0).
• It contains a single-bit input (clk) and a 4-bit output
(digit).
• A variable, temp, was employed to create the four
flip-flops necessary to store the 4-bit output signal.
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Example: One Digit Counter with Asynchronous
Reset I/P
clk
C
O
U
N
T
E
R
digit(3:0)
rst
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Example: Shift Register
• Figure shows a 4-bit shift register.
• The output bit (q) must be four positive clock
edges behind the input bit (d).
• It also contains an asynchronous reset, which
must force all flip-flop outputs to ‘0’ when
asserted.
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Example: Shift Register
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Example: Shift Register
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WAIT
• Wait Statement has three different formats.
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WAIT UNTIL
• The WAIT UNTIL statement accepts only one
signal, thus being more appropriate for
synchronous code than asynchronous.
• Since the PROCESS has no sensitivity list in this
case, WAIT UNTIL must be the first statement
in the PROCESS.
• The PROCESS will be executed every time the
condition is met.
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WAIT UNTIL Example
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WAIT ON
• WAIT ON, on the other hand, accepts multiple
signals.
• The PROCESS is put on hold until any of the
signals listed changes.
• In the example below, the PROCESS will
continue execution whenever a change in rst or
clk occurs.
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WAIT ON
• In the example below, the PROCESS will
continue execution whenever a change in rst
or clk occurs.
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WAIT FOR
• Finally, WAIT FOR is intended for simulation
only (waveform generation for test benches).
• Example: WAIT FOR 5ns;.
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Example: DFF
• DFF with Asynchronous Reset using WAIT ON
instead of IF.
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Example: One Digit Counter
• Use WAIT UNTIL to design a one digit counter
clk
C
O
U
N
T
E
R
digit(3:0)
rst
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To download this lecture visit
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END OF LECTURE-9-10
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