[SOLVED] Ecse 222 vhdl assignment #5: storage elements, clock divider, and counters

In this VHDL assignment, you will learn how to describe storage elements and sequential logic circuits in VHDL.
Then, you will design a 3-bit up-counter and simulate the counter using ModelSim.
If you need any help regarding the lab materials, you can
• Ask the TA for help during lab sessions and office hours.
• Refer to the text book. In case you are not aware, Appendix A “VHDL Reference” provides detailed information on VHDL.
• You can also refer to the tutorial on Quartus and ModelSim provided by Intel (click here for Quartus and
here for ModelSim).
• Refer to the DE1-SoC User Manual (in the Content tab on myCourses).
It is highly recommended that you first try to resolve any issue by yourself (refer to the textbook and/or the
multitude of VHDL resources on the Internet). Syntax errors, especially, can be quickly resolved by reading the
error message to see exactly where the error occurred and checking the VHDL Reference or examples in the textbook
for the correct syntax.
4 VHDL Description of Storage Elements
In the previous VHDL assignments, you have learned how to use sequential statements to describe the behavior of
combinational circuits inside the process block. Sequential statements within the process block can also be used to
describe sequential circuits such as storage elements. In digital systems, we have two types of memory elements:
latches and flip-flops (FFs). Latches are memory elements that immediately reflect any changes in the input signal
to the output signal when the level of the control signal (clk) is high. As such, latches are usually referred to as
level-triggered memory elements. Alternatively, FFs change their state when the control signal goes from either
high to low or from low to high. Note that FFs working with a control signal that goes from high to low or from
low to high are called, respectively, negative-edge-triggered and positive-edge-triggered FFs. In digital systems,

edge-triggered flip-flops are superior to level-triggered latches since FFs are more robust. For example, noise can
easily disrupt the output of a latch when the control signal is high. On the other hand, the output of FFs can be
disrupted only in presence of noise at the edge transition of the control signal. It is highly recommended, therefore,
to use FFs when designing sequential circuits.
In VHDL, process blocks are used to describe FFs. Since FFs set their state at the edge of the control signal, we need
a statement to detect the edge transition of the control signal. This can be simply done by using a IF-THEN-ELSE
statement. Assuming that clk is the control signal of a FF, a positive edge transition (i.e., a transition from ‘0’ to
‘1’) of the clk signal can be detected by the following statement: RISING_EDGE (clk). Similarly, a negative edge
transition (i.e., a transition from ‘1’ to ‘0’) can be detected by the following statement FALLING_EDGE (clk). For
example, the following process block describes a positive-edge-triggered DFF.
PROCESS ( clk )
BEGIN
IF RISING_EDGE( clock ) THEN
Q <= D;
END IF;
END PROCESS;
Since the state Q (output) changes only as a result of a positive clock edge, only the clk signal is listed in the
sensitivity list of the process. Note that there are additional ways to detect a clock edge, such as IF clk’EVENT AND
clk = ‘1’ or WAIT UNTIL clk’EVENT AND clk = ‘1’ statements. The syntax clk’EVENT uses a VHDL construct
called an attribute. An attribute refers to a property of an object such as a signal. In this case, the ’EVENT attribute
refers to any change in the clock signal. These two statements (i.e., clk’EVENT AND clk = ‘1’ and WAIT UNTIL
clk’EVENT AND clk = ‘1’) are described in detail in Examples 7.2 and 7.3 of the textbook. Note that if we use
the WAIT UNTIL statement, then the sensitivity list of the process block should be empty.
So far, we have described a positive-triggered flip-flop in VHDL. Now, let us describe a positive-triggered flip-flop
with asynchronous active-low reset (also known as clear) and asynchronous active-low set signals. When the reset
signal is ‘0’, the output of the FF is immediately set to ‘0’, regardless of the value of the control signal (i.e., clk).
Similarly, when the set signal is ‘0’, the output of the FF is immediately set to ‘1’, regardless of the value of the
control signal (i.e., clk). The sensitivity list of the process contains, therefore, the clk, reset, and set signals, since
these three signals trigger a change in the output of the FF (positive clock edge and low-activated set and reset
signals. A positive-triggered FF with asynchronous active-low reset and set signals can be described in VHDL as
follows:
PROCESS ( clk, reset, set )
BEGIN
IF reset = ‘0’ THEN
Q <= ‘0’;
ELSIF set = ‘0’ THEN
Q <= ‘1’;
ELSIF RISING_EDGE( clk ) THEN
Q <= D;
END IF;
END PROCESS;
Note that since we check the reset signal first, it has priority over the other two signals, i.e., set and clk. Similarly,
set has priority over the clk signal.

If we check for the reset and set signals at the positive edge of the clock, we obtain a positive-triggered flip-flop
with synchronous active-low reset and set signals as follows:
PROCESS ( clk )
BEGIN
IF RISING_EDGE( clk ) THEN
IF reset = ‘0’ THEN
Q <= ‘0’;
ELSIF set = ‘0’ THEN
Q <= ‘1’;
ELSE
Q <= D;
END IF;
END IF;
END PROCESS;
Note that we do not include the reset and set signals in the sensitivity list of a flip-flop circuit with synchronous
reset and set signals.
5 Design of a Storage Element
In this assignment, you are asked to design a JKFF. The inputs to the FF are J, K, and clk. The output of the FF
is Q. The operation of a JKFF is described in Section 7.6 of the textbook. Use the following entity declaration for
your implementation of the storage element circuit.
l i b r a r y IEEE;
u s e IEEE.STD_LOGIC_1164.ALL;
u s e IEEE.NUMERIC_STD.ALL;
e n t i t y firstname_lastname_jkff i s
P o r t ( clk : i n s t d _ l o g i c ;
J : i n s t d _ l o g i c ;
K : i n s t d _ l o g i c ;
Q : o u t s t d _ l o g i c ;
end firstname_lastname_jkff ;
where firstname_lastname in the name of the entity is the name of one of the students in your group.
To describe your circuit in VHDL, use a single process block. Once you have your circuit described in VHDL, you
should simulate it. Write a testbench code and perform a functional simulation for your VHDL description of the
counter. When writing a testbench for sequential circuits, a clock signal is required within the testbench. One way
to generate the clock signal in testbench, is provided below.
clock_generation : p r o c e s s
b e g i n
clk <= ‘1’;
w ai t f o r clock_period /2;
clk <= ‘0’;
w ai t f o r clock_period /2;
end p r o c e s s clock_generation ;
Note that the “clock_period” parameter should be replaced with desired clock period value. In this assignment, we
will use a clock period of 10 ns. Due to the absence of any indefinite “wait” statement in the “clock_generation”
process, you cannot use the run -all command in ModelSim; you must instead explicitly determine a simulation
duration (e.g., run 100 ns).
Once you have described your circuit in VHDL, you should write a testbench and simulate the circuit in ModelSim.
Make sure that all possible inputs to the JKFF are verified in the simulation.

6 Counters
A counter is a special sequential circuit. When counting up (by one), we require a circuit capable of “remembering”
the current count and increase it by 1 the next time we request a count. When counting down (by one), we require
a circuit capable of “remembering” the current count and subtracting 1 the next time we request a count. Counters
use a clock signal to keep track of time. In fact, each increment (or decrement) occurs when one clock period has
passed. To design an up-counter counting in increments of 1 second, we will first design a 3-bit up-counter counting
at the positive edge of the clock with an asynchronous reset (which should be active low) and an enable signal
(which should be active high). The counter counts up when the enable signal is high. Otherwise, the counter holds
its previous value. Use the following entity declaration for your VHDL description of the counter:
l i b r a r y IEEE;
u s e IEEE.STD_LOGIC_1164.ALL;
u s e IEEE.NUMERIC_STD.ALL;
e n t i t y firstname_lastname_counter i s
P o r t ( enable : i n s t d _ l o g i c ;
reset : i n s t d _ l o g i c ;
clk : i n s t d _ l o g i c ;
count : o u t s t d _ l o g i c _ v e c t o r (2 downto 0) );
end firstname_lastname_counter ;
where firstname_lastname in the name of the entity is the name of one of the students in your group.
Note that a 3-bit counter counts from 0 to 7. When the current count reaches 7, the next count will automatically
wrap around back to 0.
Once you have your circuit described in VHDL, you should simulate it. Write a testbench code and perform a
functional simulation for your VHDL description of the counter.
7 Clock Divider
A clock divider is a circuit that generates a signal that is asserted once every T clock cycles. This signal can be
used as a condition to enable the counter that you designed in Section 6. An example of the clock and output (i.e.,
“enable”) waveforms for T = 4 is:
Clock
Enable
3 2 1 0 3 2 1 3
Implementing the clock divider circuit requires a counter counting clock periods. The counter counts down from
T −1 to 0. Upon reaching the count of 0, the clock divider circuit outputs/asserts 1 and the count is reset to T −1.
For other values of the counter, the output signal of the clock divider circuit remains 0. In this lab, we want to
design a counter counting in increments of 1 second. In other words, we need to assert an enable signal every 1
second. First, find the value of T for the clock divider circuit to generate an enable signal every 1 second. Note
that the PLL, the device which supplies the clock for your design on the DE1-SoC board, works at a frequency of
50 MHz. Describe the clock divider circuit in VHDL using the following entity declaration:

l i b r a r y IEEE;
u s e IEEE.STD_LOGIC_1164.ALL;
u s e IEEE.NUMERIC_STD.ALL;
e n t i t y firstname_lastname_clock_divider i s
P o r t ( enable : i n s t d _ l o g i c ;
reset : i n s t d _ l o g i c ;
clk : i n s t d _ l o g i c ;
en_out : o u t s t d _ l o g i c );
end firstname_lastname_clock_divider ;
where firstname_lastname in the name of the entity is the name of one of the students in your group.
Hint: The following figure shows an example of the clock divider circuit. Also, note that the down-counter inside
the clock divider circuit is different from the up-counter that you designed in Section 6, and that en_out is the
NOR of the bits in the counter value.
Down Counter
From T-1 to 0
clk
reset
enable
en_out
Once you have your circuit described in VHDL, write a testbench code and perform a functional simulation for
your VHDL description of the clock divider.
8 3-bit Up-Counter Counting in Increments of 1 Second
In this part, you will design a simple counter circuit that counts in increments of 1 second using the counter and
clock divider circuits.
Reset is an asynchronous active-low input. The normal condition of the reset is high (‘1’). If reset goes low (‘0’),
the output of the counter should be 0 as long as reset remains low. Once reset goes back high, the counter should
start counting. Enable is a synchronous active-high input. When enable is high (‘1’) the counter counts every 1
second, the circuit will hold and display the current count otherwise.
You will need to create one instance of your firstname_lastname_counter and seven_segment_decoder you created
in VHDL Assignment #4. Since we measure time in increments of 1 second, the counter that you designed in
Section 6 increments only when the output signal of the clock divider circuit becomes high. The following figure
shows the high-level architecture of the circuit.

Describe the circuit counting at every 1 second in VHDL using the following entity declaration:
l i b r a r y IEEE;
u s e IEEE.STD_LOGIC_1164.ALL;
u s e IEEE.NUMERIC_STD.ALL;
e n t i t y firstname_lastname_wrapper i s
P o r t ( enable : i n s t d _ l o g i c ;
reset : i n s t d _ l o g i c ;
clk : i n s t d _ l o g i c ;
HEX0 : o u t s t d _ l o g i c _ v e c t o r (6 downto 0) );
end firstname_lastname_wrapper ;
where firstname_lastname in the name of the entity is the name of one of the students in your group.
You will now write a testbench to verify your circuit using the ModelSim simulation. Generate a simulation that
shows the functionality of the counter and the reset and enable inputs. Clearly show on the waveform that the
counter counts at 1 second intervals.
9 Questions
1. Briefly explain your VHDL code implementation of all circuits.
2. Provide waveforms for each of the individual storage elements and sequential circuits (each section) and for
the 3-bit up-counter.
3. Perform timing analysis (slow 1,100 mV, 85C model) of the 3-bit up counter and find the critical path(s) of
the circuit. What is the delay of the critical path(s)?
4. Report the number of pins and logic modules used to fit your 3-bit up counter design on the FPGA board.
10 Deliverables
You are required to submit the following deliverables on MyCourses. Please note that a single submission is required
per group (by one of the group members).
• Lab report. The report should include the following parts: (1) Names and McGill IDs of group members,
(2) an executive summary (short description of what you have done in this VHDL assignment), (3) answers to
all questions in previous section (if applicable), (4) legible figures (screenshots) of schematics and simulation
results, where all inputs, outputs, signals, and axes are marked and visible, (5) an explanation of the results
obtained in the assignments (mark important points on the simulation plots), and (6) conclusions. Note –
students are encouraged to take the reports seriously, points will be deducted for sloppy submissions. Please
also note that even if some of the waveforms may look the same, you still need to include them separately in
the report.
• Project files. Create a single .zip file named VHDL#_firstname_lastname (replace # with the number of the
current VHDL assignment and firstname_lastname with the name of the submitting group member). The
.zip file should include the working directory of the project.