LAB 3: A Debounced Switch and Counters

University of Florida EEL 3701 
Department of Electrical & Computer Engineering Revision
Page 1/7 LAB 3: A Debounced Switch and Counters
OBJECTIVES
To understand the design, function and operation of a
debounced switch and simple counter circuits.
MATERIALS
• Your entire lab kit (including your DAD/NAD)
• UF's DAD/NAD Waveforms 2015 Tutorial
• Suggested Quartus Components
• In “others | maxplus2” library
o 7474: Dual D-flip flops
• In “primitives | storage” library
o dff
• In “primitives | logic” library
o not, and2, or2, bor2, etc.
• In “primitives | pin” library
o input, output
• In “primitives | other” library
o vcc, gnd
You must adhere to the Lab Rules and Policies document for
every lab. Re-read, if necessary. All equations and K-Maps
must be included in your pre-lab document.
INTRODUCTION
NOTES ON UF-3701 BOARD AND QUARTUS
The ground and Vcc connections are the most important one for
the 3701 PLD board. Connect the two ground pin (labeled
GND on your 3701 PLD PCB) to the ground on your
protoboard. Similarly, connect the two power pins (labeled
3.3V on your 3701 PLD PCB) to the power on your breadboard.
Be very careful that you do NOT reverse these pins; if you do,
the chip will be destroyed and you will need to spend a lot of
time and money to correct the problem.
One of the pins labeled C2P (i.e., CLK2P, MAX 10 pin G9) or
C3P (i.e., CLK3P, MAX10 pin F13) should be used for any
clock input. These are the available global clock pins on our
PLD PCB.
Warning: Tri-state all unused PLD pins, as described in the
Quartus Tutorial.
Don’t forget to also ground your DAD; the DAD/NAD’s
ground wire is black.
DEBOUNCED SWITCHES
The switches that you have been using this semester are known
as single-pole-single-throw (SPST) switches. When you move
the SPST switch in a switch circuit from ON to OFF or from
OFF to ON, the resulting output bounces around between low
and high voltages for a short time. If the switch circuit output
is used as a synchronizing signal (such as a clock) in a digital
machine, weird things will happen. If the machine is a counter,
the count may seem to jump wildly. This is obviously
undesirable, so a debouncing circuit must be built. We
discussed several debouncing circuits in class. You will design
and build one of these debounced switch circuits using a singlepole-double-throw (SPDT)
switch (see Figure 0). You will
use this debounced switch circuit
in this lab and rest of the labs this
semester. The SPDT switch has
three aligned pins. The center pin
is connected to one or the other of
the outside pins, depending on
the position of the switch. You
can use your multimeter on the
resistance (Ω) setting to verify
the operation of this switch.
There are two pins on the switch
in addition to the three aligned
pins. These two pins (shown on the middle of the top image in
Figure 0) have no useful electrical purpose and are used for
mounting only. In lab 0, you mounted the SPDT switch onto a
small PCB (printed circuit board) shown in the bottom image
of Figure 0.
Your debounced switch circuit should use the two axial
resistors (sometimes incorrectly
called radial resistors) in your lab
kit. A 1 kΩ ¼ W (which you have
in your lab kit) axial resistor is
shown in Figure 1.
When flipping the SPDT switch, hold the switch down with
another finger. If this switch circuit appears to be bouncing,
you probably have poor soldering; re-solder the SPDT pins on
the small SPDT PCB.
COUNTERS
A synchronous counter is a device that progresses through a
known sequence with every clock input signal. The counter
advances to the next state/number at a rising (or falling) edge of
each clock pulse. The counter sequence is arbitrary, i.e., it may
count up, down, or in some strange sequence. The counter you
will design in this lab will have a custom count sequence with
some special additional inputs.
PRE-LAB REQUIREMENTS
1. Make a debounced switch circuit for your clock input (as
discussed in class) using your SPDT switch and other
circuitry. You may use a NAND chip or a NOR chip in
your design. (Although a 74’74 chip could be used, you are
not allowed to use this in 3701.) Another alternative is to
use the PLD itself to provide the two NAND or two NOR
gates. If you build the debouncer with NAND or NOR
chips, you should place it in a corner of your breadboard.
You are expected to know how to use the PLD for the two
NAND or two NOR gates, i.e., to use only the PLD for your
circuit designs (in addition to resistors, an SPDT switch,
[regular] SPST switches, and LED switches). Hint: This
means that you should use the PLD for the two NANDs
or NORs of the debounced switch circuit in this lab, but
also know how to do it with NAND or NOR chips.
There is no easy way to test your debounced circuit without
an oscilloscope. Luckily, the DAD/NAD has an
Figure 0: SPDT switch
Figure 1: Axial resistor.
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering Revision 3 28-Sep-19
Page 2/7 LAB 3: A Debounced Switch and Counters
oscilloscope function (called Scope). See the Oscilloscope
(Scope) section of the DAD/NAD Tutorial for help in
determining how to measure the switch bouncing.
Quartus’ simulation can NOT be used to simulate the
debounce circuit that was taught in class because there are
no resistor components available in Quartus. A voltmeter
will not help, since the bounce rate is in the order of
milliseconds. Instead, you will test your debounced circuit
with a counter designed in the next part of the pre-lab. This
will also serve as a your first counter design, to assure that
you understand the proper design technique before
attempting a more complex counter later in this lab.
Use your DAD/NAD to measure the bouncing of a normal
(SPST) switch circuit. See the Appendix for information
on settings that may be helpful to observe the switch
bouncing. Move the switch from one position to another
and get a screen shot of the bouncing with an appropriate
time base. Move the switch back to the original position
and get a second screenshot. Move the switch one more
time and get a third screen shot. It may be necessary to try
each of these a few times to see the bouncing. Put at least
three of these screenshots into your lab document and
interpret these images by explaining the number of clocks
that would occur if this switch was connected to a 5-bit
counter that counts from 0 to 31.
Now use your DAD/NAD to measure the possible
bouncing of your debounced circuit (with the SPDT
switch). Move the switch from one position to another and
get a screen shot of the bouncing with an appropriate time
base. Move the switch back to the original position and get
a second screenshot. Put both of these screenshots into
your lab document and interpret these images explaining
the number of clocks that would occur if this switch was
connected to a 5-bit counter that counts from 0 to 31.
2. Design a counter
(shown in Figure 2) to
count through the
sequence 00, 10, 11,
01, 00, … Note that
there is only a single
input (CLK) and two
active-high outputs (Q1 and Q0). (To start the counter at a
known value, use the pre-sets and pre-clears of the two flipflops.)
a. Make a next-state truth table. The “inputs” for this
table are Q1 and Q0; the “outputs” are Q1+ and Q0+.
b. Using D-flip-flops, determine the next state equations
for Di = Qi
+ = f(Q1,Q0). Use K-maps, if necessary, for
each Di to get MSOP or MPOS equations. Note: There
will be two 2-input K-Maps.
c. Design the required counter circuit in Quartus (called
Lab3_2bit_Cnt). (I suggest that you do it first on
paper, but this is not required and will not be
submitted.) I suggest that you use one of the below
two possible D- flip-flops available in Quartus.
i. Use “others | maxplus2 | 7474” for the left item in
Figure 3.
ii. Use “primitives | storage | dff” for the right item in
Figure 3.
d. Simulate the circuit and, as always, annotate this
simulation. Verify that your design counts as required
with each rising CLK edge.
3. Create a component in Quartus for 7-segment Decoder!
Test your 2-bit counter in part 2 with this component.
4. The above items, including circuit schematic (with PLD
pin numbers) and annotated Quartus simulation results
should be part of the submitted lab document. (As usual,
all pre-lab material must be submitted through Canvas
prior to the start of your lab. Be sure to also submit your
archive files.)
5. Test your 2-bit counter design, using your debounced
switch for the clock input. Use your DAD/NAD and
appropriate LED circuits for the count (Q) outputs.
a. Download your counter design to your PLD board
with nothing else connected to the board except
power and ground. Remove power from your
breadboard.
b. Move your PLD board onto a pre-wired area with your
designed debounced CLK input and the two count
outputs. Reconnect power to your breadboard.
c. Toggle (flip) the debounced CLK input switch to
verify that your counter counts as expected. If your
counter output does not exactly match the required
count sequence as you toggle the switch input (but it
worked in simulation), then your debounced switch
circuit is not designed and/or built correctly. If
necessary, verify your debounced switch circuit design
and construction. (The only way to easily test your
debounced switch circuit is with a counter.)
6. Replace the debounced CLK input circuit to your counter
and replace it with a normal (un-debounced) SPST switch
input circuit for the CLK. Write down the outputs with 10
successive clocks. Compare each successive count to what
you should get. How does counting with an un-debounced
clock input compare to counting with a debounced clock
input? Put this info in your submitted lab document.
Figure 2: Simple counter
block diagram.
Q1
Q0
CLK
 Figure 3: Two D-FF available in Quartus.
D FLIP-FLOPS
2D
2PRN
1CLK
1D
1PRN
2CLK
2CLRN
1CLRN
1QN
2Q
2QN
1Q
7474
inst
CLRN
D
PRN
Q
DFF
inst1
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering Revision 3 28-Sep-19
Page 3/7 LAB 3: A Debounced Switch and Counters
7. Design a counter (called Lab3_3bit_Cnt) that will
count forward with the following sequence:
000, 100, 010, 011, 111, 000, …
This counter will also count backward in the reverse order:
000, 111, 011, 010, 100, 000, …
A block diagram for the counter is shown if Figure 4.
Your counter can also pause the counting. These three
modes (forward [F], backward [B], pause) will be
controlled with 2-inputs, F and B. When neither forward
nor backward is true, the counter will ignore the CLK input
and hold its count value, i.e., pause.
F and B should never be simultaneously true, so your
counter should deal with this case in the most cost-effective
fashion, i.e., if you assume that a user will never make both
inputs true, design the most inexpensive circuit that you
can that accomplishes the required goals, i.e., use “don’t
cares.” Note that the counter does not include
Q2Q1Q0=%001, %101 and %110, where % is a prefix for
binary. These three counts should contribute “don’t cares”
in your next-state truth tables and K-maps.
Your counter should have a means to asynchronously set
and clear each bit. SET(L) and CLR(L) are the inputs to
asynchronously set and clear a particular counter bit.
These SET and CLR inputs will allow you to start the
counter at any desired count. (If you initialize your counter
at the count Q2Q1Q0=%001, %101 or %110, the next count
is not specified in the problem description. The next count
will be determined by the values selected for the “don’t
cares” associated with these counts.)
As you may recall, when we first started discussing circuits
with feedback, I stated that with these types of circuits it is
often easier to deal with voltages rather than with logic.
Let me suggest that you design this (and all) counter(s)
with active-high state-bits (to generate the next-state
circuits) and then generate the appropriate output circuits
with the required activation levels. In this case, use activehigh Q2, Q1, and Q0 in your design of the counter next state
circuits. But when creating the final circuit, the outputs
will be as shown in the block diagram of Figure 4.
Finally, the counter should have an additional output
indicating the count is at a “special value,” Sp. Special
should be true only when the count is “111” and F is true
or when the count is “011” when B is true.
a. Make a next-state truth table with the inputs: F, B, Q2,
Q1, Q0 and outputs Q2+ Q1+, Q0+, and Sp. (Ignore the
SET and CLR for the design. These are controlled
directly with the FF set and clear inputs.)
b. Using D-flip-flops, determine the next state equations
for Di = Qi
+=f(F,B,Q2,Q1,Q0). Use a K-map for each
output to determine MSOP or MPOS equations. Note:
There will be three 5-input K-Maps.
c. Determine the equation for the output Sp. Use a Kmap to get an MSOP or an MPOS equation.
d. Design the required counter circuit (called
Lab3_3bit_Cnt) in Quartus. (I suggest that you do
it first on paper, but this is not required and will not be
submitted.) Don’t forget to include the SET and CLR
inputs in your circuit. (In Quartus, choose D-FF’s with
asynchronous Set and Clear inputs. Both of the DFF’s shown in Figure 3 have asynchronous Set and
Clear inputs.)
e. Simulate the circuit and add annotations.
i. Verify that your design counts forward, counts
backwards and holds the count with the appropriate
input combinations.
ii. Verify that each bit can be set and cleared by using
the SETi and CLRi (i=0,1,2) inputs.
iii. What is the next count for each value of F and B
for Q2Q1Q0=%101 and %110?
iv. As always, include the circuit schematic (with PLD
pin numbers) and annotated Quartus simulation
results in your lab document. (Also as usual,
submit your archived Quartus file.)
f. Build this circuit on your breadboard. (You can undo
the 2-bit counter if you would like to, since you will
not demo this in lab.) Reprogram your PLD board,
and verify that this counter operates properly. Use a
debounced-switch circuit for the CLK input. Use
appropriate switch circuits for the other inputs and
your DAD/NAD for the count (Q2, Q1, Q0) and Sp
outputs. (Note that the DAD/NAD does not deal
directly with active-low outputs, so you will either
have to interpret the active-low outputs in reverse or
provide the active-high versions as well as the activelow outputs for the DAD/NAD.) In addition to using
the DAD/NAD for the outputs, you must also use the
7-segment display (with the Hex to-7-segment
Decoder designed in Lab 2) for the Q outputs. Use the
active-high versions of Q to directly view decimal
version of the count values.
8. As always, put your design and simulation in your lab
document and submit this file along with your design
archives.
9. Now re-design the counter using a T-FF for bit 1 (the most
significant bit) and a JK-FF for bit 0 (the least significant
Figure 4: Forward/Back counter block diagram.
3
SET
CLR
F
B
3
Q2
Q1
Q0
Sp
CLK
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering Revision 3 28-Sep-19
Page 4/7 LAB 3: A Debounced Switch and Counters
bit). This new design will require that you add to your nextstate truth table from part 2a, determine equations for the
T1, J0, and K0 inputs, draw and simulate the new circuit
diagram in Quartus, and verify with the simulation that it
counts properly. You do NOT need to build/demo this
circuit on your breadboard.
Note: Since you may find that you need to make a change with
one or both of your counters during lab, leave a spot to program
the PLD on a corner of your bread board.
IN-LAB
1. As always, bring a printout of your Summary document to
lab.
2. Demonstrate your 3-bit forward/back counter design.
a. Verify that your design counts forward, counts
backward, and holds the count with the appropriate
input combinations.
b. Verify that each bit can be set and cleared by using the
SETi and CLRi (i=0,1,2) inputs.
3. Now replace the debounced CLK input with a normal (nondebounced) switch circuit. Demonstrate your 3-bit counter
design with the non-debounced CLK.
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering Revision 3 28-Sep-19
Page 5/7 LAB 3: A Debounced Switch and Counters
APPENDIX
In order to see the switch bouncing, use the following
DAD/NAD settings.
• Time Base: 50 us/div (or 20 us/div)
• Offset: 0
• Level: 1.5 V
• Condition: Either
• Mode: Repeated, Normal
• Range: 1 V/div
Figure A.1 shows the time base at 1 ms/div. Note that the
bouncing is apparent, but the amount of bouncing cannot be
determined because of the too large time base.
Figures A.2 and A.3 show bouncing with the time base at
50 us/div and 20 us/div, respectively.
A recording of switch bouncing with different time base
settings is available at the following location on our class
website: https://mil.ufl.edu/3701/docs/Bouncing.mov.
Figure A.1: Bouncing with time base of 1 ms/div.
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering Revision 3 28-Sep-19
Page 6/7 LAB 3: A Debounced Switch and Counters
Figure A.2: Bouncing with time base of 50 us/div.
University of Florida EEL 3701 — Fall 2019 Dr. Eric M. Schwartz
Department of Electrical & Computer Engineering Revision 3 28-Sep-19
Page 7/7 LAB 3: A Debounced Switch and Counters
Figure A.3: Bouncing with time base of 20 us/div.