Laboratory Exercise 1
Switches, Lights, and Multiplexers
The purpose of this exercise is to learn how to connect simple input and output devices to an FPGA chip and
implement a circuit that uses these devices. We will use the switches SW17−0 on the DE2-series board as inputs
to the circuit. We will use light emitting diodes (LEDs) and 7-segment displays as output devices.
Part I
The DE2-series board provides 18 toggle switches, called SW17−0, that can be used as inputs to a circuit, and
18 red lights, called LEDR17−0, that can be used to display output values. Figure 3 shows a simple Verilog module
that uses these switches and shows their states on the LEDs. Since there are 18 switches and lights it is convenient
to represent them as vectors in the Verilog code, as shown. We have used a single assignment statement for all 18
LEDR outputs, which is equivalent to the individual assignments
assign LEDR[17] = SW[17];
assign LEDR[16] = SW[16];
. . .
assign LEDR[0] = SW[0];
The DE2-series board has hardwired connections between its FPGA chip and the switches and lights. To use
SW17−0 and LEDR17−0 it is necessary to include in your Quartus II project the correct pin assignments, which are
given in the DE2-series User Manual. For example, the manual specifies that on the DE2 board, SW0 is connected
to the FPGA pin N25 and LEDR0 is connected to pin AE23. On the DE2-70 board, SW0 is connected to the FPGA
pin AA23 and LEDR0 is connected to pin AJ6. Moreover, on the DE2-115 board, SW0 is connected to the FPGA
pin AB28 and LEDR0 is connected to pin G19. A good way to make the required pin assignments is to import into
the Quartus II software the file called DE2_pin_assignments.qsf for the DE2 board, DE2_70_pin_assignments.qsf
for the DE2-70 board, or DE2_115_pin_assignments.qsf for the DE2-115 board, which is provided on the DE2Series
System CD and in the University Program section of Altera’s web site. The procedure for making pin
assignments is described in the tutorial Quartus II Introduction using Verilog Design, which is also available from
Altera.
When importing the pin assignments file for the DE2-70 board, it is important to use Advanced Import Settings.
To do so, click the Advanced... button on the Import Assignments screen as shown in Figure 1. Then,
check Global assignments check box as shown in Figure 2 and press the OK button. Please note that omitting
this step on a DE2-70 board may cause a compile time error.
Figure 1. DE2-70 Import Assignments window.
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Figure 2. DE2-70 Advanced Import Settings window.
It is important to realize that the pin assignments in the .qsf file are useful only if the pin names given in the
file are exactly the same as the port names used in your Verilog module. The file uses the names SW[0] . . . SW[17]
and LEDR[0] . . . LEDR[17] for the switches and lights, which is the reason we used these names in Figure 3.
// Simple module that connects the SW switches to the LEDR lights
module part1 (SW, LEDR);
input [17:0] SW; // toggle switches
output [17:0] LEDR; // red LEDs
assign LEDR = SW;
endmodule
Figure 3. Verilog code that uses the DE2-series board switches and lights.
Perform the following steps to implement a circuit corresponding to the code in Figure 3 on the DE2-series board.
1. Create a new Quartus II project for your circuit. If using the Altera DE2 board, select Cyclone II EP2C35F672C6
as the target chip, which is its FPGA chip. Select Cyclone II EP2C70F896C6 if using the DE2-70 board.
Or, select Cyclone IV EP4CE115F29C7 if using the DE2-115 board.
2. Create a Verilog module for the code in Figure 3 and include it in your project.
3. Include in your project the required pin assignments for the DE2-series board, as discussed above. Compile
the project.
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4. Download the compiled circuit into the FPGA chip. Test the functionality of the circuit by toggling the
switches and observing the LEDs.
Part II
Figure 4a shows a sum-of-products circuit that implements a 2-to-1 multiplexer with a select input s. If s = 0 the
multiplexer’s output m is equal to the input x, and if s = 1 the output is equal to y. Part b of the figure gives a
truth table for this multiplexer, and part c shows its circuit symbol.
x
s
y
0
1
x
s
y
m
m
s m
0
1
x
y
a) Circuit
b) Truth table c) Symbol
Figure 4. A 2-to-1 multiplexer.
The multiplexer can be described by the following Verilog statement:
assign m = (∼s & x) | (s & y);
You are to write a Verilog module that includes eight assignment statements like the one shown above to
describe the circuit given in Figure 5a. This circuit has two eight-bit inputs, X and Y , and produces the eight-bit
output M. If s = 0 then M = X, while if s = 1 then M = Y . We refer to this circuit as an eight-bit wide 2-to-1
multiplexer. It has the circuit symbol shown in Figure 5b, in which X, Y , and M are depicted as eight-bit wires.
Perform the steps shown below.
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0
1
s
a) Circuit b) Symbol
0
1
0
1
m0
m6
m7
x6
x0
y6
y0
y7
x7
0
1
s
M
Y
X
8
8
8
Figure 5. A eight-bit wide 2-to-1 multiplexer.
1. Create a new Quartus II project for your circuit.
2. Include your Verilog file for the eight-bit wide 2-to-1 multiplexer in your project. Use switch SW17 on the
DE2-series board as the s input, switches SW7−0 as the X input and SW15−8 as the Y input. Connect the
SW switches to the red lights LEDR and connect the output M to the green lights LEDG7−0.
3. Include in your project the required pin assignments for the DE2-series board. As discussed in Part I,
these assignments ensure that the input ports of your Verilog code will use the pins on the FPGA that are
connected to the SW switches, and the output ports of your Verilog code will use the FPGA pins connected
to the LEDR and LEDG lights.
4. Compile the project.
5. Download the compiled circuit into the FPGA chip. Test the functionality of the eight-bit wide 2-to-1
multiplexer by toggling the switches and observing the LEDs.
Part III
In Figure 4 we showed a 2-to-1 multiplexer that selects between the two inputs x and y. For this part consider a
circuit in which the output m has to be selected from five inputs u, v, w, x, and y. Part a of Figure 6 shows how
we can build the required 5-to-1 multiplexer by using four 2-to-1 multiplexers. The circuit uses a 3-bit select input
s2s1s0 and implements the truth table shown in Figure 6b. A circuit symbol for this multiplexer is given in part c
of the figure.
Recall from Figure 5 that an eight-bit wide 2-to-1 multiplexer can be built by using eight instances of a 2-to-1
multiplexer. Figure 7 applies this concept to define a three-bit wide 5-to-1 multiplexer. It contains three instances
of the circuit in Figure 6a.
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0
1
a) Circuit
0
1
0
1
w
x
v
u
000
m
0
1
s1
s2
s0
y
m
001
010
011
100
s1
s2
s0
w
x
v
u
y
c) Symbol
m
0
0
u
v
b) Truth table
s1 s0s2
0 0
0 1
0
0
w
x
1 0
1 1
1 y0 0
1
1
0 1
1 0
1 1 1
y
y
y
Figure 6. A 5-to-1 multiplexer.
000
M
001
010
011
100
s1
s2
s0
W
X
V
U
Y
3
3
3
3
3
3
Figure 7. A three-bit wide 5-to-1 multiplexer.
Perform the following steps to implement the three-bit wide 5-to-1 multiplexer.
1. Create a new Quartus II project for your circuit.
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2. Create a Verilog module for the three-bit wide 5-to-1 multiplexer. Connect its select inputs to switches
SW17−15, and use the remaining 15 switches SW14−0 to provide the five 3-bit inputs U to Y . Connect the
SW switches to the red lights LEDR and connect the output M to the green lights LEDG2−0.
3. Include in your project the required pin assignments for the DE2-series board. Compile the project.
4. Download the compiled circuit into the FPGA chip. Test the functionality of the three-bit wide 5-to-1
multiplexer by toggling the switches and observing the LEDs. Ensure that each of the inputs U to Y can be
properly selected as the output M.
Part IV
Figure 8 shows a 7-segment decoder module that has the three-bit input c2c1c0. This decoder produces seven
outputs that are used to display a character on a 7-segment display. Table 1 lists the characters that should be
displayed for each valuation of c2c1c0. To keep the design simple, only four characters are included in the table
(plus the ‘blank’ character, which is selected for codes 100 − 111).
The seven segments in the display are identified by the indices 0 to 6 shown in the figure. Each segment is
illuminated by driving it to the logic value 0. You are to write a Verilog module that implements logic functions
that represent circuits needed to activate each of the seven segments. Use only simple Verilog assign statements
in your code to specify each logic function using a Boolean expression.
7-segment
0
1
2
3
4
5
6
decoder
c2
c1
c0
Figure 8. A 7-segment decoder.
c2c1c0 Character
000 H
001 E
010 L
011 O
100
101
110
111
Table 1. Character codes.
Perform the following steps:
1. Create a new Quartus II project for your circuit.
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2. Create a Verilog module for the 7-segment decoder. Connect the c2c1c0 inputs to switches SW2−0, and
connect the outputs of the decoder to the HEX0 display on the DE2-series board. The segments in this
display are called HEX00, HEX01, . . ., HEX06, corresponding to Figure 8. You should declare the 7-bit port
output [0:6] HEX0;
in your Verilog code so that the names of these outputs match the corresponding names in the DE2-series
User Manual and the pin assignments file.
3. After making the required DE2-series board pin assignments, compile the project.
4. Download the compiled circuit into the FPGA chip. Test the functionality of the circuit by toggling the
SW2−0 switches and observing the 7-segment display.
Part V
Consider the circuit shown in Figure 9. It uses a three-bit wide 5-to-1 multiplexer to enable the selection of five
characters that are displayed on a 7-segment display. Using the 7-segment decoder from Part IV this circuit can
display any of the characters H, E, L, O, and ‘blank’. The character codes are set according to Table 1 by using
the switches SW14−0, and a specific character is selected for display by setting the switches SW17−15.
An outline of the Verilog code that represents this circuit is provided in Figure 10. Note that we have used the
circuits from Parts III and IV as subcircuits in this code. You are to extend the code in Figure 10 so that it uses five
7-segment displays rather than just one. You will need to use five instances of each of the subcircuits. The purpose
of your circuit is to display any word on the five displays that is composed of the characters in Table 1, and be
able to rotate this word in a circular fashion across the displays when the switches SW17−15 are toggled. As an example,
if the displayed word is HELLO, then your circuit should produce the output patterns illustrated in Table 2.
7-segment
decoder
000
001
010
011
100
3
3
3
3
3
3
SW17
SW16
SW15
SW14 12–
SW11 9–
SW8 6–
SW5 3–
SW2 0–
7
0
1
2
3
4
5
6
Figure 9. A circuit that can select and display one of five characters.
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module part5 (SW, HEX0);
input [17:0] SW; // toggle switches
output [0:6] HEX0; // 7-seg displays
wire [2:0] M;
mux_3bit_5to1 M0 (SW[17:15], SW[14:12], SW[11:9], SW[8:6], SW[5:3], SW[2:0], M);
char_7seg H0 (M, HEX0);
endmodule
// implements a 3-bit wide 5-to-1 multiplexer
module mux_3bit_5to1 (S, U, V, W, X, Y, M);
input [2:0] S, U, V, W, X, Y;
output [2:0] M;
. . . code not shown
endmodule
// implements a 7-segment decoder for H, E, L, O, and ‘blank’
module char_7seg (C, Display);
input [2:0] C; // input code
output [0:6] Display; // output 7-seg code
. . . code not shown
endmodule
Figure 10. Verilog code for the circuit in Figure 9.
SW17 SW16 SW15 Character pattern
000 H E L L O
001 E L L O H
010 L L O H E
011 L O H E L
100 O H E L L
Table 2. Rotating the word HELLO on five displays.
Perform the following steps.
1. Create a new Quartus II project for your circuit.
2. Include your Verilog module in the Quartus II project. Connect the switches SW17−15 to the select inputs of
each of the five instances of the three-bit wide 5-to-1 multiplexers. Also connect SW14−0 to each instance
of the multiplexers as required to produce the patterns of characters shown in Table 2. Connect the outputs
of the five multiplexers to the 7-segment displays HEX4, HEX3, HEX2, HEX1, and HEX0.
3. Include the required pin assignments for the DE2-series board for all switches, LEDs, and 7-segment displays.
Compile the project.
4. Download the compiled circuit into the FPGA chip. Test the functionality of the circuit by setting the proper
character codes on the switches SW14−0 and then toggling SW17−15 to observe the rotation of the characters.
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Part VI
Extend your design from Part V so that is uses all eight 7-segment displays on the DE2 board. Your circuit should
be able to display words with five (or fewer) characters on the eight displays, and rotate the displayed word when
the switches SW17−15 are toggled. If the displayed word is HELLO, then your circuit should produce the patterns
shown in Table 3.
SW17 SW16 SW15 Character pattern
000 H E L L O
001 H E L L O
010 H E L L O
011 H E L L O
100 E L L O H
101 L L O H E
110 L O H E L
111 O H E L L
Table 3. Rotating the word HELLO on eight displays.
Perform the following steps:
1. Create a new Quartus II project for your circuit and select the appropriate target chip.
2. Include your Verilog module in the Quartus II project. Connect the switches SW17−15 to the select inputs of
each instance of the multiplexers in your circuit. Also connect SW14−0 to each instance of the multiplexers
as required to produce the patterns of characters shown in Table 3. (Hint: for some inputs of the multiplexers
you will want to select the ‘blank’ character.) Connect the outputs of your multiplexers to the 7-segment
displays HEX7, . . ., HEX0.
3. Include the required pin assignments for the DE2-series board for all switches, LEDs, and 7-segment displays.
Compile the project.
4. Download the compiled circuit into the FPGA chip. Test the functionality of the circuit by setting the proper
character codes on the switches SW14−0 and then toggling SW17−15 to observe the rotation of the characters.
Copyright c 2011 Altera Corporation.
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