CEC 222 Digital Electronics Lab
Spring 2015
Lab 1: Introduction to Digital Lab
Learning Objectives:

Develop familiarity with some of the lab equipment and simple components (e.g., oscilloscope,
voltmeter, and resistors).

Introduce the relationship between digital and analog signals.
Lab Overview:
In this lab you will be building a digital to analog 8 (DAC), specifically, an R–2R resistor ladder network. The
switches on your FPGA board will generate a 4-bit digital value that is converted, via your DAC circuit, to an
analog voltage that you measure with a voltmeter.
The latter part of the lab involves using an oscilloscope to determine the frequency and magnitude of
mystery signals.
YOUR NAME(S)
Lab 01
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CEC 222 Digital Electronics Lab
Spring 2015
Pre-Lab (10%)
Question 1. What is the resistor color code for the following
resistor values (assume a 5% tolerance)? Fill in the table.
1st Band
brown
Select
Value
10 k
47 M
820 
2.2 k
Select
Select
2nd Band
select
violet
Select
Select
3rd band
Select
Select
4th band
Select
Select
Select
Select
gold
Select
Question 2. What is the value and tolerance for each resistor
shown? Enter the value in Ohms.
Value ()
Enter Text
Tolerance
Resistor
Enter Text
Color
1st Band
(1st digit)
2nd Band
(2nd digit)
3rd Band
(multiplier)
Black
0
0
100
Brown
1
1
101
Red
2
2
102
Orange
3
3
103
Yellow
4
4
104
Green
5
5
105
Blue
6
6
106
Violet
7
7
107
Grey
8
8
108
White
9
9
4th Band
(tolerance)
2%
109
Gold
10-1
5%
Silver
10-2
10%
Figure 1 Resistor color codes
Enter Text
10%
Enter Text
Enter Text
In the lab we will be realizing a 4-bit digital to analog converter (DAC) via an R–2R resistor ladder network
(see Figure 3) that transforms a digital input digital pattern (i.e., decimal value) into an analog output voltage
(Vout). If the resistor values are exact the theoretical output of our ladder network DAC is
Vout  3.3 
Task 1.
Decimal Value
24
(1.1)
Noting that the 4-bit digital pattern “0000” corresponds to a decimal value of 0 and “1111” a
decimal value of 15, use Eqn. (1.1) to fill in the second and fourth columns of Table 1.
Question 3. Considering the plot of a 0 V to 3.3 V magnitude 1 kHz
square wave shown in Figure 2:
a) What is the horizontal time scale?
_______________________ seconds/division
b) What is the vertical magnitude scale?
____________________ Volts/division
Task 2.
YOUR NAME(S)
Watch the 12 min Digilent video on the oscilloscope.
Lab 01
Figure 2 A 1 kHz square wave
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CEC 222 Digital Electronics Lab
Spring 2015
Experiments (90%)
EXPERIMENT 1.
DIGITAL TO ANALOG CONVERSION
In this experiment, you will be building and testing a simple digital to analog converter (DAC) via the circuit
shown in Figure 3. Follow the steps below to complete the experiment. Note the wireing color convention that
RED = POWER (Vcc) and BLACK = GROUND (GND).
Output Voltage (Vout)
SW2
SW1
20 k
20 k
20 k
SW3
10 k
20 k
10 k
20 k
10 k
SW0
MODE jumper
Port JA
RED = Power (Vcc)
BLACK = Ground
Power switch
LED3 to LED0
SW3 – SW0 switches
Figure 3 Digital to analog conversion circuit configuration.
Step 1.a: Load the file “lab1.bit” into your BASYS 2 FPGA board

Set the MODE jumper on your FPGA board (from PC) to ROM

Connect your board to a PC (via USB cable) and turn on the board’s power

Download the “lab_1.bit” file onto your desktop, start the
“Adept” software (
click on browse (
Program the board (
), and on the row labeled “PROM”
) to select the “lab1.bit” file.
) with this file.
 Programming may take ~ 30 seconds.

You should now be able to generate a 4-bit pattern on port
“JA” via the slide switches SW3 to SW0. The pattern will also
be displayed on the LEDs next to the switches (LED3 to LED0).

TURN OFF your FPGA board before continuing!!
YOUR NAME(S)
Lab 01
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CEC 222 Digital Electronics Lab
Spring 2015
Step 1.b: Build the resistor circuit (see Figure 3) on your breadboard and connect to port “JA”

Obtain five 20 k and three 10 k resistors

Note the connectivity between the holes on a breadboard

On your breadboard, build the resistor circuit shown in Figure
3 and make the connections between your circuit (i.e.,
breadboard) and the six nodes of port JA of your FPGA.

Have the instructor or TA inspect your circuit before turning on the power to your FPGA
Step 1.c: Vary the four switches to generate all possible 4-bit binary patterns and record the resulting output
voltages using a voltmeter.

Turn on the power to your FPGA and use the voltmeter functionality of the Analog Discovery (see
Appendix A: A1) Appendix A: ) to measure the output voltage (Vout) of your DAC as you vary the 4-bit
binary pattern on port “JA” via the slide switches SW3 to SW0.
Task 3.
Record your results in column three of Table 1.
Binary Pattern
(SW3 …SW0)
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Table 1 Results of Digital to Analog Conversion
Measured Output
Theoretical Output
Decimal Value
Voltage (Vout) in Volts Voltage (Vout) in Volts
0
Enter Text
0.00
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
9
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
Enter Text
15
Enter Text
3.09
Question 4. Considering Table 1, determine which row exhibits the largest difference between measured
and theoretical output voltages. Largest difference = ________________ in Volts.
Question 5. Why is there a difference between the theoretical and measured values?
__________________________________________
YOUR NAME(S)
Lab 01
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CEC 222 Digital Electronics Lab
EXPERIMENT 2.
Spring 2015
INTRODUCTION TO THE OSCILLOSCOPE
For this experiment you will be using your Analog Discovery USB oscilloscope to determine the frequency
and magnitude of two “mystery” waveforms. The file which you loaded/programmed into your FPGA board also
generates two square wave signals on port JB, specifically, on the pins labeled C6 and B6.
Step 2.a: Determine the frequency and magnitude of the signal on port JB pin C6.

Assume that the signal on pin C6 has a frequency of around 1 kHz and amplitude of approximately 3 V.
Question 6. What is the approximate period of the waveform and hence what is a reasonable selection for
the horizontal time scale (time/division) of the oscilloscope (assume 10 divisions)?
in seconds  Reasonable horizontal scale =
Approximate period =
s/division
Question 7. What is a “reasonable” selection for the vertical scale
(V/division) of the oscilloscope (assume 10 divisions)?
Reasonable vertical scale =

V/division
Start the Digilent -> WaveForms software (
Oscilloscope
) and click
on the “Scope” icon in order to start the oscilloscope.

Connect “scope channel 1 positive” to the FPGA’s port JB pin C6
and “scope channel 1 negative” to GND (see Figure 9 for the
connectivity details).
Task 4.
Take a screenshot (Figure 4) of the window showing more than one, but, less than two periods
of the waveform. You will likely need to further adjust the horizontal and vertical scales, also, you will
need to be able to see your horizontal and vertical scale choices.
Figure 4 Screenshot of the waveform on pin C6.
YOUR NAME(S)
Lab 01
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CEC 222 Digital Electronics Lab
Spring 2015
Step 2.b: Determine the frequency and magnitude of the signal on port JB pin B6.

Assume that the signal on pin B6 has a frequency around 10 kHz and amplitude of approximately 3 V.

Repeat Step 2.a: for this signal.
Approximate period = ______________ in second  Reasonable horizontal scale =
s/division
Reasonable vertical scale = _________ V/division
Figure 5 Screenshot of the waveform on pin B6.
Report Requirements
Task 5.
Plot the values from Table 1. Specifically, plot a line for the theoretical voltages (vertical axis) vs
decimal values (horizontal axis) and a second line corresponding to the measured voltages vs decimal
values on the same graph (include a legend).
YOUR NAME(S)
Lab 01
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CEC 222 Digital Electronics Lab
Spring 2015
Figure 6 Plot of the DAC output (measured & theoretical) vs decimal value.
Question 8. What is the resolution1 of the DAC which you built (in Volts)? _____________ in Volts
Question 9. What is the magnitude and frequency of the square wave on port JB pin C6?
Magnitude = ___________ in Volts and Frequency = ________________ in Hz
Question 10.
What is the magnitude and frequency of the square wave on port JB pin B6?
Magnitude = ___________ in Volts and Frequency = ________________ in Hz
Optional Exercise(s) (+10% Extra Credit)
EXPERIMENT 3.
ADDITIONAL FUNCTIONALITY OF THE OSCILLOSCOPE
For this experiment you will be repeating the steps of Experiment 2, however, we will also explore the
effects of triggering. (Video -> Analog Discovery Tutorials: Scope 2: Triggered Acquisition)
Step 3.a: Recapture the screenshot of the signal on port JB pin C6 with edge-triggering

Set the scope to rising edge triggering and take a screenshot of the window showing more than ½, but,
less than one period. You will also need to set the (triggering) level to about 1V.
1
Smallest change in the output voltage that is realizable.
YOUR NAME(S)
Lab 01
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CEC 222 Digital Electronics Lab
Spring 2015
Figure 7 Screenshot of the waveform on pin C6 with rising edge triggering.
Question 11.

What is the width (in seconds) of first half of the square wave? ____________ in sec
Set the scope to falling edge triggering and take a screenshot of the window showing more than ½, but,
less than one period.
Figure 8 Screenshot of the waveform on pin C6 with falling edge triggering.
Question 12.
What is the width (in seconds) of second half of the square wave? ___________ in sec
Reference Material
[1] Digilent Basys2 Board Reference Manual
[2] Digilent ADEPT software (free download)
[3] Analog Discovery video tutorials
YOUR NAME(S)
Lab 01
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CEC 222 Digital Electronics Lab
Spring 2015
APPENDIX A: USING THE ANALOG DISCOVERY OSCILLOSCOPE
Figure 9 Input / Output connections for the Analog Discovery.
A1) VOLTMETER FUNCTIONALITY OF THE ANALOG DISCOVERY

Start the Digilent -> WaveForms software (
) and select
“Voltmeter” from the pull-down menu via “More Instruments.”

Connect “scope channel 1 positive” (i.e., 1+) to the signal of
interest and “scope channel 1 negative” (i.e., 1-) to GND.
YOUR NAME(S)
Lab 01
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