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, logic
analyzer, voltmeter, inverter, and resistors).

Introduce the relationship between digital and analog signals
Lab Overview:
In this lab you will be building a digital to analog converter (DAC), specifically, an R–2R resistor ladder
network. The switches on your FPGA board will generate a 4-bit digital value which is converted, via your DAC
circuit, to an analog voltage which 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)
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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)?
Value
10 k
47 M
1st Band
brown
2nd Band
3rd band
820 
2.2 k
4th band
gold
Question 2. What is the value and tolerance for each resistor
shown?
Value
Tolerance
Resistor
Figure 1 Resistor color codes
10%
Question 3. In the lab we will be realizing a 4-bit digital to analog converter (DAC) via a R–2R resistor ladder
network (see Figure 2) which 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
Noting that the 4-bit digital pattern “0000” has a decimal value of 0 and “1111” has a decimal
value of 15, use this formula to fill in the second and fourth columns of Table 1.
Question 4. 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
Figure 2 A 1 kHz square wave
YOUR NAME(S)
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CEC 222 Digital Electronics Lab
Spring 2015
Experiments (90%)
EXPERIMENT 1.
DIGITAL TO ANALOG CONVERSION
For this experiment you will be building and testing a simple digital to analog converter (DAC) (specifically,
an R–2R resistor ladder network) via the circuit shown in Figure 3. Follow the steps below in order to complete
the experiment.
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
Power switch
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, and on the row labeled “PROM” click on
browse (
board (

) to select the “lab1.bit” file. Program the
) with this file.
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).
YOUR NAME(S)
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CEC 222 Digital Electronics Lab

Spring 2015
TURN OFF your FPGA board before continuing!!
Step 1.b: Build the resistor circuit 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 a voltmeter to measure the Output Voltage of your DAC as you
vary the 4-bit binary pattern on port “JA” via the slide switches SW3 to SW0.
Task 2.
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
0.00
9
15
3.09
Question 5. Considering Table 1 which row exhibits the largest difference between measured and
theoretical output voltages, what is this difference (in Volts), and why is there a difference between the
theoretical and measured values?
YOUR NAME(S)
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CEC 222 Digital Electronics Lab
EXPERIMENT 2.
Spring 2015
INTRODUCTION TO THE OSCILLOSCOPE
For this experiment you will be using an 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 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)?
Question 7. What is a “reasonable” selection for the vertical scale (V/division)?

Take a screenshot 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 scale). Compute the magnitude and
frequency from this screenshot (i.e., you will need to see your horizontal and vertical scale choices).
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 1 for this signal.
Report Requirements
Task 3.
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.
Question 8. What is the resolution of the DAC which you built (in Volts)?
Question 9. What is your determination of the magnitude and frequency of the square wave on port JB pin
C6?
Question 10.
JB pin B6?
YOUR NAME(S)
What is your determination of the magnitude and frequency of the square wave on port
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CEC 222 Digital Electronics Lab
Spring 2015
Optional (10% Extra Credit) Exercise(s)
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 and coupling.
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.
Question 11.

What is the width (in seconds) of first half of the square wave?
Set the scope to falling edge triggering and take a screenshot of the window showing more than ½, but,
less than one period.
Question 12.
What is the width (in seconds) of second half of the square wave?
Step 3.b: Recapture the screenshot of the signal on port JB pin B6 with coupling (MSO-28 Oscilloscope only)

Set the scope to DC coupling and take a screenshot of the window showing more than one, but, less
than two periods of the waveform.
Question 13.

What are the minimum and maximum magnitudes of the signal?
Set the scope to AC coupling and take a screenshot of the window showing more than one, but, less
than two periods of the waveform.
Question 14.
What are the minimum and maximum magnitudes of the signal?
Reference Material
[ 1] Digilent Basys2 Board Reference Manual
[ 2] Digilent ADEPT software (free download)
Appendix A: Using the Analog Discovery Oscilloscope
Appendix B: Using the MSO-28 Oscilloscope
YOUR NAME(S)
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