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Activity 1.3.2 Sequential Logic: An Overview
Introduction
Along with combinational logic, sequential logic is a fundamental building block of
digital electronics. The output values of sequential logic depend not only on the
current input values (i.e., combinational logic), but also on previous output values.
Thus, sequential logic requires a clock signal to control sequencing and memory and
to retain previous outputs.
In this activity we will use the D flip-flop. We are limiting our use to this type of flipflop in this introductory unit because of its simplicity and ease of use. The D flip-flop
is just one of many different types of flip-flops that can be used to implement
sequential logic circuits.
Equipment


Paper and pencil
Circuit Design Software (CDS)
Procedure
Let’s begin the study of sequential logic by investigating the basic operations of the
D flip-flop.
1. Using the Circuit Design Software (CDS), enter the D flip-flop test circuit
shown below. Use a switch for the input T and probes for the outputs Que
and NOT_Que.
Note: The PR (preset) and CLR (clear) inputs on the 74LS74 are active low
inputs. In this circuit PR and CLR are connected to 5v (high), which makes
them both inactive.
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DE – Unit 1 – Lesson 1.3 – Activity 1.3.2 – Sequential Logic: An Overview – Page 1
a) Toggle the input T several times and record what effect this has on the
two outputs.
It switches diodes
b) What happens when the switch is moved from GROUND to 5v? What
happens when the switch is moved from 5v to GROUND? Explain.
Power shuts off then turns back on
2. Using the Circuit Design Software (CDS), modify the circuit used in step (1)
so that it matches the example shown below.
a) Start the simulation.
b) Set the input switches P and R to 5v. Again, since PR and CLR are active
low inputs, this will make them both inactive. Toggle the input T several
times. The circuit should behave exactly like the circuit in step (1).
c) Set the input switch P to GROUND and R to 5v. What is the state of the
two outputs?
Que is lit
d) Toggle the input T several times and record what effect this has on the
two outputs.
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DE – Unit 1 – Lesson 1.3 – Activity 1.3.2 – Sequential Logic: An Overview – Page 2
No effect
e) Set the input switch P to 5v and R to GROUND. What is the state of the
two outputs?
Not Que is lit
f) Toggle the input T several times and record what effect this has on the
two outputs.
No effect
Let us examine a simple binary counter. Counters are one of the most common
applications of flip-flops. The circuit that we will be observing is called a two-bit
binary counter. The counter will count from zero (00 in binary) to three (11 in binary).
1. Using the Circuit Design Software (CDS), enter the two-bit binary counter
shown below. Use a switch for the input Clock-In and probes for the outputs
A and B.
a) Start the simulation.
b) Cycle the input Clock-In (switch T) several times and record what effect
this has on the two outputs in the table below.
Clock-In
Initial Values
1st Toggle
2nd Toggle
3rd Toggle
4th Toggle
5th Toggle
6th Toggle
A
B
0
1
0
1
0
1
0
1
1
0
0
1
1
0
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c) Based on these results, explain the pattern that you observe in the two
outputs.
It starts with B, fill and empties then starts A fills and empties
2. Using the Circuit Design Software (CDS), modify the circuit used in step (1)
so that it matches that shown below.
The first modification is to replace the switch input with a CLOCK_VOLTAGE.
This change will result in the input being continuously toggled. Be sure the
CLOCK_VOLTAGE is set to 5 volts, 50% duty cycle, 1 kHz.
The second modification is to add a four-channel oscilloscope set up to view
the four signals A, B, Clock-In, and ~1Q (i.e., the Q-not output of the first flipflop). Be sure to set the oscilloscope’s time-base to 1ms/div and the verticalbases of the four channels to 10volts/div. Also, adjust the Y position of the
four channels such that the four signals are all clearly visible.
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a) Start the simulation and let it run until you have captured several periods
of each signal.
b) Using the oscilloscope’s markers, measure the period of the three signals.
Use this data to calculate the frequency for each signal. Record your data
in the table below. Be sure to use the correct units.
Signal
ClockIn
A
B
Period
1mS
2mS
4mS
Frequency
1mHz
500uHz
250uHz
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c) Based on these results, explain the relationship of the period and
frequency between the three signals. Was this expected?
Conclusion
1. List 3-5 real-world applications where you might find counters like the one
examined in this activity.
Stop lights, cross walk signs, emergency lights
2. Simple counters are just one application of sequential logic. From your life
experiences, list 3-5 examples of products that you have used that contain
sequential logic.
Strobe light,
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Going Further – Optional
Analyze the 4-bit binary counter shown below to determine the frequency and period
for the signals A, B, C, and D. Use the table shown below to record your answers.
Signal
ClockIn
A
B
C
D
Period
Frequency
1mS
1mHz
2mS
4mS
8mS
16mS
500uHz
250uHz
125uHz
.05125uHz
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DE – Unit 1 – Lesson 1.3 – Activity 1.3.2 – Sequential Logic: An Overview – Page 7