Işık University Department of Electronics Engineering
EE240
Experiment 5
Counters
1. Introduction
The input pulses, called count pulses, may be clock pulses or they may
originate from an external source and may occur at prescribed intervals of time or
at random. The sequence of states may follow a binary count or other sequence
states in a counter. Counters found in almost all equipment containing digital
logic are used for counting the number of occurrences of an event and are useful
for generating timing sequences to control operations in a digital systems.
Counters come in two categories : Asynchronous counters ( Ripple Counters ) and
Synchronous counters.
A asynchronous counter is formed with a series cascaded flip-flops. In a
asynchronous counters, the flip-flop output transition serves as a source for
triggering other flip-flops. The first flip-flop is triggered by the input pulses. The
output of the flip-flops are trigger the clock of the next flip-flop. The circuit
diagram and timing diagram of a simple asynchronous counter is shown in
Figure.5.1.
Q1 (20)
Vcc
Q2 (21)
Vcc
J
CLK
outputs
Q
J
CLK
K
Q
CLK
Q
K
Q
(a) Circuit diagram
Clock
Pulses
Q1
0
1
0
1
0
Q2
0
0
1
1
0
1
(b) Timing diagram
Figure.5.1. An example of asynchoronous counter
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0
0
1
Işık University Department of Electronics Engineering
EE240
The asynchronous counter is an up counter. A down counter can be
implemented with the same circuitry used for the counter. By connecting the
complement output instead of true output to the clock input of the next flip-flop
will form down counter.
In a synchoronous counter, the clock pulses are applied to clock inputs of all
flip-flops. Therefore each flip-flop of the counter changes state at the same time.
This advantage of this configuration is that only one gate delay is required for the
counter stages to change state. The circuit diagram and timing diagram of a
simple synchronous counter is shown in Figure.5.2.
outputs
Q1 (20)
Q2 (21)
Vcc
J
Q
J
CLK
K
Q
CLK
Q
K
Q
CLK
(a) Circuit diagram
Clock
Pulses
Q1
0
1
0
1
0
Q2
0
0
1
1
0
1
0
0
1
(b) Timing diagram
Figure.5.2. An example of synchoronous counter
This counter is an up counter. A down counter can be implemented with the
same circuitry used for the counter. By connecting the complement output instead
of true output to the J-K inputs of the next flip-flop will form down counter.
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Işık University Department of Electronics Engineering
2.
EE240
Exercises
2.1. Objective
The purpose of this experiment is to investigate asynchronous counters
and synchronous counters. We will implement up and down counter using
discrete flip-flop ICs
2.2. Material
C.A.D.E.T. Breadbord
1x74LS74 Dual D Type Positive Edge-Triggered Flip-Flop
1x74LS76 Dual J-K Flip-Flops with Set(Preset) and Reset(Clear)
1x74LS86 Quad 2-Input XOR Gate
1x74LS90 Decade Counter
Jumper Wire
2.3. Procedure
1. Set up the circuit shown in Figure.5.3. Use extra caution wiring the power
and ground connections.
outputs
Led1
Led2
switch1
Vcc=+5v
K
9
6
Q
14
12
J
Q
CL
15
CLK
CL
16
Q
PR
1
J
PR
PB2
4
7
2
Vcc=+5v
Q
11
CLK
K
10
8
3
switch2
Figure.5.3. Asynchronous up counter
2. Turn on power to the circuit.
3. Turn the switch1 and switch2 to logic-1 position.
4. Apply clock pulses to the clock input(PB2) by using pushbutton on the
5.
6.
7.
8.
C.A.D.E.T. Record your observations of this circuit operation.
Use PB2 to place a count of two on Led1 and Led2. Turn the switch1 and
switch2 to logic-1 and logic-0 position, respectively. Record your observation.
Turn the switch1 and switch2 to logic-0 and logic-1 position, respectively.
Record
your observation.
Turn off power. This counter is an up- counter. Convert the counter to the
down counter..
Repeat 2-6 for the down counter.
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Işık University Department of Electronics Engineering
EE240
9. Remove the wire from PB2 to clock input. Connect the clock input to pulse
generator on the C.A.D.E.T. Apply a square wave of 10 kHz. Observe the
clock input, led1, and led2 outputs by using an oscilloscope. Record your
observation.
10. Set up the circuit shown in Figure.5.4. Use extra caution wiring the power
and ground connections.
outputs
Led1
Led2
Switch1
5
12
2
11
D
CL
Q
9
CLK
CLK
Q
Q
CL
3
Q
PR
3
D
PR
2
10
4
1
6
8
13
1
Switch2
PB2
Figure.5.4.Synchronous up counter
11. Turn on power to the circuit.
12. Turn the switch1 and switch2 to logic-1 position.
13. Apply clock pulses to the clock input(PB2) by using pushbutton on the
C.A.D.E.T. Record your observations of this circuit operation.
14. Use PB2 to place a count of two on Led1 and Led2. Turn the switch1 and
switch2 to logic-1 and logic-0 position, respectively.
Record your
observation.
15. Turn the switch1 and switch2 to logic-0 and logic-1 position, respectively.
Record your observation.
16. Turn off power. This counter is an up-counter. Convert the counter to the
down counter..
17. Repeat 11-16 for the down counter.
18. Remove the wire from PB2 to clock input. Connect the clock input to pulse
generator on the C.A.D.E.T. Apply a square wave of 10 kHz. Observe the
clock input, led1, and led2 outputs by using an oscilloscope. Record your
observation
19. Set up the circuit shown in Figure.5.5. Notice the unconventional arrangment
of the power pins
20. Turn on power to this circuit. Turn the switch1 to logic-1. The LED should
display a zero.
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Işık University Department of Electronics Engineering
EE240
21. Turn switch1 to logic-0. Use PB2 as the count input and the seven segment
display on the C.A.D.E.T. as the output. Record your observations of the
operation of this circuit.
U?
PB2
Vcc=+5v
switch1
14
1
2
3
6
7
A
B
QA
QB
QC
QD
12
9
8
11
A
B
C
D
R0(1)
R0(2)
R9(1)
R9(2)
74LS90
Figure.5.5 Decade counter
22. Set the counter to some non-zero count and turn switch1 to logic-1. Record
your observations.
23. Turn off power and remove the 74LS90 and it’s circuitry.
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Işık University Department of Electronics Engineering
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