Digital Circuits for High School Students (Part 3.5)

Prepared by Charles S. Tritt, Ph.D.
July 27, 1998

Equipment (for each pair of students working together)

1 Digi Designer
1 Set of wires (these should come with the Digi Designer)
1 7402 Quad NOR
1 7474 Dual D flip-flop
1 7475 4-bit latch
1 7476 Dual JK flip-flop

Introduction

The outputs of sequential digital circuits depend on both their present input and their previous inputs. These circuits exhibit memory. Synchronous sequential circuits are the most common type of sequential circuits. Synchronous circuits have a clock input that coordinates their operation. In this exercise you will build a simple asynchronous RS flip-flop, observe the behavior of a 7474 D flip-flip and compare it to the behavior of a 7475 4-bit latch and observe the behavior of a 7476 JK flip-flip. The pin outs for the chips you will be using are shown in Figure 1.

Figure 1 -- Pin outs for the 7402, 7474, 7475 and 7476 chips.

Safety Considerations

Dangerous line voltages are present at the outlets on your work bench. Do not inert anything other than approved power plugs into these outlets.

The voltages and currents produced by the Digi Designer are generally safe. However, you should not short +5V (Vcc) or logic High outputs directly to ground or intentionally make yourself part of a logic circuit.

Chip pins and the tips of the small wires may be sharp. Handle with care. Chips can become very hot if wired incorrectly. Be careful.

Procedures

Build Your Own RS Flip-flop

Insert a 7402 chip into the bread board on your Digi Designer. Connect power and ground to pins 14 and 7, respectively. You will be wiring the circuit shown in Figure 2 (a).

Figure 1 -- An RS flip-flop can be constructed using two NOR gates from a 7402.

Make the following connections to your chip:

Pin Function Connection

1 Q output J1, LED

2 Reset input J21, normally low pulser

3 First feedback path Pin 4, NOT Q output

4 NOT Q output J2, LED

5 Set input J19, normally low pulser

6 Second feedback path Pin 1, Q output

Pin	Function	Connection
1	Q output	J1, LED
2	Reset input	J21, normally low pulser
3	First feedback path	Pin 4, NOT Q output
4	NOT Q output	J2, LED
5	Set input	J19, normally low pulser
6	Second feedback path	Pin 1, Q output

Confirm that your circuit produces the outputs shown in Figure 2 (b). Does this behavior make sense in terms of the names of the inputs (Set and Reset)?

Disassemble your circuit and remove the 7402 from your Digi Designer.

Type D Flip-flop

Insert a 7474 chip into the bread board on your Digi Designer. Connect power and ground to pins 14 and 7, respectively. Make the following connections to your chip:

Pin Function Connection

1 Clear (active low) input +5 volts, logic high

2 D input J19, normally low pulser

3 Clock input J13, Digi Designer Clock output

4 Preset (active low) input +5 volts, logic high

5 Q output J3, LED

Pin	Function	Connection
1	Clear (active low) input	+5 volts, logic high
2	D input	J19, normally low pulser
3	Clock input	J13, Digi Designer Clock output
4	Preset (active low) input	+5 volts, logic high
5	Q output	J3, LED

Also connect the Digi Designer clock output to an LED (J1) and the pulser output (D input) to another LED (J2). Set the Digi Designer clock to 1 Hz. The output, Q, of the flip-flop should change to reflect the input, D, only when the clock input goes from low to high. Observer you circuit carefully and confirm it is functioning as expected.

Disassemble your circuit and remove the 7474 from your Digi Designer.

Data Latch

Insert a 7475 chip into the bread board on your Digi Designer. Connect power and ground to pins 5 and 12, respectively (note these are not the standard positions for these connections). Make the following connections to your chip:

Pin Function Connection

2 Data input J19, normally low pulser

13 Enable (or gate) input J21, normally low pulser

16 Q output J3, LED

Pin	Function	Connection
2	Data input	J19, normally low pulser
13	Enable (or gate) input	J21, normally low pulser
16	Q output	J3, LED

Also connect pulser J19 (Enable) an LED (J1) and pulser J21 (Data input) to another LED (J2).The output, Q, of the latch should change to reflect the Data input only when the Enable input input is high. Observer you circuit carefully and confirm it is functioning as expected. Compare the behavor of the 7475 to the 7474 you used in the previous circuit.

Disassemble your circuit and remove the 7475 from your Digi Designer.

Type JK Flip-flop

Insert a 7476 chip into the bread board on your Digi Designer. Connect power and ground to pins 5 and 13, respectively. Make the following connections to your chip:

Pin Function Connection

1 Clock input J13, Digi Designer Clock output

2 Preset (active low) input +5 volts, logic high

3 Clear (active low) input +5 volts, logic high

4 J, input J19, normally low pulser

15 Q output J2, LED

16 K, input J21, normally low pulser

Pin	Function	Connection
1	Clock input	J13, Digi Designer Clock output
2	Preset (active low) input	+5 volts, logic high
3	Clear (active low) input	+5 volts, logic high
4	J, input	J19, normally low pulser
15	Q output	J2, LED
16	K, input	J21, normally low pulser

Also connect the Digi Designer clock output to an LED (J1). Set the Digi Designer clock to 1 Hz. The output, Q, of the flip-flop should change in response to the J and K inputs when the clock input goes from low to high. When used alone, the J input sets the flip-flop and the K input clears the flip-flop. When both J and K are high the flip-flop toggles (changes state). Observer you circuit carefully and confirm it is functioning as expected.

Disassemble your circuit and remove the 7476 from your Digi Designer.

Figure 2 was taken from Digital Design, 2^nd ed. by M. Morris Mano.