Digital Circuits for High School Students (Part 4)

Prepared by Charles S. Tritt, Ph.D.
July 26, 1998

Equipment (for each pair of students working together)

1 Digi Designer
1 Set of wires (these should come with the Digi Designer)
1 74x08 Quad 2-input AND chip
2 74x73 Dual JK Flip-flop chip
1 74x169 Synchronous 4-bit Up/Down Counter chip

Introduction

Counters are simple sequential circuits made up of flip-flops. In this exercise you will observe the behavior of a 74169 4-bit, up/down counter and build your own counters using 7473 JK flip-flop 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

Using a MSI Counter Chip (74169)

This counter will count either up or down depending on the level of the U/D' input. This counter changes state on rising clock edges. It has an active low parallel load input and has two active low enable inputs. The pin out diagram for the 74169 is given in Figure 1.

74169 chip

Figure 1 -- Pin outs for the 74169 up/down binary counter chip.

Insert your 74169 into the bread board on your Digi Designer. Connect power and ground to pins 16 and 8, respectively. Set the Digi Designer clock to 1 Hz. Make the following connections from left to right across the lower edge of the chip:

Pin Function Connection

1 U/D' (up/not down) input J20, normally high pulser

2 Ck (clock) input J14, clock

3 A input J18, switch

4 B input J17, switch

5 C input J16, switch

6 D input J15, switch

7 P' (active low enable) input Ground, logic False

Pin	Function	Connection
1	U/D' (up/not down) input	J20, normally high pulser
2	Ck (clock) input	J14, clock
3	A input	J18, switch
4	B input	J17, switch
5	C input	J16, switch
6	D input	J15, switch
7	P' (active low enable) input	Ground, logic False

Make the following connections from right to left across the upper edge of the chip:

Pin Function Connection

9 Load (active low) input J22, normally high pulser

10 T' (active low enable) input Ground, logic False

11 Q_D output J1, lamp monitor

12 Q_C output J2, lamp monitor

13 Q_B output J3, lamp monitor

14 Q_A output J4, lamp monitor

15 Ripple Carry (active low) output No connection

Pin	Function	Connection
9	Load (active low) input	J22, normally high pulser
10	T' (active low enable) input	Ground, logic False
11	Q_D output	J1, lamp monitor
12	Q_C output	J2, lamp monitor
13	Q_B output	J3, lamp monitor
14	Q_A output	J4, lamp monitor
15	Ripple Carry (active low) output	No connection

Test your circuit and complete the table in the attached work sheet. It should normally count up in binary. Holding pulser button S7 down should cause it to count down. Pressing pulser button S8 should cause whatever value is set by switches S3 through S6 to be loaded into the counter. Releasing pulser button S8 should then re-enable counting from the loaded value.

Building a Ripple Counter

In the next two exercises you will use 7473 dual JK flip-flop chips. The pin outs for the 7473's are shown in Figure 2. Note the unusual locations for the power and ground connections (pins 4 and 11, respectively) on this chip.

7473 chip

Figure 2 -- Pin outs for the 7473 dual JK flip-flop chip.

Build the 4-bit binary ripple counter circuit shown in Figure 3. Use two 7473 dual JK flip-flop chips to build this circuit. Connect all the CLR (asynchronous clear) flip-flop inputs together and connect them to one of the normally high pulser outputs on your Digi Designer. Use one of the Digi Designer switches for the Enable input and use the Digi Designer clock (set at 1 Hz) as the Pulses source. Connect the counter outputs A₁ through A₄ to lamp monitors J4 through J1, respectively. Test your counter and record the results on the attached work sheet.

Ripple Counter

Figure 3 -- Use two 7473 chips to provide the 4 JK flip-flops needed for this 4-bit ripple counter circuit.

Building a Synchronous Counter

Build the 4-bit binary synchronous counter circuit shown in Figure 4. Use two 7473 dual JK flip-flop chip and one 7408 quad AND chip. Connect all the CLR (asynchronous clear) flip-flop inputs together and connect them to one of the normally high pulser outputs on your Digi Designer. Use one of the Digi Designer switches for the Enable input and use the Digi Designer clock (set at 1 Hz) as the Pulses source. Connect the counter outputs A₁ through A₄ to lamp monitors J4 through J1, respectively. Test your counter and record the results on the attached work sheet.

Synchronous Counter

Figure 4 -- Use two 7473 chips and a 7408 chip to build this 4-bit synchronous counter.

Chip pin out diagrams in this document were taken from Fairchild/National Instruments chip data sheets. Figures 3 and 4 were taken from Digital Design, 2^nd ed. by M. Morris Mano.