POTPOURRICHAPTER

We will cover a wide variety of subjects in this chapter. Not all of these subjects are digital electronics in the strictest sense of the word; however, all of the subjects covered are pertinent to digital systems.

11.0 INTRODUCTION

Upon completion of this chapter you should be able to:

• Identify the uses of the 555IC.

• Describe an opto-isolator and it's uses.

• Understand the need for and application of DIP relays.

• Define ROM and explain some typical ROMapplications.

• Understand what programmable logic devices are andname the leading types of programmable logic devices.

11.1 OBJECTIVES

11.2 DISCUSSION

11.2.0 The 555 Timer

The accent in this chapter is on variety. You will study a fairly wide variety of devices and their application in digital circuits. The early sections of this chapter will deal with devices which generate timing signals and allow interfacing between digital circuits. The later portions of the chapter will deal with ROMs and programmable logic devices. These devices give the circuit designer alternatives to MSI circuits when implementing logic functions.

The 555 timer is one of the more versatile devices ever imple-mented as an IC. The device contains 23 transistors, 2 diodes and 16 resistors on a single chip configured as an eight pin DIP. The closely related 556 IC puts two 555 ICs onto a single chip in a 14 pin DIP. The circuit diagram and pinout diagram for the 555 are shown in Figure 11-1.

The 555 IC has two basic modes of operation, the astable mode and the monostable mode. In the monostable mode the device operates as a one-shot. The one-shot can be operated as a simple one-shot or a retriggerable one-shot. Typical applications are timers, missing pulse detectors, switch debouncing, and touch switches.

The 555 may also be operated in the astable or free-running mode. In this mode the device acts as an oscillator. Typical applications include LED or lamp flashers, pulse train generation, logic clocks, and tone generation. The applications discussed here are only a few of the applications of the 555 that you would likely encounter in digital circuits. A truly amazing number of 555 applications have been developed and entire books about 555 applications are available for the curious student.

An opto-isolator is a device which incorporates two distinct devices into one package. The input side of this device is an LED, most frequently an IR LED. The output section of this device is an NPN phototransistor. A phototransistor works exactly like a regular NPN bipolar junction transistor except that the base of the transistor is driven by a light source. This is possible since the base drive is not a high current.

An opto-isolator is used where logic circuits need to be electrically isolated from each other. The opto-isolator can perform this function and has the advantage of providing high voltage and noise isolation in a very small package. Typical applications for opto-isolators are logic type/level conversion, and interfacing digital and analog circuits with a large voltage difference in their operating characteristics.

The focus of this book has been digital switching where both the inputs and outputs of

a circuit are at some digital logic

11.2.1 Opto-Isolators

11.2.2 DIP Relays

11.2.3Programmable Logic Devices

level. Many devices that are controlled by digital logic do not operate at any convenient logic level. One method of driving this type of device is the DIP relay. A dip relay is much like any other relay except that the coil of the DIP relay is designed to operate from 5 VDC so that the relay can be used with logic circuits. This type of construction allows switching AC loads using TTL circuits with a minimum number of components.

The DIP relay is usually a reed relay which is a compact device easily integrated into solid state circuits. Reed relays are commonly encountered in telephone circuits. The importance of DIP relays is that they allow circuit designers to switch high current AC and DC devices with TTL logic levels.

Programmable logic devices give us alternatives to MSI components for implementing logic equations. The three major types of programmable logic devices are PALs, PLAs and ROMs. PAL stands for programmable array logic. A PAL is formed from a programmable AND array using a fixed OR array on the output. An example of this architecture is shown in Figure 11-2.

The PAL is programmed by "blowing" fuses in the AND array. The limitation of PALs is that not all of the AND gate outputs are available to each of the OR gate inputs.

A ROM (Read Only Memory) is constructed of a fixed AND array which, is used to fully decode the ROM inputs, and a program-mable OR array which provides the ROM outputs. Figure 11-3 shows

a ROM architecture.

ROMs are often described by the number of input combinations or addresses and the number of outputs. For example, a IK x 8 ROM has 10 (IK = 1024) inputs and 8 outputs . The ROM can be used as a simple memory array where values are stored at certain ROM addresses. The addresses are determined by the ROM inputs while the value stored is determined by the OR array.

The ROM can be used to perform logic functions by using the ROM inputs as the logic equation inputs and programming the ROM array so that the outputs correspond to the logic outputs for the given input conditions. This allows the designer to implement a separate independent logic function for each output. The basic limitation of ROMS in performing logic functions is their inability to provide the number of inputs and

outputs needed to perform a specific logic function. This problem arises since ROMs have a fixed number of inputs and outputs. For example, the IK x 8 ROM cannot implement a logic function with 11 inputs and 5 outputs even though the device has more inputs and outputs than the logic function requires.

ROMs are available as masked ROMs which are custom pro-grammed at the factory to a users specification, PROMs (Program-mable Read Only Memory) which can be user programmed once, UV-EPROMs (Ultra-Violet Erasable PROMs) which can be electrically programmed and then erased with ultra-violet light for reuse, and EEPROMs (Electrically Erasable PROMs) which can be electrically programmed and erased.

PLAs or programmable logic arrays offer the ability to program both the AND and OR arrays. The architecture of a PLA is shown in Figure 11-4.

The PLA, has more flexibility than either ROMS or PALs. The device is programmed by blowing fuses in the gate arrays. The biggest disadvantage of PLAs is that they are about half as fast as TTL bipolar circuits since the logic signals must travel through two programmable arrays instead of one. Further, the advantage of being able to access all of the AND gate outputs cannot be used in implementing many logic equations.

This chapter has focused on a variety of digital devices and their application. Digital uses of the 555 timer were highlighted. The opto-isolator and some of it's applications were introduced. DIP relays were discussed and their use in switching ac loads mentioned. The last section of the chapter dealt with programmable logic devices as alternatives to MSI circuits.

11.3 SUMMARY

1. Is the 555 timer a digital or analog circuit?

2. When would an opto-isolator be useful?

3. Name some uses of a DIP relay.

4. Name three types of programmable logic devices.

5. Explain the difference between PALs, PLAs and ROMs.

11.4 REVIEW QUESTIONS

LAB EXERCISE 11.1The 555 Timer

Objectives

Materials

Procedure

In this lab exercise you will study the 555 IC. You will use the 555 in both the monostable and astable modes. We will discuss some applications of the 555.

C.A.D.E.T. 555 Timer IC

Resistors 1 Megohm-2

Capacitors 10 Microfarrad-1,0.01 Microfarrad-1,0.22 Micro-farrad-1

Jumper Wires

1. Place the 555 IC onto the C.A.D.E.T. breadboard, you will usethe 555 as a one-shot in this part of the lab exercise.

2. Wire the circuit shown in Figure 11-5. This is a mono-stable multivibrator.

3. Turn on power to the circuit. Dl should light and allother lights should be off.

4. Use PB2 as the switch input and LI1 as the circuit output.Observe the circuit operation and record your obser-

vations. Note that the pulse width of the output pulse is about equal to R X C.

5. Turn off power. Remove the wiring for the previouscircuit and leave the 555 on the C.A.D.E.T. breadboard. Now wire the circuit shown in Figure 11-6. This is the free-running or astable circuit.

6. Turn on power to the C.A.D.E.T., LI1 should flash HI and LO acouple of times a second. Observe the operation of this circuit.Note that the frequency of the output is equal to about 1.44/((R1+2R2)C1). The time that the output is HI is about.7(R1+R2)C1. The time that the output is LO is about .7(R2)(C1).This means that the resistors can be used to set the duty cycle ofthe output.

7. Leave the circuit connected while you answer thefollowing questions.

1. Is the circuit of Figure 11.5 retriggerable?

2. Name one use of the astable circuit?

1. Can a 555 produce a 50% duty cycle output?

2. Is a 555 a digital circuit?

Questions

LAB EXERCISE 11.2 DIP Relays

Objectives Materials

In this lab exercise you will study the use of DIP relays. Dip relays are frequently used to switch non-TTL loads. Circuit designers are able to switch AC loads under the control of electronic logic circuits by using DIP relays as the switching element. You will study the use of a common DIP relay.

C.A.D.E.T.

DIP Reed Relay (Radio Shack 275-244)

Jumper Wires

Diode IN4148

100 KD. Resistor

Digital Multimeter

1. Place the DIP relay onto the C.A.D.E.T. breadboard. Wire the circuit shown in Figure 11-7. This circuit implements a DIP relay.

2. Use both the fixed +5V supply and the 1.3 to 15V supply. Set the positive supply for +12V before connecting the circuit. (See instructions for Exercise 10.1).

3. Switch LSI to HI. Turn on power. Use LSI as the input to therelay. Use LI8 to observe the relay output. Notice that the relayis switching 12V not 5V. Also note that the diode is used to shortinductive voltage spikes that occur when the relay turns off.Record your observations of the circuit operation.

4. Turn off power. Connect the ammeter between the LSI inputand the lead coming from the low side of the coil (pins 1 and 14are the coil). Turn on power. Switch LSI to LO. Record thecurrent reading on the ammeter.

5. Turn off power. Leave the circuit connected while youanswer the following questions

1. Could you use a normal TTL circuit to drive the relay?Why?

2. Name one application of relays.

3. If a normal TTL device cannot drive a relay directly thenwhat is the advantage of having a 5V relay coil?

Questions

LAB EXERCISE 11.3 The Opto- Isolator

Objectives

Materials

Procedure

In this lab exercise you will study the opto-isolator. An opto-isolator is a device which contains a light source/light sensor pair. The sources commonly used are LEDs, tungsten lamps, and neon lamps. Common sensors are phototransistors, photo-diodes, light activated SCRs, light activated TRIACs, and photoresistors. Other names for opto-isolators are optocoupler, photo-isolated coupler and photon isolators. The device we will use in this lab exercise is a 4N35 which has a LED source and phototransistor sensor. Opto-isolators are very useful for electrically isolating two circuits and for converting voltage levels at circuit interfaces.

C.A.D.E.T. 1 K Ohm

Resistor 100K Ohm

Resistor Digital

Multimeter

1. Place the opto-isolator onto the C.A.D.E.T. breadboard. Wire the circuit shown in Figure 11-8. This circuit will use the 4N35 to convert from TTL to CMOS voltage level.

2. Use both the fixed +5V supply and the 1.3 to 15V supply. Setthe positive supply for +12V before connecting the circuit. Ifnecessary review instructions in Exercise 10.1.3. through 4. (as marked in book).

3. Turn on power. Use LSI as the circuit input and LI1 as theoutput. Observe and record the operation of this circuit.

4. Remove the connection between LI1 and pin 5 of the 4N35. Connect a voltmeter to the output on pin 5. Switch LSI HI and LO and record the voltmeter reading.

5. Turn off power. Leave the circuit connected while you answer the following questions.

1. Name three types of light sources.

2. Name two types of light detectors.

3. Define opto-isolator.

4. Does the circuit of Figure 11-8 invert the input signal?

5. Does the circuit of Figure 11-8 convert from TTL toCMOS?

Questions

LAB EXERCISE 11.4Implementing

Logic Functionswith ROMs

Objectives

Materials

Procedure

In this lab exercise you will study implementing logic functions with ROMs. We will focus our study on the 2864 EEPROM. This PROM will operate from a 5 V supply and stores 8K bytes. You will use the 2864 to convert from hexadecimal to ASCII. ROMS are normally used to implement logic equations when a large number of the ROM storage cells can be used.

C.A.D.E.T. 2864 EEprom

DIP Switch (Position)

Jumper Wires Resistors 1

k ohm (4)

1. Place the 2864 onto the C.A.D.E.T. breadboard. Wire Power to pin 28 and ground to pin 14. Now wire the circuit shown in Figure 11-9. This circuit will allow you to program the EEPROM.

2. Turn on power. The IC should remain cool to the touch. You willnow program the equation into ROM. LS1-LS8 are your datainputs. The switch register made from the logic switch is thefour low order bits of the address input to the ROM. PB2 enablesthe IC while PB1 causes the ROM to store the data entered onLS1-LS8

3. Load the data from Table 11-1 into the ROM. To do thisyou will need to set the address and data lines to theirproper values then press and hold PB2. Next momentarily press PB1 while still holding PB2 down.Release PB2 and the ROM has programmed the addressselected. Continue this process until all the values areentered.

4. Turn off power to the ROM. Remove the I/O lines at theirconnection to LS1-LS8 and connect the free ends to LI1-LI8respectively. Remove the lead at pin 27 and connect it to pin 22.Remove the connection between pin 22 and Vcc and connect pin27 to Vcc. The circuit is now ready to convert from hexadecimalto ASCII.

5. Turn on power. The controls to the ROM work much thesame as before; however, the ROM will now retrieve theinformation when PB1 is pressed momentarily. Recordthe output of this circuit for all sixteen combinations ofthe inputs.

Questions

ADDRESS OUTPUT

6. Leave this circuit connected while answering the following questions.

1. How many memory locations does the 2864 have?

2. How many bits can each location store?

3. Would you normally use this ROM for this application?