Download CHAPTER 11

CHAPTER
POTPOURRI
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:
11.1 OBJECTIVES
• 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
applications.
and
explain
some
typical
ROM
• Understand what programmable logic devices are and
name the leading types of programmable logic devices.
11.2 DISCUSSION
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.
11.2.0 The 555
Timer
The 555 timer is one of the more versatile devices ever implemented 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 deboun cing, 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.
11.2.1 Opto-Isolators
The focus of this book has been digital switching where
both the inputs and outputs of a circuit are at some digital logic
11.2.2 DIP Relays
11.2.3
Programmable
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 programmable 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 programmed at the factory to a users specification, PROMs (Programmable Read Only Memory) which can be user programmed once, UVEPROMs (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?
11.4 REVIEW
QUESTIONS
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.
LAB EXERCISE 11.1
The 555 Timer
Objectives
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 Microfarrad-1
Jumper Wires
Materials
1.
Place the 555 IC onto the C.A.D.E.T. breadboard, you will use
the 555 as a one-shot in this part of the lab exercise.
2.
Wire the circuit shown in Figure 11-5.
stable multivibrator.
3.
Turn on power to the circuit.
other lights should be off.
This is a mono-
Procedure
Dl should light and all
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 previous
circuit 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 a
couple 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 of
the output.
7.
Leave the circuit connected while you answer the
following questions.
1.
Is the circuit of Figure 11.5 retriggerable?
2.
Name one use of the astable circuit?
3.
Can a 555 produce a 50% duty cycle output?
4.
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 the
relay. Use LI8 to observe the relay output. Notice that the relay
is switching 12V not 5V. Also note that the diode is used to short
inductive 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 input
and the lead coming from the low side of the coil (pins 1 and 14
are the coil). Turn on power. Switch LSI to LO. Record the
current reading on the ammeter.
5.
Turn off power. Leave the circuit connected while you
answer 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 then
what is the advantage of having a 5V relay coil?
Questions
LAB EXERCISE 11.3
The Opto- Isolator
Objectives
In this lab exercise you will study the opto-isolator. An optoisolator 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
Materials
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.
Procedure
2.
Use both the fixed +5V supply and the 1.3 to 15V supply. Set
the positive supply for +12V before connecting the circuit. If
necessary 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 the
output. 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 to
CMOS?
Questions
LAB EXERCISE 11.4
Implementing
Logic Functions
with ROMs
Objectives
Materials
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)
Procedure
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 will
now program the equation into ROM. LS1-LS8 are your data
inputs. The switch register made from the logic switch is the
four low order bits of the address input to the ROM. PB2 enables
the IC while PB1 causes the ROM to store the data entered on
LS1-LS8
3.
Load the data from Table 11-1 into the ROM. To do this
you will need to set the address and data lines to their
proper values then press and hold PB2. Next mo
mentarily press PB1 while still holding PB2 down.
Release PB2 and the ROM has programmed the address
selected. Continue this process until all the values are
entered.
4.
Turn off power to the ROM. Remove the I/O lines at their
connection to LS1-LS8 and connect the free ends to LI1-LI8
respectively. Remove the lead at pin 27 and connect it to pin 22.
Remove the connection between pin 22 and Vcc and connect pin
27 to Vcc. The circuit is now ready to convert from hexadecimal
to ASCII.
5.
Turn on power. The controls to the ROM work much the
same as before; however, the ROM will now retrieve the
information when PB1 is pressed momentarily. Record
the output of this circuit for all sixteen combinations of
the inputs.
ADDRESS
OUTPUT
6.
Leave this circuit
following questions.
connected
while
answering
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?
Questions
the