Alciatore D.G., Histand M.B. Introduction to Mechatronics and Measurement Systems

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Figure 6.41 555 pin-out. (Courtesy

of Texas Instruments, Dallas, TX)

Figure 6.42 Monostable multivibrator (one-shot).

trigger

outpu

reset

threshold

comparator

trigger

comparator

–

2/3V

1/3V

control

flip-flop

6.12 Special Purpose Digital Integrated Circuits 241

The circuit generates a single pulse of desired duration when it receives a trigger

signal, hence it is also called a one-shot. The time constant of the resistor-capacitor

combination determines the length of the pulse. In Figure 6.43 , the sequence of

operation is as follows: When the circuit is powered up or after the reset is activated,

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Figure 6.43 One-shot timing.

trigger

output

2/3V

capacitor

voltage

242 CHAPTER 6 Digital Circuits

the output will be low ( Q  0), transistor Q

is saturated (ON) shorting capacitor C,

and the outputs of both comparators are low. When the trigger pulse goes below

(1/3) V

, the trigger comparator goes high, setting the flip-flop. Now the output is

high ( Q  1), transistor Q

cuts off, and the capacitor begins to charge with time

constant   R

C. The time constant is the time required for the capacitor voltage to

reach 63.2% of its full charge value V

(see Section 4.9). When the capacitor volt-

age reaches (2/3) V

, the threshold comparator resets the flip-flop, which resets the

output low ( Q  0) and discharges the capacitor again. If the trigger is pulsed while

the output is high, it has no effect. The length of the pulse is given approximately

( Question 6.52 ) by

Δ T ≈ 1.1R

C (6.36)

As alternatives to the 555 one-shot circuit, the TTL logic family also includes

devices that can be used as one-shots. Examples are the 74121 and the 74123. They

also require an external resistor and capacitor to control the pulse width.

Another important circuit that can be constructed with a 555 timer is an

astable multivibrator, also called a square-wave generator or an oscillator. The

schematic for this circuit is shown in Figure 6.44 . When the circuit is powered,

the capacitor C charges through the series resistors R

and R

with a time constant

( R

 R

) C. When the voltage on C reaches (2/3) V

, the flip-flop resets, turning

on the discharge transistor that discharges the capacitor through resistor R

with

time constant R

C. When the voltage on C drops to (1/3) V

c c

, the flip-flop sets caus-

ing the cycle to repeat. The result is a signal consisting of repeating and regular

pulses.

Figure 6.45 shows how the capacitor voltage ( V

) and the output signal ( Q ) vary

over time as the astable multivibrator cycle repeats. From Section 4.9, an RC circuit,

which is a first-order system with time constant   RC, charges in response to a

step input V

as follows:

t V 1 e

–⫽

ccC

(6.37)

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Figure 6.44 Astable multivibrator.

output

–

555

reset

Figure 6.45 Astable multivibrator capacitor voltage and output signal.

capacitor

voltage (V

)

2/3 V

1/3 V

output (Q)

6.12 Special Purpose Digital Integrated Circuits 243

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244 CHAPTER 6 Digital Circuits

The time it would take the capacitor to charge from 0 V to 2/3V

, through series

resistors R

and R

, can be found from:

---

1 e

+ C

----------------------- ---

–=

(6.38)

Solving this equation gives:

+ C

---

ln–=

(6.39)

Similarly, the time to charge from 0 V to 1/3V

would be:

+ C

---

ln–=

(6.40)

Therefore, the period of charging through series resistors R

and R

from 1/3V

2/3V

, during which time the output is high, is:

 t

 t

 (R

 R

) C ln (2) (6.41)

Similarly, the period of discharge through resistor R

from 2/3V

to 1/3V

, during

which time the output is low, can be found to be ( Question 6.54 ):

 R

C ln (2)

(6.42)

Therefore the total period for the pulse cycle is:

T  T

 T

 ln(2)(R

 2R

)C ≈ 0.693 (R

 2R

)C (6.43)

and the cycle frequency is:

f 



_____________

ln(2)(R

 2R

≈

1.443

__________

 2R

(6.44)

To obtain a near-symmetrical square wave, the ON and OFF times for the pulse

widths, T

and T

need to be as close to equal as possible. This can be achieved by

making resistance value R

much larger than resistance value R

(see Class Discus-

sion Item 6.14 ).

■ CLASS DISCUSSION ITEM 6.14

Astable Square-Wave Generator

How could you attempt to produce a perfectly symmetric square wave with the

circuit in Figure 6.44 ? Are there any potential problems with this approach? Also,

if the capacitor has a partial charge on it before power is applied to the circuit, what

effect would this have on the circuit output?

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6.13 Integrated Circuit System Design 245

6.13 INTEGRATED CIRCUIT

SYSTEM DESIGN

For most digital design applications, integrated circuits can be used as building

blocks to create the desired functionality. A myriad of ICs on the market provide

almost every conceivable digital function. Listings of ICs along with data sheets

describing their functionality can be found online or in manufacturer TTL or logic

data books. The digital tachometer example in Design Example 6.1 illustrates a digi-

tal design solution employing several commercially available ICs. Internet Link 6.9

is an excellent resource providing a review of digital electronics fundamentals, logic

devices, circuit analysis and design, and application examples.

Digital Tachometer

Internet Lin

6.9All about

circuits—

Vol. IV—digital

(continued )

Our objective is to design a system to measure and display the rotational speed of a shaft.

A simple method to measure rotational speed is to count the number of shaft rotations during

a given period of time. The resulting count will be directly proportional to the shaft speed.

A number of different sensors can detect shaft rotation. A proximity sensor that uses

magnetic, optical, or mechanical principles to detect some feature on the shaft is one exam-

ple. We can use an LED-phototransistor pair as an optical sensor and place a small piece

of reflective tape on the shaft. Each time the tape passes by the photo-optic pair, the 7414

Schmitt trigger inverter provides a single pulse to a counting circuit. The following figure

illustrates the sensor and signal conditioning circuit.

5 V

330 Ω

1 kΩ

5 V

7414

output pulse

for each complete

rotation of the shaft

rotating

shaft

reflected light

reflective tape

Now we need a circuit to count and display the pulses over a given interval of time T. The

following figure illustrates all the required components. The 7490 decade counter counts the

pulses and is reset by a negative edge on signal R after the time period T. The period T is set

by a resistor-capacitor combination using a 555 oscillator circuit. If the count can exceed nine

during the period T, additional 7490s must be cascaded to provide the full count. Just prior to

counter reset, the output is stored by 7475 data latches that are enabled by a brief pulse on sig-

nal L. The latches are necessary to hold the previous count for display while the counter begins

a new count cycle. One of the two 74123 one-shots is positive edge-triggered by the clock

signal CK to generate a latch pulse L of length  t. Note that the latch and reset pulse widths

must be small ( t  T ) to maintain count accuracy (see Class Discussion Item 6.15 ). The

trailing edge of the latch pulse triggers the second one-shot, which is negative edge-triggered,

to produce a delayed reset pulse R for the counter. The 7447 LED decoder and driver converts

DESIGN

EXAMPLE 6.1

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246 CHAPTER 6 Digital Circuits

the latched BCD count into the seven signals required to drive the LED display. The display

reports the number of pulses that have occurred during the counting period T.

The shaft speed in revolutions per minute is related to the displayed pulse count by

rpm =

pulse count/ppr

___________

where ppr is the number of pulses per revolution generated by the sensor (e.g., 1 for a single

piece of reflective tape).

4-bit

decade

counter

7490

data

latches

7475

LED

decoder

and

driver

7447

LED

display

resistor

bank

I: input pulses

CK: clock timer

L: latch enable

R: counter reset

one-shot

pulse generators

clock

generator

555

74123

input

pulses

from

shaft

sensor

clock signal: CK

reset signal: R

latch signal: L

input pulses: I

Δt

■ CLASS DISCUSSION ITEM 6.15

Digital Tachometer Accuracy

What effect does the choice of ⌬ t have on the accuracy of the counts displayed by

the digital tachometer presented in Design Example 6.1 ?

■ CLASS DISCUSSION ITEM 6.16

Digital Tachometer Latch Timing

One potential problem with the digital tachometer circuit in Design Example 6.1

could occur if latching happens at the exact instant when the counter is increment-

ing in response to an input pulse. Explain why. This problem can be resolved by

blocking the input pulses during the latch with a logic gate. What would you need

to add to the circuit schematic to accomplish this?

(continued )

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6.13 Integrated Circuit System Design 247

(continued )

In Design Example 3.4, we introduced a discrete component power driver for mechatronic

system peripheral devices. The large semiconductor manufacturers provide specialized

integrated circuits that vastly aid the mechatronic designer in many applications. Example

ICs include relay drivers, lamp drivers, motor drivers, and solenoid drivers. In this chapter,

we learned how to design logic circuits to provide digital control outputs. These outputs

can then be interfaced to peripheral power devices to control mechanical power. Periph-

eral power requirements are quite varied, creating a need for interface ICs that have a large

degree of adaptability. Specialized interface ICs offer some benefits that are difficult for the

designer to include using discrete components. Example features include short circuit pro-

tection at outputs, glitch-free power-up, inductive flyback protection, and negative transient

protection.

Consider a situation where digital outputs are provided via an 8-bit bus that could be

used, for example, to control eight separate devices. A good choice for a peripheral driver is

the National Semiconductor DP7311 octal latched peripheral driver. This device can provide

up to 100 mA DC at each output at a maximum operating voltage of 30 V to drive LEDs,

motors, sensors, solenoids, or relays. It has open-collector outputs that require external pull-

up resistors when interfaced to high-current external discrete drivers capable of controlling

much higher currents than the IC itself. Let us consider two possible configurations.

The first configuration shows a typical output (one of eight) for a high-side (load

between supply and collector), large-current bipolar power transistor driver. The data in line

is active high, and the clear (CLR) and strobe (STR) lines are active low. The latches are

cleared (the outputs are turned off) when CLR goes low, and the data in values are latched

when STR goes low.

1 of 8

outputs

data in

(1 of 8)

latch

CLR

STR

control

logic

DP7311

30 V max

load

Digital Control of Power to a Load Using Specialized ICs

DESIGN

EXAMPLE 6.2

■ CLASS DISCUSSION ITEM 6.17

Using Storage and Bypass Capacitors in Digital Design

It is standard practice to include one or more large tantalum or electrolytic storage

capacitors (e.g., 100  f) across the power supply lines within a digital system and

to include a small ceramic bypass or decoupling capacitor (e.g., 0.1  f) between

the V

and ground leads supplying power to each IC. Why?

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248 CHAPTER 6 Digital Circuits

An alternative high-current n-channel MOSFET design is shown next:

1 of 8

outputs

load

supply

DP7311

(continued )

Figure 6.46

IEEE standard symbols for digital ICs. (Courtesy of Texas

Instruments, Dallas, TX)

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Questions and Exercises 249

6.13.1 IEEE Standard Digital Symbols

The IEEE standard symbols for illustrating digital input and output conditions are

shown in Figure 6.46 . When drawing or reading digital schematics, it is important to

recognize these conventions. You will find these symbols used in some manufactur-

ers’ TTL data books.

QUESTIONS AND EXERCISES

Section 6.2 Digital Representations

6.1. In computers, integers are sometimes represented by 16 bits. What is the largest posi-

tive base 10 integer that can be represented by a 16 bit binary number?

6.2. Convert each of the following base 10 integers to their equivalent binary representa-

tions. Show your work.

a. 128

b. 127

6.3. Convert each of the following base 10 integers to their equivalent hexadecimal repre-

sentations. Show your work.

a. 128

b. 127

6.4. Perform each of the following binary arithmetic operations and check your results

with decimal equivalents. Show your work.

a. 1101  1001

b. 1101  1001

c. 1101  1001

d. 111  111

e. 111  111

Section 6.3 Combinational Logic and

Logic Classes

6.5. Draw the schematics of logic circuits that produce the following logic expressions:

a . A  B

b . A · B

6.6. Create an inverter using a single NAND gate. Draw the schematic.

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250 CHAPTER 6 Digital Circuits

6.7. Write out a simplified Boolean expression and construct a truth table for each of the

following circuits. Assume 0  low  0 V and 1  high  5 V.

5 V

(a)

Hint: Try to write out a quasi-logic statements (see Sections 6.6.1 and 6.6.2 ) or a

truth table first, based on how the transistors function. Assume ideal transistors

operating either in cutoff or saturation.

(b)

(c)

(d)

Section 6.4 Timing Diagrams

6.8. Construct a timing diagram showing the complete functionality of a NOR gate.

6.9. Construct a timing diagram showing the complete functionality of an XOR gate.

6.10. Construct a timing diagram showing the results of the truth table in Example 6.2.

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