Semiconductor device technology and digital system design

Computers and Electronics in Agriculture, 1 (1985) 5--29 Elesevier Science Publishers B.V., Amsterdam --Printed in The Netherlands

5

Review SEMICONDUCTOR DEVICE TECHNOLOGY AND DIGITAL SYSTEM DESIGN

EDWARD W. PAGE Department of Computer Science, Clemson University, Clemson, SC 29631 (U.S.A.) (Accepted 26 June 1985)

ABSTRACT Page, E.W., 1985. Semiconductor device technology and digital system design. Comput. Electron. Agric., 1: 5--29. Since the introduction of the first commercial integrated circuits nearly 25 years ago, the semiconductor industry has undergone what is now recognized as a revolution in electronics technology. Modern technology is now yielding complex circuits consistin~ of nearly half a million transistors on a single integrated circuit chip with a price of a few hundred dollars. Integrated circuit technology has provided increasingly capable functions at ever decreasing prices, thus resulting in ever widening areas of application. Digital system design has become a very challenging and stimulating endeavor. This paper provides insight into the nature of digital system design and integrated circuit technology. Additionally, it surveys the types of building blocks available in the commercial marketplace and discusses industry trends.

INTRODUCTION Since 1 9 6 1 , w h e n the first i n t e g r a t e d circuits were p r o d u c e d c o m m e r cially, t h e r e has been a seemingly endless flow o f new i n t e g r a t e d circuit devices. As early as 1 9 6 4 , G o r d o n E. M o o r e , n o w C h a i r m a n o f t h e B o a r d a n d Chief E x e c u t i v e Officer o f Intel C o r p o r a t i o n ( S a n t a Clara, CA), n o t e d that: the n u m b e r o f e l e m e n t s in a d v a n c e d i n t e g r a t e d circuits h a d d o u b l e d every y e a r since 1 9 5 9 and p r e d i c t e d t h a t this t r e n d w o u l d c o n t i n u e . No significant d e p a r t u r e s f r o m this p r e d i c t i o n , o f t e n referred to as M o o r e ' s law, haw~ been seen as e v i d e n c e d by the r e c e n t g r a p h o f Fig. 1 c o m p i l e d by H a y e s ( 1 9 8 4 ) . The earliest devices, w h i c h c o n t a i n e d r o u g h l y 1 t o 12 gates, are referred t o as small-scale i n t e g r a t i o n (SSI) devices. Medium-scale (MSI) i n t e g r a t i o n refers t o i n t e g r a t e d circuit c o n t a i n i n g 13 t o 100 gates and largescale i n t e g r a t i o n (LSI) refers to devices c o n t a i n i n g f r o m 1 0 0 to a b o u t 1 0 0 0 gates per chip. I n t e g r a t e d circuits c o n t a i n i n g m o r e t h a n 1 0 0 0 gates are classified as V L S I (very-large-scale i n t e g r a t i o n ) devices. M o d e r n t e c h n o l o g y will s o o n yield m i c r o p r o c e s s o r chips c o n t a i n i n g as m a n y as half a million transistors ( Z o r p e t t e , 1 9 8 5 ) a n d m e m o r i e s capable o f storing m o r e t h a n 1 million bits o f i n f o r m a t i o n (Posa, 1985).

10 6

-

256K-bit RAM •

f • 32-bit 64K-bit RAM •/~=~-u ~ microprocessor / " 16-bit / mlcroprocessor 4K bit RAM @//• 87bi t microprocessor

o o

4

-

to

~4o0 0)-~4 o

iK-bit RAM ~ 4 ? b i t -- ~ S I microprocessor lo 2

r)~ /-ssl 1960

I 1970

I 1980

I 1990

YEAR

Fig. 1. Growth of maximum achievable integrated circuit component density (Hayes, 1984).

The continuing rapid decline in the cost of functions provided by integrated circuit technology has resulted in ever widening application areas. There is hardly any segment of our everyday environment that has not been impacted by microelectronics. Integrated circuit technology continues to challenge the imagination and ingenuity of system designers. System design has become more demanding and, at the same time, more rewarding. Modern system designers must be able to design system architectures employing a wide spectrum of integrated circuits from basic gates to microprocessors. This paper provides an introduction to the nature of digital system design using commercially available semiconductor devices. It begins with a review of basic logic circuits and presents practical design examples using commercially available SSI, MSI and LSI components. Also represented is an overview of semiconductor device technology as well as recent trends within the industry. DESIGN USING SMALL-SCALE INTEGRATION Advances in electronics technology are impacting virtually every segment of the agricultural industry. Examples of recent applications of electronics in agriculture include livstock monitoring and identification, automatic inspection of fruits and vegetables, soil condition monitoring, and automated crop harvesting. Such applications require that quantities be monitored, manipulated arithmetically and stored. Measurements may be represented in either analog or digital form. An analog measurement is represented by a second variable that is di-

rectly proportional to the quantity being measured. Measurements of temperature, for example, may be represented by the height of a column of mercury or the degree of bending in a bimetallic strip. The speed of an automobile may be represented by the deflection of the speedometer needle. Air pressure or velocity may be represented by a voltage. Analog quantities vary over a continuous range of values. An analog quantity can therefore assume an infinite n u m b e r of values within a specified range. In a digital system, variables can assume only a finite number of values. Quantities in a digital system, therefore, do not vary over a continuous range but in discrete steps. Almost all modern digital systems use binary digital signals to represent the digits 0 and I in the binary number system. T h e binary number system is convenient for representation of digital quantities since signals can assume only two values. The two values can be represented by a number of physical devices such as switches, relays, transistors, punched holes and the polarity of magnetized areas on a magnetic tape. Numbers are expressed in the binary number system as a string of O's and l's. Binary arithmetic requires only a few simple rules and algorithms and is easily implemented electronically. Many agricultural applications of electronics deal with physical variables such as temperature, pressure, h u m i d i t y and fluid flow. Commerically available sensors for measuring physical variables typically provide analog outputs. However, digital systems are better suited for manipulating and storing information. Consequently, systems designed for agricultural applications frequently employ both analog and digital components. Because of the greater versatility of digital systems, designers tend to use digital components for all functions except those for which analog components are essential.

Basic logic circuits Signals within a digital system exist in one of two states: 0 or 1. A 0 might be represented by a voltage that is 0.8 V or less, while a 1 might be represented by a voltage that is 2 V or greater. Digital systems are constructed from simple elements called logic gates. A logic gate is a device with one or

(a)

(b) a

0 0 1 i

0 0 0 1

Fig. 2. Basic logic gates: (a) AND;

0 0 1 i

b

(c)

f

0 1 0 1

a

0 I ] ]

(b) OR;

0 1

(e) NOT

gate.

m o r e i n p u t terminals and a single o u t p u t terminal t h a t will p r o d u c e an o u t p u t o f 0 or 1 as d e t e r m i n e d by the gate inputs. T h e r e are t h r e e f u n d a m e n t a l t y p e s o f logic gates f r o m w h i c h a n y digital s y s t e m can be c o n s t r u c t e d : AND, O R and N O T gates. An A N D gate has t w o or m o r e inputs and p r o d u c e s a 1 o u t p u t o n l y w h e n all o f t h e inputs are 1. An O R gate has t w o or m o r e inputs and p r o d u c e s a 1 w h e n a n y o f the i n p u t s are 1. When all inputs are 0, an O R gate will p r o d u c e a 0 o u t p u t . T h e N O T gate, or inverter, has a single i n p u t and a single o u t p u t . A N O T gate p r o d u c e s an o u t p u t o f 1 w h e n the i n p u t is 0 and an o u p u t o f 0 w h e n the i n p u t is 1. T h e i n p u t / o u t p u t r e l a t i o n s h i p for a gate can be described b y a truth table. A t r u t h table specifies t h e gate o u t p u t f o r e v e r y possible i n p u t c o m b i n a t i o n . Figure 2 shows b o t h the graphic s y m b o l and the t r u t h table for AND, O R and N O T gates. An e x a m p l e will illustrate h o w logic gates are used. S u p p o s e we have a sensor t h a t will p r o d u c e an o u t p u t o f 1 if it is in c o n t a c t with a liquid and an o u t p u t o f 0 otherwise. T w o such sensors can be used to m o n i t o r t h e level o f liquid w i t h i n a tank. Assume t h a t o n e sensor $1 is p o s i t i o n e d at the desired u p p e r level and a n o t h e r sensor $2 is placed at the desired l o w e r level. T h r e e possible c o n d i t i o n s can o c c u r as illustrated in Fig. 3. T h e circuit o f Fig. 4 will p r o d u c e an o u t p u t o f 1 w h e n e v e r the liquid is e i t h e r t o o low or t o o high and a 0 w h e n e v e r t h e liquid level is w i t h i n a c c e p t a b l e limits. T h e o u t p u t f r o m $1 is c o n n e c t e d directly to an i n p u t o f the OR gate. This

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(a)

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(b)

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(c)

Fig. 3. Sensing water levels within a tank: (a) too low; (b) within limits; (c) too high.

Fig. 4. Circuit to detect out-of-limits condition.

forces the output of the OR gate high (i.e., to state 1) whenever the liquid level is at or above S, . The output from S2 is connected to an inverter which. in turn, is connected to the other input of the OR gate. Whenever S1 produces a low (i.e., a 0 indicating that the liquid level is too low) the inverter produces a 1 and the OR gate thus produces a 1 output. Only when S, z 0 and S2 = 1 will the output of the OR gate be 0. The OR gate output can be used to actuate an output device such as a warning light or a siren. The output of the OR gate, of course, is a digital signal ranging between 0 and a few volts. Frequently the output device may require 12 VDC (volts direct current) or even 110 VAC (volts alternating current) for actuation. In such cases, a variety of devices such as transistors, optical isolators, solid-state relays, or thyristors (Artwick, 1980) can be used to transform the gate output into the desired signal. Several additional basic gates are shown with their corresponding truth tables in Fig. 5. Although these gates can be constructed from an interconnection of AND, OR, and NOT gates, they are available and are widely used as independent structures. A NAND gate is simply an AND with a q

Fig.

5. Additional

SIVE-NOR

gate.

logic

gates:

(a) NAND;

(b) NOR:

(c)

EXCLUSIVE-OR;

(d) EXCLU-

10 built-in i n v e r t e r on t h e o u t p u t . Likewise, a N O R gate is an O R gate w i t h an i n v e r t e d o u p u t . In fact, the N A N D gate a n d t h e N O R gate are quite p o p u l a r w i t h designers since a n y desired logic circuit can be i m p l e m e n t e d using o n l y N A N D gates or o n l y N O R gates. An E X C L U S I V E - O R gate gives an o u t p u t o f 1 o n l y w h e n its t w o i n p u t s are d i f f e r e n t while an E X C L U S I V E - N O R gate gives an o u t p u t o f 1 o n l y w h e n its t w o i n p u t s are t h e same. T h e E X C L U S I V E - O R a n d the E X C L U S I V E - N O R t h e r e f o r e p r o d u c e c o m p l e m e n t a r y (inverted) o u t p u t s f o r identical inputs. As an e x a m p l e o f t h e use o f E X C L U S I V E - O R gates, c o n s i d e r the circuit c o n f i g u r a t i o n in Fig. 6. A s s u m e t h a t t h e signals L1, L2 a n d L3 are status lines f r o m a m a c h i n e a n d s h o u l d be identical w h e n the m a c h i n e is f u n c t i o n i n g n o r m a l l y . T h e t w o E X C L U S I V E - O R gates c o n n e c t e d as s h o w n will n o t o n l y d e t e c t a d i s a g r e e m e n t in t h e status lines, b u t will ident i f y the disagreeing line as well. S u p p o s e line I is high w h e n the o t h e r t w o are low. In this case, h0 = 1 a n d hi = 0. T h e o u t p u t o f t h e t w o E X C L U S I V E O R gates w h e n v i e w e d as the b i n a r y n u m b e r h l h o is 1, the n u m b e r o f the disagreeing line. C o n s i d e r w h a t h a p p e n s w h e n L3 = 0 w i t h b o t h L1 a n d L2 = 1. In this case, h0 = 1 a n d h l = 1 a n d the o u t p u t o f t h e t w o gates r e p r e s e n t s b i n a r y 3. A final gate s h o u l d be discussed f o r c o m p l e t e n e s s . This gate, t h e tris t a t e ~ or t h r e e - s t a t e gate, is i m p o r t a n t n o t b e c a u s e it p e r f o r m s a logic operL1 E2

T

L3

? h1 h0 Fig. 6. Use of EXCLUSIVE-OR gates to monitor status lines.

D

~

ID, if C=l ~isconnected, if C=O

C Fig. 7. A tri-state gate. '

Tri-State is a registered trademark of National Semiconductor Corporation.

11

a t i o n b u t because it provides a way o f c o n t r o l l i n g the c o n n e c t i o n and disc o n n e c t i o n o f signal paths. T h e tri-state gate, as s h o w n in Fig. 7, has a single d a t a i n p u t D and a c o n t r o l i n p u t C. When C = 1, the gate is said to be e n a b l e d and the d a t a o n the i n p u t simply passes to the o u t p u t . When C is 0, n o t o n l y is t h e d a t a at the i n p u t i n h i b i t e d f r o m passing t o the o u t p u t , the o u t p u t is e f f e c t i v e l y d i s c o n n e c t e d and carries n o signal. Tri-state gates are t y p i c a l l y used to c o n n e c t c o m p o n e n t s to a bus as in a c o m p u t e r system. T h e use o f tri-state gates p e r m i t s any n u m b e r o f devices t o be physically c o n n e c t e d to the bus y e t electronically isolated f r o m the bus until individually enabled. Tri-state gates can f r e q u e n t l y be used in logic designs in lieu o f basic gates as illustrated in Fig. 8. This simple n e t w o r k will select one o f the i n p u t signals and p e r m i t it to pass to the o u t p u t . If select = 0, signal A passes t o the o u t p u t ; if select = 1, signal B passes t o the o u t p u t . A device o f this t y p e t h a t passes a selected i n p u t signal t o the o u t p u t is k n o w n as a multiplexer.

B

SELECT

Fig. 8. Multiplexer implementation using tri-state gates.

T h e logic circuits t h a t we have c o n s i d e r e d t h u s far are k n o w n as combinational logic circuits since t h e o u t p u t can be d e s c r i b e d by a table t h a t specifies the o u t p u t f o r every possible c o m b i n a t i o n o f inputs. T h e p r e s e n t o u t p u t o f a c o m b i n a t i o n a l n e t w o r k is strictly a f u n c t i o n o f the present inputs. In the n e x t section we shall e x a m i n e sequential logic circuits. T h e p r e s e n t o u t p u t o f a sequential circuit d e p e n d s u p o n the p r e s e n t inputs as well as the p r e s e n t internal state. T h e internal state, in t u r n , d e p e n d s u p o n the past inputs t o the circuit. While a c o m b i n a t i o n a l n e t w o r k has n o memory, a sequential n e t w o r k m u s t i n c o r p o r a t e m e m o r y e l e m e n t s to store the internal state.

12

Flip -flops A flip-flop is a device having two stable states that can store a single bit o f information. Flip-flops are used to i m pl em ent logic circuitry for applications such as counters, sequencers and memories which require some f o rm of in f o r m a t i on storage. Although there are several different types o f flip-flops, t h e y share t w o c o m m o n properties: (1) A flip-flop will remain in its present stable state until an input signal causes it to change states. (2) A flip-flop has t w o c o m p l e m e n t a r y o ut put s (i.e., when one is high the o t her is low). One t y p e o f flip-flop is the RS flip-flop illustrated in Fig. 9. It has two inputs, R and S, t hat can cause a state change and two c o m p l e m e n t a r y outputs, Q and Q. The o u t p u t Q is simply Q inverted. When Q is high, the flip-flop stores a 1 and is said to be set. When Q is low, the flip-flop stores a 0 and is said to be reset. An R S flip-flop changes states according to the following rules: (1) When R and S are 0 the state will n o t change. (2) When S -- 1 and R = 0 in coincidence with a pulse on the clock input, the flip-flop will change to the set state. (3) When S -- 0 and R = 1 in coincidence with a pulse on the clock input, the -flip-flop will change to the reset state. (4) The co n d i t i on with bot h S = 1 and R = 1 is disallowed. (In this case the behavior o f flip-flop becomes unpredictable.)

CLOCK Fig. 9. RS flip-flop. Notice that the change to a new state takes place in coincidence with a pulse on the clock input referred to as a clock pulse. We may think of the R and S inputs as determining what the n e x t state of the flip-flop will be and the clock pulse as specifying when the transition will take place. Applications employing flip-flops typically will have the o u t p u t s of some flip-flops c o n n e c t e d through combinational circuitry to the inputs o f o th er flip-flops. A flip-flop o u t p u t may even be c o n n e c t e d to its own R or S input. What prohibits a change in one flip-flop's state from immediately propagating to the input of a n o t h e r and causing an u n w a n t e d change in th at flip-flop's state? Designers overcome this potential probl em through

13 the use of edge-triggered flip-flops. With edge triggering, the state change occurs during the transition of the clock pulse. A flip-flop may respond to a low-to-high clock pulse (positive edge triggering) or a high-to-low clock pulse (negative edge triggering). The clock transition requires only a few nanoseconds. In this short time, a flip-flop cannot experience a state change and have the new state influence other flip-flops that it may be, connected to. Edge triggering assures reliable and predictable circuit oper.. ation by allowing state transition to take place only within a very short time span. Thus, new states will not affect any control inpUts until after the clock pulse disappears. A system employing a collection of flip-flops would have all flip-flops connected to the same clock input. When the clock transition occurs, those flip-flops that are to change state will switch to the new state. No further state transitions can occur until the next clock transition. As a practical example, Fig. 10 shows a 4-bit register implemented using RS flip-flops. The data presently on the S inputs will be loaded into the flip-flops when a clock pulse appears and will become the Q outputs. Thus., any 4-bit value can be stored for as long as desired. The circuit of Fig. 10 can be extended to store binary values of arbitrary size. DATA

/

.....~

IN

A.

Q

~

\

Q

Q

F

W LOAI) PULSE

/ V DATA

OUT

Fig. 10. Four-bit register using RS flip-flops.

:LOCI'< Fig. II. J K flip-flop.

14

Another frequently used flip-flop is the J K flip-flop shown in Fig. 11. The J and K inputs act the same as the S and R inputs on an R S flip-flop with one exception: J and K are allowed to be high simultaneously. When J = 1 and K = 1, in coincidence with a clock pulse, the flip-flop will change its state. When a flip-flop makes a transition from its present state to the complementary state, it is said to toggle. Figure 12 shows a binary counter implemented with J K flip-flops. This counter will count up to 7 pulses on the clock input; the 8th pulse resets the count to 0. To understand the operation of the counter, assume that all three flip-flops are initially reset. Since the J and K inputs of flip-flop #1 are connected to a constant logical 1, flip-flop #1 will toggle on each clock pulse. Note that the J and K inputs of the others are connected to the Q o u t p u t of the flip-flop to the immediate right. Therefore, the other flip-flops will toggle only when the flip-flop to the right is set and a clock pulse occurs. Flip-flop #1 then toggles with every clock pulse; flip-flop #2 toggles with every second clock pulse and flip-flop #3 toggles with every fourth clock pulse. The present count is the state of the three flipflops (Q3 Q2 Q1) interpreted as a binary number. By adding flip-flops, a counter of arbitrary size can be constructed. Special decoder chips may be employed to convert the count to decimal and display it in a lightemitting diode (LED) display unit of the type used in electronic calculators.

I T A

i Q3

Q2 e2~

33

J2

K2

A K1

r I

CLOCK v

Fig. 12. Binary counter using JK flip-flops.

CLOCK

Fig. 13. D flip-flop.

15

A t h i r d f r e q u e n t l y used flip-flop is the D flip-flop s h o w n in Fig. 13. Unlike R S and JK flip-flops t h a t have t w o c o n t r o l inputs, the D flip-flop has o n l y one c o n t r o l input. T h e D flip-flop has a single simple rule o f operation: the new state Q will be the state o f the D i n p u t at the t i m e the clock occurs. Thus, the D flip-flop simply stores the present D input. In a d d i t i o n to the c o n t r o l inputs and the clock input, c o m m e r c i a l l y available flip-flops have inputs to establish the initial state. A clear i n p u t forces a flip-flop t o the Q = 0 state while a preset i n p u t forces a flip-flop to the Q := 1 state. These inputs, w h e n c o n n e c t e d to resistor-capacitor n e t w o r k s , can force a s y s t e m e m p l o y i n g flip-flops t o start in any desired state. ,As a final e x a m p l e o f applications e m p l o y i n g flip-flops, Fig. 14 shows a s e q u e n c e r t h a t p r o d u c e s an alternating s e q u e n c e o f pulses on o u t p u t s O1 and 02. This s e q u e n c e r c o u l d be used to c o n t r o l a simple process consisting o f t w o alternating activities. An e x a m p l e is an a u t o m a t i c fruit ins p e c t i o n system w h e r e the first activity is to m o v e a piece of fruit into p o s i t i o n and the s e c o n d is to activate the inspection unit. Once started, this process w o u l d r e p e a t until a stop b u t t o n is pressed. F o r simplicity, assume t h a t the pulse g e n e r a t o r generates a stream o f equally spaced pulses of the p r o p e r f r e q u e n c y and d u r a t i o n and t h a t the run and stop b u t t o n s p r o d u c e a single pulse w h e n pressed. Also assume t h a t b o t h flip-flops are initially reset. When the start b u t t o n is pressed, the D flip-flop b e c o m e s set and, in t u r n , enables the train o f pulses f r o m the pulse g e n e r a t o r to pass t h r o u g h the AND gate and into the clock i n p u t o f the JK flip-flop. The JK flip-flop will begin to toggle with each pulse f r o m the pulse generator. Since b o t h flip-flops were assumed to be reset initially, ~)2 -- 1 initially and

PULSE GENERA'POR

I

- -

A

Q] CLEAR D

I STOP

Fig. 14. A simple sequencer.

Q1

16

the first pulse causes a pulse to appear at o u t p u t O1. Since the pulse will toggle the J K flip-flop, the second pulse will cause a pulse to appear at O2. The sequencer will continue to produce pulses at O1 and 02 in an alternating fashion until the stop button is pressed. MEDIUM-SCALE INTEGRATION TECHNOLOGY

Medium-scale integration technology began to emerge during the latter years of the 1960%. As standard circuits of 20 to 100 gates complexity began to appear, the previously clear distinctions between system design, logic design and device selection began to blur and become heavily interdependent. Far more importance was placed upon selecting an appropriate mix of MSI and SSI devices to perform the desired function. The higher level of integration provided by MSI resulted in less costly designs offering increased reliability and decreased power consumption. The availability of a wide variety of MSI chips opened many new possibilities in system design. The art of logic design became more demanding but much more stimulating and rewarding. This section highlights a few of the hundreds of available MSI devices that seem particularly appropriate for agricultural applications. ICL8211. As an example of the variety of MSI building blocks available, consider the ICL8211 offered by Intersil, Inc., Cupertino, CA (Intersil, 1984). This device is intended for precise voltage detection and generation. A particularly useful application of the ICL8211 chip is to detect low battery voltage as may be required in portable or remotely operated equipment. The circuit of Fig. 15 provides a simple technique for detecting a depleted or discharged battery. The external resistors are chosen according to a formula provided by the manufacturer to establish the desired threshold voltage below which the light-emitting diode will turn on. A

-,7

S

ICL8211 ±

150KI

~

3ED AMP

.IT

1

Fig. 15. Low-voltage battery indicator (Intersil, 1984).

17 L M 5 5 5 . T h e L M 5 5 5 timer, o f f e r e d by N a t i o n a l S e m i c o n d u c t o r , Santa Clara, CA (National S e m i c o n d u c t o r , 1 9 8 0 ) as well as a n u m b e r o f o t h e r m a n u f a c t u r e r s , is used t o generate t i m e delays or oscillation. When used in the t i m e delay m o d e , the delay is c o n t r o l l e d b y an e x t e r n a l resistor and c a p a c i t o r selected t o m e e t the application r e q u i r e m e n t s . When used as an oscillator, the f r e q u e n c y o f oscillation as well as the d u t y cycle (i.e., the ratio o f o n t i m e to the p e r i o d o f oscillation) are d e t e r m i n e d by t w o e x t e r n a l resistors and a c a p a c i t o r . Figure 16 shows a 50% d u t y cycle oscillator emp l o y i n g the LM555. This c o n f i g u r a t i o n c o u l d be used as the pulse g e n e r a t o r f o r the s e q u e n c e r of Fig. 14.

i 2

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Fig. 16. Oscillator with 50% duty cycle (National Semiconductor, 1980). L M 1 8 3 0 . A final e x a m p l e o f an MSI building b l o c k is the L M 1 8 3 0 fluid d e t e c t o r o f f e r e d b y National S e m i c o n d u c t o r (National S e m i c o n d u c t o r , 1980). This chip can d e t e c t the p r e s e n c e (or absence) o f w a t e r or o t h e r c o n d u c t i v e fluids and has obvious a p p l i c a t i o n to areas such as irrigation and a q u a c u l t u r e . T h e p r e s e n c e o f a liquid is d e t e c t e d by c o m p a r i n g the resistance o f the fluid b e t w e e n t w o p r o b e s with an internal r e f e r e n c e resistance. In case the fluid resistance should be significantly d i f f e r e n t f r o m the 13 k ~ internal r e f e r e n c e , an e x t e r n a l r e f e r e n c e resistance can be used with the L M 1 8 3 0 . T h e p r o b e m a y be as simple as t w o steel rods or t w o parallel plates o f c o p p e r . When sensing fluid level in a c o n d u c t i n g t a n k , the tank can be g r o u n d e d and a steel r o d a few c e n t i m e t e r s long can be used as the p r o b e . When t h e resistance b e t w e e n the p r o b e s e x c e e d s the r e f e r e n c e resistance, the o u t p u t o f t h e L M 1 8 3 0 will be a square wave with a 50% d u t y cycle capable o f driving an L E D or a l o u d s p e a k e r . If the L M 1 8 3 0 is to drive a logic gate, as r e q u i r e d o f the sensors in the e x a m p l e of Fig. 4, an e x t e r n a l c a p a c i t o r can be e m p l o y e d t o p r o v i d e a c o n s t a n t voltage o u t p u t . Figure 17 shows the L M 1 8 3 0 used to d e t e c t a low fluid level which is i n d i c a t e d b y driving an L E D with a square wave o f a p p r o x i m a t e l y 6 kHz.

18

LED

Fig. 17. Low-level warning device with LED indication (National Semiconductor, 1980). MICROPROCESSORS A microprocessor is the processing and control portion of a computer implemented as a single integrated circuit chip. By employing the microprocessor with m e m o r y , i n p u t / o u t p u t devices, and a few ancillary chips, a complete computer can be constructed. A computer based upon a microprocessor is referred to as a microcomputer. The first microprocessor, the 4004, was introduced by Intel Corp., Santa Clara, CA, in 1971. The 4004 is a 4-bit microprocessor that was designed for calculator applications. By 1972, several 8-bit microprocessors reached the market place. Digital designers built microcomputers to serve application areas that had previously relied upon some combination of SSI, MSI and custom LSI chips. Microprocessor applications began to proliferate because they were inexpensive, they offered reduced design time and they offered greater design flexibility. The greater design flexibility is a result of the programmable nature of the microprocessor. The ability to change the operation of a system by altering the software is an attractive alternative to extensive hardware modifications that are normally required to modify hardwired designs. It should be n o t e d that 'micro' refers to size and cost and not to capability. As early as 1978, several microprocessors were introduced that would rival minicomputers in terms of performance.

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20 In 1976, Intel introduced the 8048 -- a c o m p l e t e c o m p u t e r implemented on a single integratetl circuit chip (Page, 1984). The 8048 quickly began to find use in a wide spectrum of applications ranging from toys to satellites. Though not as fast as other popular microprocessors such as the Zilog (Cupertino, CA) Z-80, 8048-based systems offered much capability in only a few chips. The system of Fig. 18, for example, features an 8-bit microprocessor, 64 bytes of read/write m e m o r y , 1024 bytes of ultraviolet erasable program m e m o r y , a timer t h a t can also be used as an event counter, 16 digital i n p u t / o u t p u t pins, two testable input pins, an interrupt capability, sixteen 8-bit analog inputs and two 8-bit analog outputs. VLSI technology has continued to provide increased functional capabilities of microprocessors, microcomputers, memories and support chips such as m e m o r y management units, floating-point processors and network interfaces. Today, several 32-bit microprocessors with mainframe-like features are commercially available. Examples are the Motorola Semiconductor Products (Phoenix, AZ) MC68020 which provides a processing rate approaching 2 to 3 million instructions per s, and the National Semiconductor NS32032 which offers an instruction rate of 1 million instructions per s. A number of manufacturers already offer 256K-bit m e m o r y chips, and 1-million-bit m e m o r y chips will soon become available. The 32-bit microprocessors and dense m e m o r y chips are being used in applications far different from their 8-bit predecessors of a decade ago. Whereas the earliest microprocessors f o u n d applications as controllers or as low-speed computational devices, the 32-bit microprocessors are finding applications in areas such as high-performance graphics workstations, fault-tolerant computers and robotics. The demand for the simpler, slower microprocessors and microcomputers continues to be strong for a host of applications not requiring high-performance processing. The trend in this area is to integrate more and more functions on a single chip. The Intel 8096 microcontroller, for example, is a single-chip processor specially designed for high-speed control functions (Intel, 1984). It is a 16-bit microcomputer featuring 8K bytes of program m e m o r y , 232 bytes of internal read/write m e m o r y , a 10-bit analog to digital converter, four 16-bit software timers, five 8-bit i n p u t / o u t p u t ports, a serial port, and a pulse-width modulated output. SEMICONDUCTOR DEVICE TECHNOLOGY p--n J u n c t i o n Semiconductor devices are based upon the electrical properties of the p--n junction. The p- and n-types of material are formed by adding impurities to silicon through a process known as doping. If silicon is doped with a material such as phosphorus, an n-type region having a surplus of electrons is formed. Likewise, a p-type region is formed by doping silicon with a material such as boron. A p-type region is characterized by a dearth

21

o f electrons. When p and n regions are f o r m e d side b y side, a p--n j u n c t i o n results. A p--n j u n c t i o n has t h e special p r o p e r t y o f p e r m i t t i n g c u r r e n t flow in o n l y o n e direction. If the p--n j u n c t i o n is biased so t h a t the voltage applied to the p side is higher t h a n t h e voltage on t h e n side, a c u r r e n t will flow. This current. is a result o f the f l o w o f e l e c t r o n s in t h e n-material t o w a r d the p-materia] as well as the flow o f positive charges t o w a r d t h e n-material. T h e positive charges are r e f e r r e d to as holes. A l t h o u g h a hole is simply a slot f o r an e l e c t r o n , it does carry a n e t charge a n d c o n t r i b u t e s to c u r r e n t flow just as an e l e c t r o n t h a t moves in t h e o p p o s i t e direction. When the p--n j u n c t i o n is biased so t h a t the voltage applied to the p region is l o w e r t h a n the voltage applied to the n region, c u r r e n t flow is r e d u c e d essentially to zero. U n d e r this c o n d i t i o n , o n l y a small c u r r e n t k n o w n as leakage c u r r e n t flows and t h e p--n j u n c t i o n is said to be reverse biased. A s e m i c o n d u c t o r device consisting o f a single p--n j u n c t i o n is called a diode. Because o f the p r o p e r t y o f p e r m i t t i n g c u r r e n t flow in o n l y one d i r e c t i o n , diodes are used in m a n y diverse applications f r o m p o w e r supplies to c o m p u t e r logic circuits. Transis tots T h e p - n j u n c t i o n provides the basis f o r the c o n s t r u c t i o n o f transistors. By sandwiching some p - t y p e material b e t w e e n t w o n - t y p e regions, an npn transistor is f o r m e d . In a similar fashion, a p n p transistor can be f o r m e d by placing a region o f n - t y p e material b e t w e e n t w o p - t y p e regions. A t r a n s i s t o r has t h r e e terminals k n o w n as the emitter, the base and the collector. In an npn transistor, the e m i t t e r emits e l e c t r o n s which flow across the base i n t o the collector. T h e base c o n t r o l s the flow o f e l e c t r o n s f r o m the e m i t t e r to t h e c o l l e c t o r . T o u n d e r s t a n d the o p e r a t i o n o f a transistor, c o n s i d e r t h e a c t i o n o f an n p n transistor u n d e r the i n f l u e n c e o f e x t e r n a l l y applied voltages as s h o w n in Fig. 19. Here the b a s e - e m i t t e r j u n c t i o n is f o r w a r d biased and the collectorbase j u n c t i o n is reverse biased. Since t h e emitter-base j u n c t i o n is f o r w a r d biased, we w o u l d e x p e c t t o see a h e a v y f l o w o f e l e c t r o n s f r o m the e m i t t e r

I EMITTER BASE COLLECTOR

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22 into the base and a nearly negligible flow of holes from the base to the emitter. B e c a u s e / t h e collector-base junction is reverse biased, we might expect to see only a small leakage current across the collector-base junction. However, by making the base region very thin, the bulk of the electrons leaving the emitter pass through the base region w i t h o u t recombining with holes and are swept into the collector. By arranging the external bias voltages so t h a t the base potential can be controlled by an external signal, the current flowing between the emitter and the base can be altered. If the base voltage is reduced to zero, the current from the emitter to the base is halted. Since no electrons would be injected into the base by the emitter, there would be no current flow across the collector-base junction either. If the base voltage is increased, more electrons will flow across the base and into the collector. By changing the base potential, the collector current can be switched on and off, thereby forming a basis for the implementation of digital logic circuits. Alternative designs use the emitter to control the flow of collector current and modern integrated circuits employ transistors having two or three emitters sharing a c o m m o n base and collector. There are only two major fabrication technologies for digital integrated circuits: bipolar and MOS (metal-oxide-semiconductor). Both processes employ p--n junctions to fabricate transistors; however, the two processes yield integrated circuits with vastly different characteristics. A very readible discussion of semiconductor theory as well as fabrication technology is presented by Meindl (1977).

Bi-polar transistors. When transistors are fabricated by forming npn (or pnp) regions in a silicon substrate, the transistor is referred to as a bi-polar transistor. The term bi-polar is used since both positive and negative charges (electrons and holes) contribute to current flow. Bi-polar transistors are the mainstay of the modern electronics industry. Figure 20 shows the cross-section of a bi-polar transistor.

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23 M O S transistors. The o t h e r m a j o r t y p e o f transistor is r e f e r r e d to as a MOS-

F E T (metal o x i d e s e m i c o n d u c t o r field e f f e c t transistor). As illustrated in Fig. 21, a typical M O S F E T is f o r m e d by creating t w o islands o f n - t y p e material in a piece o f lightly d o p e d p material k n o w n as the substrate. One o f the n - t y p e regions is r e f e r r e d t o as the source and the o t h e r is re. f e r r e d to as the drain. B e t w e e n the s o u r c e and the drain is a region o f substrate k n o w n as the channel. A thin layer o f insulation (usually silicon d i o x i d e ) is f o r m e d over t h e channel and a layer o f m e t a l is d e p o s i t e d on t o p o f the insulation. This f o r m s a third e l e c t r o d e k n o w n as the gate. Alt h o u g h the gate is s e p a r a t e d f r o m t h e c h a n n e l b y the layer o f insulation, it is c o u p l e d to the silicon b y an electric field. A charge on the gate can t h e r e f o r e i n f l u e n c e t h e m o t i o n o f charge carriers in the channel. T h e n a m e f o r this t y p e o f transistor was derived f r o m the materials used t o f o r m it (the m e t a l gate, o x i d e insulation and s e m i c o n d u c t o r substrate) as well as the m a n n e r in which t h e gate is c o u p l e d to the silicon (the electric field).

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Fig. 21. N-channel MOS transistor: (a) symbol; (b) cross section. As t y p i c a l l y e m p l o y e d , the s o u r c e and the substrate are held at g r o u n d and the drain is m a i n t a i n e d at a positive voltage. With n o voltage applied t o the gate, t h e r e is a high resistance p a t h t h r o u g h the c h a n n e l since the p - t y p e substrate is lacking in electrons. When a positive voltage is applied t o the gate, e l e c t r o n s f r o m t h e substrate a c c u m u l a t e in the c h a n n e l region. These e l e c t r o n s drastically l o w e r the resistance f r o m the source to the drain and large c u r r e n t s can flow. Unlike bi-polar transistors in which b o t h holes and e l e c t r o n s c o n t r i b u t e to c u r r e n t flow, t h e r e is essentially o n l y o n e t y p e o f charge carrier t h a t c o n t r i b u t e s to c u r r e n t flow in a M O S F E T . When the s o u r c e and the drain are b o t h n - t y p e material, the charge carriers causing c u r r e n t flow t h r o u g h the channel are electrons. Such devices are r e f e r r e d to as n-channel or NMOS devices. Transistors f a b r i c a t e d using p - t y p e material f o r the source and drain and n - t y p e material f o r t h e substrate are r e f e r r e d to as p-channel or PMOS devices. In a PMOS device, holes p r o v i d e the c o n d u c t i n g p a t h f r o m the s o u r c e t o the drain.

24 Combining n-channel and p-channel transistors on the same substrate in a complementary fashion results in CMOS (complimentary MOS) technology. With CMOS technology, logic circuits are implemented with pairs of transistors arranged so that one is always turned off while the other is on. This arrangement result in appreciable current flow only momentarily when an input signal causes the transistor pair to switch states. When the transistor pair is stable, current flow through them is barely measurable. CHARACTERISTICS OF DIGITAL LOGIC FAMILIES The primary bi-polar logic families that today's system designers must be aware of are TTL (transistor-transistor logic) and ECL (emitted-coupled logic). Circuits implemented in bi-polar technology tend to be relatively fast but also tend to consume relatively large amounts of power. Most current SSI and MSI circuits employ TTL technology. When faster switching times are important, ECL is employed at the expense of higher power consumption. The MOS technologies have been preferred over bi-polar for LSI and VLSI applications because they offer significantly higher component densities. Additionally, MOS circuits typically consume less power than equivalent bi-polar circuits. The dominate MOS logic family is CMOS. The TTL and CMOS families are the most important ones for designers to be familiar with.

T T L logic circuits The TTL family makes use of an assortment of bi-polar transistors as the basis for a rich variety of SSI and MSI circuits. Standard TTL logic gates are identified by a 54/74 numbering convention. Industry qualified parts use numbers beginning with 7400 while military standard parts are numbered beginning at 5400 and meet more stringent specifications. The basic logic gates in the 54/74 series are housed in a 14-pin package. Pin 14 is used for the power supply while pin 7 is the ground connection. Several examples of 54/74 series gates are shown in Fig. 22. There are several requirements that designers must keep in mind when using TTL gates. The supply voltage is 5 V with a tolerance of 5% for the 7400 series and 10% for the 5400 series. The fan-out for standard TTL gates is 10. This means that the o u t p u t of one gate can serve as an input to as m a n y as ten other TTL gates w i t h o u t the need for signal amplification. As illustrated in Fig. 23, the minimum voltage that a TTL gate will provide for a logical 1 is 2.4 V. Likewise, the m a x i m u m voltage that a TTL gate will provide for a logical 0 is 0.4 V. On input, a TTL gate is guaranteed to recognize a signal of 2 V or greater as a logical 1 and a signal less than 0.8 V as a logical 0. The noise margin for a TTL gate is at least 0.4 V. This means that a low o u t p u t of 0.4 V can fluctuate between 0

25

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and 0.8 V and still be recognized as a logical 0 by another TTL gate. Similarly, a high o u t p u t of 2.4 V can fall as l o w as 2 V and still be recognized as a logical 1. The voltage range b e t w e e n 0.8 and 2.0 V s h o u l d occur o n l y w h e n a gate is switching states. A TTL gate will typically exhibit a propagation delay of 10 ns and will c o n s u m e a p p r o x i m a t e l y 12 mW. Special types of TTL circuits featuring higher switching speeds and lower p o w e r c o n s u m p t i o n are available. CMOS A s e c o n d popular family of devices is CMOS. In recent years several s e m i c o n d u c t o r manufacturers have placed increasing emphasis u p o n CMOS and it is e x p e c t e d to a c c o u n t for half o f the integrated circuit market by the end of the decade. CMOS is a nearly ideal logic family. Advantages of CMOS include l o w p o w e r c o n s u m p t i o n , low propagation delays and high noise i m m u n i t y . Unlike TTL, which requires a p o w e r supply of 5 V _+ 10%, CMOS circuits will operate over the range of 3 to 15 V. Furthermore, the p o w e r required for static operation is in the order of 10 nW

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Fig. 23. TTLlogiclevelcharacteristics. per gate. As the rate at which gates are switched on and off increases, so does the power consumption. Even when switched at 1 MHz, typical power consumption is less than 10 mW per gate. Electronic wrist watches, for example, use CMOS technology and can operate for months on the power supplied by a miniature battery. The lap-top computers introduced approximately two years ago use CMOS circuitry and can typically operate for 20 h from four AA batteries. In addition to the low power consumption, CMOS circuits possess high i m m u n i t y to noise that may produce voltage spikes within a system. The noise margin for CMOS is typically 45% of the supply voltage. Therefore, a CMOS circuit operating with a 15 V power supply can tolerate spurious voltage spikes of 6.75 V w i t h o u t generating an incorrect logic level. The first family of CMOS devices was the 4000 series originated by RCA (Somerville, NJ) in the mid-1960's and is now offered by several semiconductor manufacturers. The 4000 series includes a variety of SSI and MSI parts. In 1978, National Semiconductor introduced the 54C/74C product line which provides a large number of circuits that are logically equivalent to the 54/74 series parts. CMOS technology is currently employed to produce integrated circuits ranging from SSI to VLSI. Intel, for example, offers a 256K CMOS m e m o r y chip (Posa, 1985) and Motorola

27 P r o d u ct Division (Oak Hill, TX) is producing a 32-bit CMOS microprocessor (Zorpette, 1985). Designers must exercise care when mixing T T L and CMOS circuits and even when mixing CMOS circuits from the 4000 series with 54C/74C series devices. Many CMOS circuits feature T T L compatible outputs which facilitates the problems associated with combining different logic families. A good discussion of the interfaces required to mix CMOS chips with devices fr o m o th er families can be f o u n d in the National S e m i c o n d u c t o r Application Note 77 (National S e m i c o n d u c t o r , 1978). SEMICUSTOM CIRCUITS Standard, pre-packaged circuits offer a rich variety of functions ranging from simple SSI gates to VLSI microprocessors and associated devices such as memories, disk controllers and n e t w o r k interfaces. Yet, there is a lagrge void n o t covered by standard building blocks. This void is created by the need for application-specific devices. Until recently, application specific circuits were i m pl e m ent ed from standard, pre-packaged circuits or in the form of custom chips. The use of pre-packaged chips does not provide the high level of integration needed for m any of t o d a y ' s commercial products. While the use of custom chips provides vastly increased packaging density, increased reliability and decreased power consumption as co mp ar ed with the use of standard chips, most users do not have the need for the chip volume necessary to justify custom design. To fill the void between standard chips and custom designs, s e m i c o n d u c t o r manufacturers have begun to provide several alternatives for designing semicustom. application-specific integrated circuits. Programmable logic arrays, gate arrays and cell libraries are the pr edom inat e approaches to semicustom designs.

Programmable logic devices At the lowest level of c o m p l e x i t y are programmable logic devices ( P L D ' s ) The simplest form of a PLD is a grid-like pattern of AND and OR gates that can be i n t e r c o n n e c t e d to i m pl e m ent a desired logic function. The required in ter co nnect i ons are programmable by the user. Once programmed, the in ter co n n ect i ons can n o t be altered. One popular family of PLD's was invented by Monolithic Memories, Inc. (Sunnyvale, CA); members of this family are know n as PAL's (programmable logic arrays). PLD's are used most effectively in the replacem e n t of SSI and MSI parts. Although PLD's are typically used to replace 5 to 10 SSI and MSI devices, some of the more recent PLD chips, such as the AMP AL2 2 V 10 provided by Advanced Micro Devices (Sunnyvale, CAI can be used to replace 500 to 1000 gates. Programmable logic devices have f o u n d a niche in the marketplace because of off-the-shelf availability,

28 user programmability, low cost and the short development cycle required to apply them. Gate arrays

A gate array is a matrix of basic logic and peripherial cells that can be made into a custom-designed integrated circuit with the addition of from 1 to 3 layers of metal interconnections. Although a gate array can potentially contain thousands of gates, typical designs are limited to less than 500 gates to facilitate the testing process. A prime example of the use of gate arrays is the Sinclair ZX81 personal computer (marketed in the US as the Timex Sinclair TS 1000) which owes its small size to the use of four gate arrays. Additionally, a number of mainframe computers rely heavily upon the use of gate arrays. To implement semicustom chips from gate arrays, designers use computer aided design (CAD) workstations featuring high-resolution graphics to establish the cell interconnections, to simulate circuit operation and to generate testing procedures. All of the design work can be completed at the designers facility. The required interconnections are given to the semiconductor manufacturer who, in turn, produces the chip. Using a CAD workstation, system designers can go from a logic diagram to a completed integrated circuit in about 8 weeks. The typical development cost for gate arrays ranges from $5000 to $25 000. Gate arrays were originally developed by Texas Instruments, Inc. (Dallas, TX). Other semiconductor manufacturers offering gate arrays include American Microsystems, Inc. (Santa Clara, CA), Signetics Corp. (Sunnyvale, CA), and Interdesign, Inc. (Scotts Valley, CA). Cell libraries

The cell library approach is used to obtain larger, high-density semicustom circuits than those obtainable from gate arrays. With this approach, the semiconductor manufacturer provides a library of standard cells from which designers select the cells needed for a given application and specify the required interconnections. A cell library typically contains 7400 series TTL equivalent circuits or 4000 series CMOS equivalent circuits along with a variety of devices including PLD's and even microprocessors and memories. Users can create new cells such as analog devices or particular interfaces and merge them with standard cells. Like designing with gate arrays, this process is highly automated through use of CAD workstations that permit cell selection, design, circuit simulations and test generation. Some of the companies that offer cell libraries are: American Microsystems, Inc. (Santa Clara, CA), Zymos (Sunnyvale, CA), Signetics (Sunnyvale, CA) and Harris Semiconductor (Melbourne, FL).

29 CONCLUSIONS S e m i c o n d u c t o r device t e c h n o l o g y has i n d e e d u n d e r g o n e a r e v o l u t i o n which, in t u r n , has p r o c r e a t e d t h e e x t e n s i v e use o f e l e c t r o n i c s in virtually e v e r y a s p e c t o f m o d e r n s o c i e t y . M o r e o v e r , similar a d v a n c e s can be expected: f o r t h e f o r e s e e a b l e f u t u r e . It a p p e a r s t h a t s e m i c o n d u c t o r device t e c h n o l o g y will s o o n o u t s t r i p t h e ability o f t h e s e m i c o n d u c t o r i n d u s t r y to e f f e c t i v e l y utilize the t e c h n o l o g y at h a n d . T h e increasing c a p a b i l i t y a n d decreasing, costs o f s e m i c o n d u c t o r devices is c o n t i n u i n g to o p e n m a n y n e w and exciting possibilities f o r t h e a p p l i c a t i o n o f e l e c t r o n i c s in agriculture. T o d a y ' s s y s t e m designers m u s t h a v e a k n o w l e d g e o f t h e capabilities a n d characteristics o f the wide s p e c t r u m o f available i n t e g r a t e d circuits f r o m SSI devices to V L S I m i c r o p r o c e s s o r s a n d p e r i p h e r a l chips. While T T L has b e e n the m a i n s t a y o f t h e s e m i c o n d u c t o r i n d u s t r y , CMOS is q u i c k l y b e c o m i n g the d o m i n a n t circuit t e c h n o l o g y . C u s t o m a n d s e m i c u s t o m circuits, o n c e t h o u g h t b y m a n y to b e c o m e o b s o l e t e b e c a u s e of the e m e r g e n c e of the m i c r o p r o c e s s o r , are e x p e r i e n c i n g r a p i d g r o w t h because m i c r o p r o c e s s o r s have m a d e highly a u t o m a t e d chip design a reality.

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