SOME QUESTIONS AND ANSWERS ABOUT MICROPROCESSORS
A. Larson, Assistant Professor Department of Electrical Engineering United States Air Force Academy Quarters 4210C, CO United States of America
ABSTRACT A recent development in integrated circuit technology, the microprocessor, has given the control engineer a powerful tool and a unique opportunity to employ that tool. It is the purpose of this paper to familiarize control engineers with microprocessors and their capabilities, and to suggest uses in the control field. I.
Digital computers have been used extensively in applying control theory to practical problems. The advent of minicomputers costing less than $10,000 was a primary factor in this development. Now, integrated circuit technology permits fabrication of microcomputers for less than $1,000, with small systems possible for under $100. This significant development permits the application of modern control theory to an entire new class of problems. For this reason, the modern control engineer must be aware ef microprocessors, their capabilities, and their potential appl ications. This paper will attempt to answer several questions about microprocessors, such as: (1) What is a microprocessor? (2) What unique advantages do microprocessors have over other computers? (3) What capabilities do modern microprocessors have? (4) What control applications suit microprocessors? (5) What type of microprocessors will be available in the near future? 11.
THE RATIONALE BEHIND MICROPROCESSORS
Microprocessors represent an important quantum jump in our electronics technolay. System designers need to understand them, and their effect on systems design.
Understanding microprocessors and their impact is facilitated by examining the rationale behind their development. Since some readers may not be entirely familiar with the hardware of digital computers, a discussion of basics will serve as a common starting point. A.
The Basic "Four Box" Computer
Nearly everyone is familiar with the classic "four box" digital computer, shown in Figure 1. It is basically an information processing system. Information representing data is stored, usually as binary numbers, in the memory. This data is manipulated in the arithmetic unit, which contains circuitry to add, subtract, etc., and usually one or more temporary data holding locations called registers. The manipulation of the data in memory is commanded by the control unit, which sends timing signals to cause data to be moved from memory to the arithmetic unit, manipulated, and placed back in memory. This control unit is told what actions to take by a set of instructions, which are stored in memory just as is the data. The control unit repetitively fetches an instruction from memory, examines it, moves any required data from memory to the arithmetic unit, commands the operation indicated by the instruction, and returns data to memory if required. This is
called the fetch computer.
execute cycle of the
All this operation on data presupposes that the data and the instructions to manipulate it (the program) were placed in memory. Also, the results must somehow be returned to the outside world. This is the function of the input/output unit. These four functional blocks are found in most general purpose digital computers. These computers have become very powerful tools to our society. The question of concern here is how improving technology has lowered the cost of implementing this tool, and what this means to control system designers and society as a whole. B. Implementing The Computer On An Integrated Circuit Chip Integrated circuit (IC) technology has provided the ability to produce complex electronic circuits at very low cost, because they are mass produced by automated assembly lines. In volume, costs per integrated circuit are usually less than $10, and often under $1. The key to producibility of any particular circuit is volume. Volume production of digital IC's has permitted design and cor.struction of computers which cost less and out perform their predecessors. As IC technology has developed, the fraction of the computer realizable on one IC has grown. However, as larger portions of the computer are realized, more pins are needed per IC for interconnection, at least initially. But as soon as enough of the computer is placed on an IC, the number of pins per IC begins to decrease. The general shape of the curve is shown in Figure 2. Examination of Figure 2 shows that the entire computer could be realized with a small number of pins per IC. Given the existing fact that the feasible number of pins per IC is rather small, it seems better to use them to package the entire computer rather than some small portion thereof. But before proceeding, the question of producibility requires asking, "Should the whole computer be on one integrated circuit?" Examine first the input/output unit of the basic computer. Since each user will want to connect his unit the way he wants to, it seems volume would be very limited if the input/output unit were placed on the same IC as the other units. It would seem better to just provide a basic data path in and out of the
IC and provide other IC's so the user can connect his computer to whatever he wants, be it a teletype, a line printer, the digital output of an instrument, a transducer, or anything else. Next, examine the memory. How big a memory a user needs for his applications will vary widely. If it is included in the IC, it would limit volume, unless some small amount of memory were provided, to which the user could add more if he desired. So it seems that at least the main memory should be external to the IC. The other two blocks, the arithmetic and control units, will be required by all users. If it is possible to realize them on one IC the unit should be very producible. The combination oe these two units is usually called a CPU, or central processing unit. Depending on the IC technology used and the exact architecture selected, this CPU may require 1, 2, or a few IC's to implement. This is a microprocessor. Definition: Microprocessor A microprocessor is the CPU portion of a computer realized on one or a very few integrated circuit chips. If we now add memory and input/output units, we have a microcomputer. Definition: Microcomputer A microcomputer is a computer realized by adding memory and input/output units to a microprocessor. Microprocessors and microcomputers have been available commercially since 1971 (4 bit units) and 1972 (8 bit units). This year could be called" The Year Of The Microprocessor" because of the number of IC manufacturers announcing and/or delivering microprocessors. Ill.
THE IMPACT OF MICROPROCESSORS ON SYSTEMS
Microprocessors represent a quantum jump in technology as significant as that from tubes to transistors or that from discrete components to integrated circuits. This is a result of several unique characteristics of microprocessors and microcomputers. Fundamentally, microprocessors are massproduced electronic components. Mass production means 1.
Corrunonality Availabil i ty
production as an integrated circuit means 4. 5. 6. 7.
IC reliability Small size Lightweight Low power
These characteristics permit significant improvement in system design. These improvements will touch small scale systems, such as consumer products. Microprocessors are being used in stoves, and are being considered for use in secure door locks and telephones. The number of applications are uncountable. So the implication of the microprocessor technology is that: Microprocessors will affect the way in which nearly all electronic systems are designed. A.
Microprocessor Based Systems
It is both convenient and informative to divide electrical systems into two types: those that are connected with the distribution and control of power, and those which are concerned with information. Further, information systems may be classified as information transfer systems, which move information represented by electrical signals from a source to a destination, and information acquisition systems, which cannot only transfer information from source to destination, but derive new information through the use of a block called a processor. A general block diagram of a data acquisition system is shown in Figure 4. (4) The input information is first converted to electrical form, then conditioned, and transmitted to the processor. The processor acts on this information according to some form of algorithm, generating new information. This new information is sent to the output transducer, which converts it from electrical to some other form. This data acquisition system is very general. It fits, for example, nearly all instrumentation systems. It likewise fits the controller and the state estimator in automatic control systems. Importantly, the processor block may be a microprocessor or a microcomputer. Their low cost permits the use of the "smart" data acquisition type system where it was not economical until the advent of microprocessor technology. The implications of applying
this technology to this class of systems is investigated in the next section. B. control System Applications Of Microprocessors Design of electronic systems generaly b~ gins with the theoretical derivation of a means to do the desired t .a sk. This theory is then worked into a set of design equations or an algorithm, from which a hardware (and/or software) design is developed. This merging of theory and hardware in the engineering design is critical. A theory which cannot be implemented is useless to the engineer. Changes in hardware may require new or modified theory. The interchange is very crucial to the success of any engineering effort. Unfortunately, a great many engineers have been frustrated by their discovery that the theory they learned in school was not practical when it came down to applying it to their problems. Hence, a great cry has arisen for "Relevance!" Microprocessors give the engineer a very versatile tool for implementing some of the theory he has learned. On-line digital control of processors has been a very active area for some time. Unfortunately, even the low cost of minicomputers has often been too high a price to pay for smaller systems. For example, automatic carburation control of the automobile so as to minimize pollution while. maximizing fuel economy can be theoretically attacked using optimal control theory. Recent studies indicate that fuel economy might be improved by a factor of 1.8 to 2.0. (1) However, putting a $3000 computer into a $4000 automobile is impractical. But putting a $10 or even a $100 microcomputer into the automobile is most practical. It is this small system usage which is now demanding the conversion of our control theory into practice on a greatly enlarged scale. This small scale usage is where microprocessors are first being applied. This is an obvious application involving simply the size scaling of a system. While technically this is no major challenge, one point should be borne in mind by designers: These small systems will often interact with nontechnical people. So care must be used . to implement a reliable, easy to use system, so as to foster a high regard for the control profession. This small system usage now permits an attack on several problems corrunon to many
people. Besides the uses in transportation, major strides can be made in the medical area. One exciting area is in the automatic control of prosthetic limbs and systems to give sight to the blind. (2) But larger control systems, too, can benefit. Several specific areas lend themselves to the use of dedicated microprocessors. Some suggestions are the areas of filtering, state estimation, control law implementation, and transducers. Digital filtering is well advanced. Filters are required to combat the noise found in nearly all processes. Typically, one computer served to perform filtering, state estimation and control computations. These were necessarily done sequentially, at a sacrifice in speed. Now, one or more microprocessors may be economically dedicated to each function. Filters typically inv olve computations which proceed in stages, and process vector data. It is now practical to implement each filter stage with a parallel operating set of microprocessors which each process are one vector component. State estimation via any of the wellknown techniques requires computation. Associated with this is a necessary computational delay which must be taken into account. Again, use of parallel microprocessors can reduce this delay significantly and improve the accuracy of sta.te estimation.
The designers of microprocessors hav e borrowed heav ily from the architectures of minicomputers. Figure 5 gives a summary of some of the currently available microprocessors. A survey of their capabilities shows that these microprocessors compare favorably with minicomputers. The single IC 8 bit processors (8008, 8080, MC6800, PPS8, F8, MK5065, 2650, and Micro Controller) offer the control engineer suitable choices for small systems. Where IC count is less important, "bit sliced" architecture microprocessors offer flexibility. These microprocessors are an n-bit partition of a CPU, and are designed to be cascaded to form a microprocessor of m x n bits, using m IC·s. Further, these microprocessors use microprogramming (3) to implement an instruction set. For many control applications, the entire control function may be implemented in this microprogram. The GPCP, 6701, SPB0400, and 3002 processors use this architecture. Further, the bipolar processors (6701, SPB0400, and 3002) offer instruction times in the hundreds of nanosecond range, making them potentially much faster than current minicomputers. It seems, then, that some v ery powerful tools are at hand. But even better ones are coming, as projected in the next section. V.
The computations inv olved in implementing the control law also lend themsel v es to use of dedicated microprocessors. Again, several microprocessors could be used to increase the speed of computation, allowing more rapid conv ergence to the desired operating point. Transducers are always a difficult problem in implementing a control system. Response characteristics are typically nonlinear. Now it is practical to preprocess the output of a sensor transducer and to compensate that output for nonlinear and temperature effects. Likewise, actuator transducers may be independently controlled by a dedicated microprocessor. In short, microprocessors open a new area of control application in small systems, and offer improved performance in larger systems. So it is important for the control engineer to understand what sort of computer capabilities microprocessors offer. This is cov ered in the next
While microprocessors will not sol v e all system design problems, the engineers will be able to better attack many of these problems with them. It is interesting and exciting to speculate upon what the technology will bring within a very few years. Since microprocessors are built as integrated circuits, a good question to ask is, "What can the IC manufacturers do?" One measure of what they can do is how much memory they can put on one integrated circuit. Figure 6 shows the size memory in terms of thousands of bits that can be placed upon one integrated circuit. The many data points allow a reasonably reliable projection of the curve. In 1974 it was possible to purchase one small integrated circuit on which can be stored 4096 bits of information. By the end of 1975 this will hav e increased to 16,38 4 bits of information. And by late 1976 64K bits of information will be a v ailable from one IC. The cost per bit of these memories is
projected to approach approximately .1 cent per bit as shown by Figure 7. So it appears the designer will have low cost, large size memory IC's available within the immediate future. As far as the microprocessors that are coming, engineers should realize that already the single IC 8 bit parallel microprocessors are in their second generation. The first processors were realized using an IC technology called PMOS. They were relatively slow. The lntel 8008, for example, had a 20 microsecond instruction time. The second generation microprocessors introduced in 1974 were approximately 10 times as fast, having a 2 microsecond instruction time. These devices were built using N channel (NMOS) IC technology. The third generation of single IC microprocessors is probably in the laboratories of the IC manufacturers at the current time. l
scale and large scale systems. Microprocessors already compete with minicomputers in performance, and will exceed them in the near future. So the tools are at hand for a new round of applying control theory to the problems of our peoples. Let the control community seize the opportunity, and use this new tool wisely. REFERENCES (1) Armstrong, Larry, "Microprocessors Steer to Detroit", Electronics, vol 47, No. 8, April 18, 1974, pp 65-66 (2) Dob e lle, W. H. et aI, "Data Processing, LSI Will Help to Bring Sight to the Blind", Electronics, January 24, 1974, pp 81-86 (3)
Husson, Samir S. Microprogramming Principles and Practices, pp 284
Thomas, Roland E. and Daniel E. Buehler, ~igna~s and Systems, USAF Academy, Colorado, 1974
Figure 8 attempts to project the instruction speed or processing time of these single IC microprocessors of the third generation. There are many problems with making such a projection. First, it is very difficult to define instruction time for a computer because different computers designed different ways operate totally differently and a general comparison is frequently not valid. Also, there are relatively few data points on Figure 7 so a fan of instruction cycle times has been projected. Somewhere in the 1976-1978 time frame single IC microprocessors should have instruction cycle times of about 100 nsecs. These processors will most likely be built wi th a bipolar IC technology, possibly 12L. Such high speed processors, particularly when operated in parallel, could provide average instruction times on the order of a few nanoseconds for a particular system. These newer processors will also offer improved features, such as floating point arithmetic, and hardware multiply and divide. VI.
Basic Elements of a Computer
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Microprocessors offer control engineers a significantly better computational tool than in the past. The low cost, low power, and small size permit new applications of control theory to both small
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