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Introducing Microprocessors

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Introducing Microprocessors .Introducing Microprocessors A new series on understanding microprocessors and how they work. By Mike Tooley he general learning objectives for 1.3.2 State the function of each of the The "logic gate equivalent" referred Tpart one of Introducing Micro­ principal internal elements of a repre­ to in the table provides us with a rough processors are that readers should be sentative microcomputer system. measure of the complexity of in­ able to: 1.3.3 Explain the bus system used to tegrated circuit and simply gives the (a) understand the terminology used to link the internal elements of a equivalent number of standard logic describe microcomputer and micro­ microcomputer system. gates. A logic gate is a basic circuit ele­ processor based systems 1.3.4 Distinguish between the following ment capable of performing a logical (b) identify the major logic families types of bus; address, data and control. function (such as AND, OR, NAND or and scale of integration employed NOR). A basic logic gate (e.g. a stand­ within integrated circuits Related Theory ard TTL two-input AND) would typi­ (c) draw a diagram showing the ar­ Explain the binary and hexadecimal cally employ the equivalent of six tran­ chitecture of a representative micro­ number systems. sistors, three diodes and six resistors. computer system and state the function Convert binary, hexadecimal and At this stage, readers need not worry of the principal internal elements decimal numbers over the range 0 to too much about the function of a logic (d) understand binary and 65535 (decimal). gate as we shall be returning to this hexadecimal number systems and con­ Explain how negative numbers are later on. Readers need only be aware vert from/to decimal represented in microprocessor sys­ that such devices form the basis of cir­ The specific objectives for this part are tems. cuits which can perform logical as follows: Perform addition and subtraction of decisions. binary and hexadecimal numbers over Integrated circuits are encapsulated 1.1 Terminology the range 0 to 255 (decimal). in a variety of packages but the most 1.1.1 State the meaning of any of the Note: Rather than publish a complete popular type (and that with which most terms listed in the glossary 1.1.2 Distin­ glossary of terms, we shall be produc­ readers will already be familiar) is the guish between the terms; computer, ing a mini-glossary for each part; intro­ plastic dual in-line (DIL) type. These microprocessor, microcomputer, and ducing new terms as we progress are available with a differing number of single-chip microcomputer. through the series. pins depending upon the complexity of the integrated circuit in question and, Integrated Circuits in particular, the need to provide ex­ The vast majority of today's electronic ternal connections to the device. systems rely on the use of integrated Conventional logic gates, for ex­ 1.2 Integrated Circuit circuits in which hundreds of ample, are often supplied in 14- pin or Terminology and Logic thousands of components are fabri­ 16-pin DIL packages while Families cated on a single chip of silicon. A rela­ microprocessors (and their more com- 1.2.1 Define the terms SSI, MSI, LSI tive measure of the number of in­ and VLSI as applied to integrated cir­ dividual semiconductor devices within cuits. the chip is given by referring to its 1.2.2 State the characteristics of "scale of integration", as shown below: CMOS and TTL semiconductor tech­ nologies. Scale of. Logic gate 1.2.3 State the basic properties of integration Abbreviation equivalent CMOS and TTL logic families. Small SSI 1 to10 Medium MSI 10 to 100 1.3 System Architecture Large LSI 100 to 1000 1.3.1 Draw a block diagram showing Very large VLSI 1000 to 10000 the internal architecture of a repre­ Super large SLSI 10000 to 100000 Fig. 1 Some common DJL package sentative microcomputer system. outlines (top view). E & TT March 1988 31 Introducing Microprocessors plex support devices) often require 40- CMOS devices are sometimes also pins or more. Fig. 1 shows some com­ given a suffix letter; A to denote the mon DIL package outlines together "original" (now obsolete) unbuffered with pin numbering. It should be noted series, and B to denote the improved that these are TOP views of the (buffered) series. A UB suffix denotes devices, i.e. they show how the device an unbuffered B-series device. appears when viewed from the com­ ponent side of the PCB, NOT from the Problem 1.2 underside. To which logic families do each of the In each case, the pins of the IC are following devices belong? numbered sequentially (starting at the (a) 7407 indentation) moving in an anti-clock­ (b) 74LS74 wise direction. Thus, in the case of a (c) 4052 14-pin DIL package viewed from the a. Using CMOS technology. top, pins one and 14 appear respective­ Power supplies ly on the left and-right-hand side of the Most TTL and CMOS logic systems indentation. are designed to operate from a single Problem 1.1 supply voltage rail of nominally + 5V. Fig. 2 shows the outline of an 8-pin With TTL devices, it is important for DIL device viewed from above. Iden­ this voltage to be very closely regu­ tify the pin numbers for this device. lated. Typical TTL IC specifications call for regulation of better than 5% INPUTS{: OUTPUT (i.e. the supply voltage should not fall outside the range 4.75V to 5.25V). CMOS logic devices are fortunately very much more tolerant of their supp­ ly voltage than their TTL counterparts. Most CMOS devices will operate from 0 b. Using TTL technology. a supply rail of anything between + 3V Fig. 2. An 8-pin DIL package. Fig. 3. Representative circuitry of and + 15V. This, coupled with a mini­ two-input AND gates. mal requirement for supply current (a Logic Families CMOS gate typically requires a supply The integrated circuit devices on which must be stressed that they have IDEN­ current of only a few microamps in the modern digital circuitry depends TICAL logical functions. quiescent state) makes them eminently belong to one of the other of several suited to portable battery powered "logic families". Readers should note TTL logic equipment. this term refers to the type of semi con­ The most common range of conven­ TTL devices require considerably ductor technology employed in the tional TTL logic devices is known as more supply current than their CMOS fabrication of the integrated circuit. the "74" series. These devices are, not equivalents. A typical TTL logic gate The semiconductor technology surprisingly, distinguished by the prefix requires a supply current of around employed in the fabrication of an in­ number 74 in their coding. Thus 8mA; approximately 1000 times that of tegrated circuit is instrumental in devices coded with the numbers 7400, its CMOS counterparts when operat­ determining its characteristics. This 7408, 7432, and 74121 are all members ing at a typical switching speed of encompasses such important criteria of this family which is often referred to 10kHz. as supply voltage, power dissipation, as "Standard TTL". Other versions (or switching speed, and immunity to "subfamilies") of standard TTL exist noise. and these are distinguished by ap­ CMOS versus TTL The most popular logic families, at propriate letters placed after the "74" Having spent some time in discussing least as far as the more basic general coding. Where "Low Power Schottky" the merits of CMOS and TTL devices purpose devices are concerned, are technology is used in the manufacture it is now worth summarizing three of Complementary Metal Oxide Semi­ of a TTL logic gate, for example, the important differences between conductor (CMOS) and Transistor device coding include the letters "LS". these logic families in tabular form: Logic (TTL), TTL also has a number Thus a 74LSOO device is a Low Power of sub-families including the popular Schottky version of a standard 7400 Problem 1.3 Low Power Schottky (LS-TTL) device. Which logic family would be most variants. suited for use in a piece of equipment which has to operate over long periods For the curious we have shown the CMOS Logic from dry batteries? internal circuitry of representative The most popular CMOS family is the CMOS and TTL two-input AND gates "4000" series and devices have an ini­ Microprocessors in Fig. 3. Despite the obvious dis­ tial prefix of 4. Thus 4001, 4174, 4501 Having dealt with the basic building similarity of these two arrangements, it and 4574 are all CMOS devices. blocks of digital circuits we have at last 32 E & TTMIII'Ch 1988 Introducing Microprocessors ~ - CMOS TTL puter installation. The vast saving in hardware development can usefully be Speed of operation Relatively slow Fast devoted to the software aspects of a I (power consumption (power consumption project and future modifications ca n increases as switching substantially simply involve the substitution 0 f speed increases) constant) "firmware" (ROM based software). Power consumption Negligible Appreciable Bits, Bytes and Buses (but see above) An 8-bit microprocessor fetches an d outputs data m groups of 8- bit s Supply voltage Operates over a wide Must be closely (known as "bytes"). This data is move d voltage range regulated at, or around on eight separate line s (typically 3V to 15V) near5V (labelled DO to D7) which collective! y fo r m a " data bus " . For the cunous, the arrived at the point at which we can in­ something of a minor miracle - a device word "bus" is a contraction of the troduce the microprocessor. which could replace countless other Greek work "omnibus" which simply Microprocessors are VLSI and SLSI chips and which could address a stag­ means "to all", thus aptly describing integrated circuit devices which are gering 16K of memory! By modern the concept of a system of wiring which capable of accepting, decoding and ex­ standards, the 8008 is extremely crude links together all of the components of ecuting instructions presented to them but it was not long before Intel intro­ a microprocessor system using a com­ in binary coded form.
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