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1、Microcomputer SystemsElectronic systems are used for handing information in the most general sense; this information may be telephone conversation, instrument read or a companys accounts, but in each case the same main type of operation are involved: the processing, storage and transmission of infor

2、mation. in conventional electronic design these operations are combined at the function level; for example a counter, whether electronic or mechanical, stores the current and increments it by one as required. A system such as an electronic clock which employs counters has its storage and processing

3、capabilities spread throughout the system because each counter is able to store and process numbers. Present day microprocessor based systems depart from this conventional approach by separating the three functions of processing, storage, and transmission into different section of the system. This p

4、artitioning into three main functions was devised by Von Neumann during the 1940s, and was not conceived especially for microcomputers. Almost every computer ever made has been designed with this structure, and despite the enormous range in their physical forms, they have all been of essentially the

5、 same basic design. In a microprocessor based system the processing will be performed in the microprocessor itself. The storage will be by means of memory circuits and the communication of information into and out of the system will be by means of special input/output(I/O) circuits. It would be impo

6、ssible to identify a particular piece of hardware which performed the counting in a microprocessor based clock because the time would be stored in the memory and incremented at regular intervals but the microprocessor. However, the software which defined the systems behavior would contain sections t

7、hat performed as counters. The apparently rather abstract approach to the architecture of the microprocessor and its associated circuits allows it to be very flexible in use, since the system is defined almost entirely software. The design process is largely one of software engineering, and the simi

8、lar problems of construction and maintenance which occur in conventional engineering are encountered when producing software. The figure1.1 illustrates how these three sections within a microcomputer are connected in terms of the communication of information within the machine. The system is control

9、led by the microprocessor which supervises the transfer of information between itself and the memory and input/output sections. The external connections relate to the rest (that is, the non-computer part) of the engineering system. Fig.1.1 Three Sections of a Typical MicrocomputerAlthough only one s

10、torage section has been shown in the diagram, in practice two distinct types of memory RAM and ROM are used. In each case, the word memory is rather inappropriate since a computers memory is more like a filing cabinet in concept; information is stored in a set of numbered boxes and it is referenced

11、by the serial number of the box in question. Microcomputers use RAM (Random Access Memory) into which data can be written and from which data can be read again when needed. This data can be read back from the memory in any sequence desired, and not necessarily the same order in which it was written,

12、 hence the expression random access memory. Another type of ROM (Read Only Memory) is used to hold fixed patterns of information which cannot be affected by the microprocessor; these patterns are not lost when power is removed and are normally used to hold the program which defines the behavior of a

13、 microprocessor based system. ROMs can be read like RAMs, but unlike RAMs they cannot be used to store variable information. Some ROMs have their data patterns put in during manufacture, while others are programmable by the user by means of special equipment and are called programmable ROMs. The wid

14、ely used programmable ROMs are erasable by means of special ultraviolet lamps and are referred to as EPROMs, short for Erasable Programmable Read Only Memories. Other new types of device can be erased electrically without the need for ultraviolet light, which are called Electrically Erasable Program

15、mable Read Only Memories, EEPROMs. The microprocessor processes data under the control of the program, controlling the flow of information to and from memory and input/output devices. Some input/output devices are general-purpose types while others are designed for controlling special hardware such

16、as disc drives or controlling information transmission to other computers. Most types of I/O devices are programmable to some extent, allowing different modes of operation, while some actually contain special-purpose microprocessors to permit quite complex operations to be carried out without direct

17、ly involving the main microprocessor. The microprocessor processes data under the control of the program, controlling the flow of information to and from memory and input/output devices. Some input/output devices are general-purpose types while others are designed for controlling special hardware su

18、ch as disc drives or controlling information transmission to other computers. Most types of I/O devices are programmable to some extent, allowing different modes of operation, while some actually contain special-purpose microprocessors to permit quite complex operations to be carried out without dir

19、ectly involving the main microprocessor. The microprocessor , memory and input/output circuit may all be contained on the same integrated circuit provided that the application does not require too much program or data storage . This is usually the case in low-cost application such as the controllers

20、 used in microwave ovens and automatic washing machines . The use of single package allows considerable cost savings to e made when articles are manufactured in large quantities . As technology develops , more and more powerful processors and larger and larger amounts of memory are being incorporate

21、d into single chip microcomputers with resulting saving in assembly costs in the final products . For the foreseeable future , however , it will continue to be necessary to interconnect a number of integrated circuits to make a microcomputer whenever larger amounts of storage or input/output are req

22、uired.Another major engineering application of microcomputers is in process control. Here the presence of the microcomputer is usually more apparent to the user because provision is normally made for programming the microcomputer for the particular application. In process control applications the be

23、nefits lf fitting the entire system on to single chip are usually outweighed by the high design cost involved, because this sort lf equipment is produced in smaller quantities. Moreover, process controllers are usually more complicated so that it is more difficult to make them as single integrated c

24、ircuits. Two approaches are possible; the controller can be implemented as a general-purpose microcomputer rather like a more robust version lf a hobby computer, or as a packaged system, signed for replacing controllers based on older technologies such as electromagnetic relays. In the former case t

25、he system would probably be programmed in conventional programming languages such as the ones to9 be introduced later, while in the other case a special-purpose language might be used, for example one which allowed the function of the controller to be described in terms of relay interconnections, In

26、 either case programs can be stored in RAM, which allows them to be altered to suit changes in application, but this makes the overall system vulnerable to loss lf power unless batteries are used to ensure continuity of supply. Alternatively programs can be stored in ROM, in which case they virtuall

27、y become part of the electronic hardware and are often referred to as firmware. More sophisticated process controllers require minicomputers for their implementation, although the use lf large scale integrated circuits the distinction between mini and microcomputers, Products and process controllers

28、 of various kinds represent the majority of present-day microcomputer applications, the exact figures depending on ones interpretation of the word product. Virtually all engineering and scientific uses of microcomputers can be assigned to one or other of these categories. But in the system we most s

29、tudy Pressure and Pressure Transmitters. Pressure arises when a force is applied over an area. Provided the force is one Newton and uniformly over the area of one square meters, the pressure has been designated one Pascal. Pressure is a universal processing condition. It is also a condition of life

30、on the planet: we live at the bottom of an atmospheric ocean that extends upward for many miles. This mass of air has weight, and this weight pressing downward causes atmospheric pressure. Water, a fundamental necessity of life, is supplied to most of us under pressure. In the typical process plant,

31、 pressure influences boiling point temperatures, condensing point temperatures, process efficiency, costs, and other important factors. The measurement and control of pressure or lack of it-vacuum-in the typical process plant is critical.The working instruments in the plant usually include simple pr

32、essure gauges, precision recorders and indicators, and pneumatic and electronic pressure transmitters. A pressure transmitter makes a pressure measurement and generates either a pneumatic or electrical signal output that is proportional to the pressure being sensed.In the process plant, it is imprac

33、tical to locate the control instruments out in the place near the process. It is also true that most measurements are not easily transmitted from some remote location. Pressure measurement is an exception, but if a high pressure of some dangerous chemical is to be indicated or recorded several hundr

34、ed feet from the point of measurement, a hazard may be from the pressure or from the chemical carried.To eliminate this problem, a signal transmission system was developed. This system is usually either pneumatic or electrical. And control instruments in one location. This makes it practical for a m

35、inimum number of operators to run the plant efficiently.When a pneumatic transmission system is employed, the measurement signal is converted into pneumatic signal by the transmitter scaled from 0 to 100 percent of the measurement value. This transmitter is mounted close to the point of measurement

36、in the process. The transmitter output-air pressure for a pneumatic transmitter-is piped to the recording or control instrument. The standard output range for a pneumatic transmitter is 20 to 100kPa, which is almost universally used.When an electronic pressure transmitter is used, the pressure is co

37、nverted to electrical signal that may be current or voltage. Its standard range is from 4 to 20mA DC for current signal or from 1 to 5V DC for voltage signal. Nowadays, another type of electrical signal, which is becoming common, is the digital or discrete signal. The use of instruments and control

38、systems based on computer or forcing increased use of this type of signal.Sometimes it is important for analysis to obtain the parameters that describe the sensor/transmitter behavior. The gain is fairly simple to obtain once the span is known. Consider an electronic pressure transmitter with a rang

39、e of 0600kPa.The gain is defined as the change in output divided by the change in input. In this case, the output is electrical signal (420mA DC) and the input is process pressure (0600kPa). Thus the gain. Beside we must measure Temperature Temperature measurement is important in industrial control,

40、 as direct indications of system or product state and as indirect indications of such factors as reaction rates, energy flow, turbine efficiency, and lubricant quality. Present temperature scales have been in use for about 200 years, the earliest instruments were based on the thermal expansion of ga

41、ses and liquids. Such filled systems are still employed, although many other types of instruments are available. Representative temperature sensors include: filled thermal systems, liquid-in-glass thermometers, thermocouples, resistance temperature detectors, thermostats, bimetallic devices, optical

42、 and radiation pyrometers and temperature-sensitive paints.Advantages of electrical systems include high accuracy and sensitivity, practicality of switching or scanning several measurements points, larger distances possible between measuring elements and controllers, replacement of components(rather

43、 than complete system), fast response, and ability to measure higher temperature. Among the electrical temperature sensors, thermocouples and resistance temperature detectors are most widely used.单片机系统广义地说，微处理系统是用于处理信息的，这种信息可以是电话交谈，仪器读数或企业帐户，但是各种情况下都涉及相同的主要操作：信息处理、存储和传递。在常规的电子设计中，这些操作都是以功能平台方式组合起来的，

44、例如计数器，无论是电子还是机械的，都要存储当前值，并按要求将该值增1。诸如采用计数器的电子钟之类的任一系统要使其存储和处理能力遍布整个系统，因为每个计数器都能存储和处理一些数字。当前微处理化系统与上述的常规方法不同，它将处理，存储和传输三个功能分离形成不同的系统单元。这种形成三个主要单元的分离方法是冯-诺依曼在20世纪40年代所设想出来的，并且是针对微计算机的设想。从此几乎所有制成的计算机都是用这种结构设计的，尽管包含宽广的物理形式，从根本上来说他们均是具有相同的基本设计。在微处理器系统中，处理是由微处理器本身完成的。存储是利用存储器电路，而进入和出自系统的信息传输则是利用特定的输入/输出（I

45、/O）电路。要在一个微处理器化时钟中找出执行计数功能的一个特殊硬件是不可能的，因为时间存储在存储器中，而在固定的时间间隔下由微处理器控制增值。但是，规定系统运转过程的软件包含实现计数器功能的单元。由于系统几乎完全由软件所定义，所以对微处理器结构和其辅助电路这种看起来非常抽象的处理方法使其在应用时非常灵活。这种设计过程主要是软件工程，而且在生产软件时，就会遇到产生于常规工程中相似的构造和维护问题。 图1.1 微型计算机的三个组成部分图1.1显示出了微型计算机中这三个单元是如何按照机器中的信息通信方式而联接起来的。该系统由微处理器控制，它管理自己与存储器和输入/输出单元的信息传输。外部的连接与工程

46、系统的其余部分（即非计算机部分）有关。尽管图中显示的只有一个存储单元，实际中有RAM和ROM两种不同的存储器被使用。由于概念上的计算机存储器更像一个公文柜，上述的“存储器”一词是非常不恰当的；信息存放在一系列已标号的“箱子”中，而且可按问题由“箱子”的序列号进行信息的参考定位。微计算机常使用RAM（随机存取存储器），在RAM中数据可被写入，并且在需要时可被再次读出。这种数据能以任一所希望的次序从存储器中读出，不必按写入时的相同次序，所以有“随机”存取存储器。另一类型ROM（只读存储器）用来保持不受微处理器影响的固定的信息标本；这些标本在电源切断后不会丢失，并通常用来保存规定微处理器化系统运转过

47、程的程序。ROM可像RAM一样被读取，但与RAM不一样的是不能用来存储可变的信息。有些ROM在制造时将其数据标本放入，而另外的则可通过特殊的设备由用户编程，所以称为可编程ROM。被广泛使用的可编程ROM可利用特殊紫外线灯察除，并被成为EPROM，即可察除可编程只读存储器的缩写。另有新类型的期器件不必用紫外线灯而用电察除，所以称为电可察除可编程只读存储器EEPROM。 微处理器在程序控制下处理数据，并控制流向和来自存储器和输入/输出装置的信息流。有些输入/输出装置是通用型的，而另外一些则是设计来控制如磁盘驱动器的特殊硬件，或控制传给其他计算机的信息传输。大多数类型的I/O装置在某种程度下可编程，

48、允许不同形式的操作，而有些则包含特殊用途微处理器的I/O装置不用主微处理器的直接干预，就可实施非常复杂的操作。 假如应用中不需要太多的程序和数据存储量，微处理器、存储器和输入/输出可全被包含在同一集成电路中。这通常是低成本应用情况，例如用于微波炉和自动洗衣机的控制器。当商品被大量地生产时，这种单一芯片的使用就可节省相当大的成本。当技术进一步发展，更强更强的处理器和更大更大数量的存储器被包含形成单片微型计算机，结果使最终产品的装配成本得以节省。但是在可预见的未来，当需要大量的存储器或输入/输出时，还是有必要继续将许多集成电路相互联结起来，形成微计算机。 微计算机的另一主要工程应用是在过程控制中。

49、这是，由于装置是按特定的应用情况由微机编程实现的，对用户来说微计算机的存在通常就更加明显。在过程控制应用中，由于这种设备以较少的数量生产，将整个系统安装在单个芯片上所获取的利益常比不上所涉及的高设计成本。而且，过程控制器通常更为复杂，所以要将他们做成单独的集成电路就更为困难。可采用两种处理，将控制器做成一种通用的微计算机，正像较强版本的业余计算机那样；或者做成“包裹”式系统，按照像电磁继电器那样的较老式的技术进行设计，来取代控制器。对前一种情况，系统可以用常规的编程语言来编程，正如以后要介绍的语言那样；而另一种情况，可采用特殊用途的语言，例如那种使控制器功能按照继电器相互连接的方法进行描述。两种情况下，序均能存于RAM，这让程序能按应用情况变化时进行相应的变化，但是这使得总系统易受掉电影响而工作不正常，除非使用电池保证供电连续性。另一种选择是将程序在ROM中，这样他们就变成电子“硬件”的一部分并常被称为“固件”。