单片机毕业外文翻译.doc

外文资料翻译The single-chip microcomputer is the culmination of both the development of the digital computer and the integrated circuit arguably the two most significant inventions of the 20th century.Some employ the split program/data memory of the Harvard architecture, shown in Fig.A-1, others follow the philosophy, widely adapted for general-purpose computers and microprocessors, of making no logical distinction between program and data memory as in the Princeton architecture, shown in Fig.A-2. In general terms a single-chip microcomputer is characterized by the incorporation of all the units of a computer into a single device, as shown in Fig3-5A-3. These two types of architecture are found in single-chip microcomputer. Fig. A-1 A Harvard type Fig. A-2 A conventional Princeton computerRead only memory (ROM)ROM is usually for the permanent, non-volatile storage of an applications program .Many microcomputers and microcontrollers are intended for high-volume applications and hence the economical manufacture of the devices requires that the contents of the program memory be committed permanently during the manufacture of chips. Clearly, this implies a rigorous approach to ROM code development since changes cannot be made after manufacture .This development process may involve emulation using a sophisticated development system with a hardware emulation capability as well as the use of powerful software tools.Some manufacturers provide additional ROM options by including in their range devices with (or intended for use with) user programmable memory. The simplest of these is usually device which can operate in a microprocessor mode by using some of the input/output lines as an address and data bus for accessing external memory. This type of device can behave functionally as the single chip microcomputer from which it is derived albeit with restricted I/O and a modified external circuit. The use of these ROM less devices is common even in production circuits where the volume does not justify the development costs of custom on-chip ROM; there can still be a significant saving in I/O and other chips compared to a conventional microprocessor based circuit. More exact replacement for ROM devices can be obtained in the form of variants with piggy-back EPROM (Erasable programmable ROM ) sockets or devices with EPROM instead of ROM .These devices are naturally more expensive than equivalent ROM device, but do provide complete circuit equivalents. EPROM based devices are also extremely attractive for low-volume applications where they provide the advantages of a single-chip device, in terms of on-chip I/O, etc. with the convenience of flexible user programmability.Random access memory (RAM)RAM is for the storage of working variables and data used during program execution. The size of this memory varies with device type but it has the same characteristic width (4,8,16 bits etc.) as the processor ,Special function registers, such as stack pointer or timer register are often logically incorporated into the RAM area. It is also common in Harvard type microcomputers to treat the RAM area as a collection of register; it is unnecessary to make distinction between RAM and processor register as is done in the case of a microprocessor system since RAM and registers are not usually physically separated in a microcomputer.Central processing unit (CPU)The CPU is much like that of any microprocessor. Many applications of microcomputers and microcontrollers involve the handling of binary-coded decimal (BCD) data (for numerical displays, for example) ,hence it is common to find that the CPU is well adapted to handling this type of data .It is also common to find good facilities for testing, setting and resetting individual bits of memory or I/O since many controller applications involve the turning on and off of single output lines or the reading the single line. These lines are readily interfaced to two-state devices such as switches, thermostats, solid-state relays, valves, motor, etc.Parallel input/outputParallel input and output schemes vary somewhat in different microcomputer; in most a mechanism is provided to at least allow some flexibility of choosing which pins are outputs and which are inputs. This may apply to all or some of the ports. Some I/O lines are suitable for direct interfacing to, for example, fluorescent displays, or can provide sufficient current to make interfacing other components straightforward. Some devices allow an I/O port to be configured as a system bus to allow off-chip memory and I/O expansion. This facility is potentially useful as a product range develops, since successive enhancements may become too big for on-chip memory and it is undesirable not to build on the existing software base.Serial input/outputSerial communication with terminal devices is common means of providing a link using a small number of lines. This sort of communication can also be exploited for interfacing special function chips or linking several microcomputers together .Both the common asynchronous synchronous communication schemes require protocols that provide framing (start and stop) information .This can be implemented as a hardware facility or U(S) ART (Universal (synchronous) asynchronous receiver/transmitter) relieving the processor (and the applications programmer) of this low-level, time-consuming, detail. t is merely necessary to selected a baud-rate and possibly other options (number of stop bits, parity, etc.) and load (or read from) the serial transmitter (or receiver) buffer. Serialization of the data in the appropriate format is then handled by the hardware circuit.Timing/counter facilities Many application of single-chip microcomputers require accurate evaluation of elapsed real time .This can be determined by careful assessment of the execution time of each branch in a program but this rapidly becomes inefficient for all but simplest programs .The preferred approach is to use timer circuit that can independently count precise time increments and generate an interrupt after a preset time has elapsed .This type of timer is usually arranged to be reloadable with the required count .The timer then decrements this value producing an interrupt or setting a flag when the counter reaches zero. Better timers then have the ability to automatically reload the initial count value. This relieves the programmer of the responsibility of reloading the counter and assessing elapsed time before the timer restarted ,which otherwise wound be necessary if continuous precisely timed interrupts were required (as in a clock ,for example).Sometimes associated with timer is an event counter. With this facility there is usually a special input pin , that can drive the counter directly. Timing components The clock circuitry of most microcomputers requires only simple timing components. If maximum performance is required, a crystal must be used to ensure the maximum clock frequency is approached but not exceeded. Many clock circuits also work with a resistor and capacitor as low-cost timing components or can be driven from an external source. This latter arrangement is useful is external synchronization of the microcomputer is required. Programming languages Higher level PLC programming languages have been around for some time, but lately their popularity has mushrooming. As Raymond Leveille, vice president & general manager, Siemens Energy &Automation .inc; Programmable controls are being used for more and more sophisticated operations, languages other than ladder logic become more practical, efficient, and powerful. For example, its very difficult to write a trigonometric function using ladder logic. Languages gaining acceptance include Boolean, control system flowcharting, and such function chart languages as Graphcet and its variation .And theres increasing interest in languages like C and BASIC.PLCs in process controlThus far, PLCs have not been used extensively for continuous process control .Will this continue? The feeling that Ive gotten, says Ken Jannotta, manger, product planning, series One and Series Six product ,at GE Fanuc North America,is that PLCs will be used in the process industry but not necessarily for process control.Several vendors -obviously betting that the opposite will happen -have introduced PLCs optimized for process application .Rich Ryan, manger, commercial marketing, Allen-bradley Programmable Controls Div., cites PLCss increasing use such industries as food ,chemicals ,and petroleum. Ryan feels there are two types of applications in which theyre appropriate. one, he says, is where the size of the process control system thats being automated doesnt justify DCSdistributed control system.With the starting price tags of chose products being relatively high, a programmable controller makes sense for small, low loop count application .The second is where you have to integrate the loop closely with the sequential logical .Batch controllers are prime example ,where the sequence and maintaining the process variable are intertwined so closely that the benefits of having a programmable controller to do the sequential logical outweighs some of the disadvantages of not having a distributed control system.Bill Barkovitz, president of Triconex, predicts that all future controllers that come out in the process control system business will embrace a lot of more PLC technology and a lot more PLC functionality than they ever did before.Communications and MAPCommunications are vital to an individual automation cell and to be automated factory as a whole. Weve heard a lot about MAP in the last few years ,and a lot of companies have jumped on the bandwagon. Many, however, were disappointed when a fully-defined and completed MAP specification didnt appear immediately .Says Larry Komarek: Right now, MAP is still a moving target for the manufacturers, a specification that is not final .Presently, for example. people are introducing products to meet the MAP2.1standard .Yet2.1-based products will be obsolete when the new standard for MAP3.0 is introduced.Because of this, many PLC vendors are holding off on full MAP implementations. Omron, for example, has an ongoing MAP-compatibility program; but Frank Newburn, vice president of Omrons Industrial Division ,reports that because of the lack of a firm definition ,Omrons PLCs dont yet talk to MAP.Since its unlikely that an individual PLC would talk to broad MAP anyway, makers are concentrating on proprietary networks. According to Sal Provanzano, users fear that if they do get on board and vendors withdraw from MAP, theyll be the ones left holding a communications structure thats not supported.Universal I/OWhile there are concerns about the lack of compatible communications between PLCs from different vendors, the connection at the other end-the I/O-is even more fragmented .With rare exceptions, I/O is still proprietary .Yet there are those who feel that I/O will eventually become more universal .GE Fanuc is hoping to do that with its Genius smart I/O line. The independent I/O makers are pulling in the same direction. Many say that I/O is such a high-value item that PLC makers will always want to keep it proprietary .As Ken Jannotta, says: The I/O is going to be a disproportionate amount of the hardware sale. Certainly each PLC vendor is going to try to protect that. For that reason, he says, PLC makers wont begin selling universal I/O system from other vendor. if we start selling that kind of product, says jannotta, what do we manufacture?With more intelligent I/O appearing, Sal Provanzano feels this will lead to more differentiation among I/O from different makers. Where the I/O becomes extremely intelligent and becomes part of the system, he says, it really is hard to define which is the I/O and which is CPU. It really CPU, if you will, is equally integrated into the system as the I/O.Connecting PLC I/O to PCsWhile different PLCs probably will continue to use proprietary I/O, several vendors make it possible to connect5 their I/O to IBM PC-compatible equipment. Alle-bradeley, Could, and Cincinnati Milacron already have, and rumor has it that GE is planning something along these same lines .4Bill Ketelhut, manage of product planning at GE Fanuc North America ,sees this sort of thing as alternative to universal I/O.I think the trend ,instead of twoard universal I/O, will be multiple host interface , he says .Jodie Glore ,director of marking, Square D Automation Products, Views it as another indication that PLCs are, and have been for some time, industrial computers.中文译文单片机是电脑和集成电路发展的巅峰，有据可查的是他们也是20世纪最有意义的两大发明。一些厂家用哈弗结构，这种特性在硬件中分离了程序内存和数据内存，依据同样的原理广泛的适用于一般目的的电脑和微电脑。一些厂家则采用像Princeton结构，不划分程序内存和数据内存。这两种特性在单片机中得到了充分的体现。图A-1 哈弗结构图A-2 传统的普林斯顿结构只读存贮器(ROM) ROM是通常的永久性的，非应用程序的易失性存储器。许多微机和单片机用于大批量的应用，因此，要经济的制造设备就要在制造期间的把程序存储器中的内容永久性的刻录在芯片中。这意味着对ROM的开发是一种制约，因为制造之后不能修改ROM代码。这一发展过程可能涉及仿真,使用硬件仿真功能以及强大的软件工具使用先进的开发系统。一些制造商在其提供的设备包括的范围（或专用）用户可编程内存.其中最简单的通常是设备能够运行于微处理器模式通过使用一些输入/输出作为地址线额外的ROM选项和数据总线访问外部内存。这种类型的设备可以表现为单芯片微型计算机尽管有限制的I/O和外部修改这些设备的电路。小内存装置的应用在永久性内存的制造中是非常普遍的；但此I/O和其它芯片相比，基于微处理器的传统电路仍然可以在我节省大量成。更多更新换代的ROM设备，可在与形式多样的“背驮式” EPROM（可擦除可编程只读存储器）插座或存储器相兼容，而不是ROM器件。这些器件价格当然比同等ROM设备贵，但没有完整的等效电路。他们提供的单芯片器件对于容量要求低的优势，这些基于EPROM的设备也非常有吸引力，在以下方面的板载I/O等，方便用户灵活编程。随机存取存储器（RAM）RAM是程序执行中的工作变量和数据的存储器。存储依据设备的大小不同而类型有所不同，但作为处理器具有相同的特征宽度（4,8,16 比特等）。特殊功能寄存器，如堆栈指针或定时器寄存器，往往被以逻辑形式纳入内存区域。在哈弗结构的微型电脑通常做为集中内存。在微机中，由于RAM和寄存器没有被物理上划分，所以像在微处理系统中没必要把它们分别看待。中央处理单元（CPU）CPU 跟微处理器很相像。微电脑和微控制器的许多应用涉及到二进制编码的十进制数据处理（BCD）（数字显示为例）。因而， CPU非常普遍的被用来很好的处理这种类型的数据。因为许多控制应用涉及打开和关闭的单输出线或在单线，这也经常被用来对设施进行的测试，设置和重置个别位的内存或I/O的应用程序。这些线很容易和二进制的设备相连接使用，如开关，恒温器，固态继电器，阀门，电机等。并行输入/ 输出在不同的微机中并行输入和输出的机制有所不同。大多数设立的是一个机制，至少允许灵活的选择让那些引脚输出，那些引脚输。这可能适用于所有或部分端口。有些I/O线适合直接连接到设备，例如，荧光显示器，也可以提供足够的电流，使接口和其他设备直接相连。一些设备允许一个I/O端口和其他组件作为系统总线配置来扩展片外存储器和I/O口。这个设施可能用作一个产品系列的延伸，因为连续放大器对片上存储器可能太大，这建立在现有的软件基础上是不合适的。串行输入/输出串行通信是指使用少量的通讯线与终端设备的链接。这种通讯也可被用来连接特殊功能的芯片和几个微型机。异步和同步通信模式都要求提供帧（启动和停止）的信息协议。这可以作为一个硬件设施或U(S)ART（通用执行（同步）异步接收器/发送器）来减缓像处理器（和应用程序）中低级别细节费时。它只需要选择一个波特率及其他可能的选择（停止位，奇偶校验等），载入（或读取）串行发送器（或接收）的缓冲器。然后由硬件电路完成适当格式的数据序列化处理。定时/计数器设施许多应用的单片机需要对过去的真实时间准确的评价。这可以由每个程序中的执行时间分支认真评估，但除最简单的程序外，他的工作效率不高。首选方法是使用计时器电路，能独立计算精确的时间增量，并生成一个预设的时间后中断的时间。这种类型的定时器通常在所要求的数量可重载中应用。计时器然后减少此值产生中断或设置标记时，计数器到达零。更好的计时器有自动加载初始值的功能。这将缓解重新加载计数器和评估所用的时间，计时器重新启动之前这是必要的。有时候与定时器相关的是一个事件计数器。这个设备通常有一个特殊的输入引脚，可直接驱动计数器。定时元件大多数微型计算机时钟电路只需要简单的计时元件。如果要求最高性能，必须使用晶体以确保最大时钟频率接近，但不会超出。许多时钟电路，还具有电阻和低电容工作成本定时元件，也可以从外部源驱动。这后一种安排是有用的在微机外部同步是必需的时候。 编程语言更高水平的PLC编程语言已经推行有一段时间了