1、 In general terms a single-chip microputer is characterized by the incorporation of all the units of a puter into a single device, as shown in Fig3-5A-3. Fig.3-5A-1 A Harvard type Fig.3-5A-2. A conventional Princeton puter Reset Interrupts Power Fig3-5A-3. Principal features of a microputer Read onl
2、y memory （ROM）.ROM is usually for the permanent, non-volatile storage of an applications program .Many microputers 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 mitted permanently
3、 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 powe
4、rful 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 addres
5、s and data bus for accessing external memory. This type of device can behave functionally as the single chip microputer from which it is derived albeit with restricted I/O and a modified external circuit. The use of these ROMless devices is mon even in production circuits where the volume does not j
6、ustify the development costs of custom on-chip ROM2;there can still be a significant saving in I/O and other chips pared 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 ）soc
7、kets or devices with EPROM instead of ROM 。These devices are naturally more expensive than equivalent ROM device, but do provide plete circuit equivalents. EPROM based devices are also extremely attractive for low-volume applications where they provide the advantages of a single-chip device, in term
8、s 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
9、the processor ,Special function registers, such as stack pointer or timer register are often logically incorporated into the RAM area. It is also mon in Harard type microputers to treat the RAM area as a collection of register; it is unnecessary to make distinction between RAM and processor register
10、 as is done in the case of a microprocessor system since RAM and registers are not usually physically separated in a microputer .Central processing unit （CPU）.The CPU is much like that of any microprocessor. Many applications of microputers and microcontrollers involve the handling of binary-coded d
11、ecimal （BCD） data （for numerical displays, for example） ,hence it is mon to find that the CPU is well adapted to handling this type of data .It is also mon to find good facilities for testing, setting and resetting individual bits of memory or I/O since many controller applications involve the turni
12、ng 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/output. Parallel input and output schemes vary somewhat in different microputer; in most a
13、 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 interfaci
14、ng other ponents 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 bee too big for on-chip memory and it is undesirable not to
15、 build on the existing software base.Serial input/output .Serial munication with terminal devices is mon means of providing a link using a small number of lines. This sort of munication can also be exploited for interfacing special function chips or linking several microputers together .Both the mon
16、 asynchronous synchronous munication 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-
17、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
18、.Timing/counter facilities. Many application of single-chip microputers 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 bees inefficient for all but simplest programs .The preferred appr
19、oach 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
20、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 prec
21、isely 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 ponents. The clock circuitry of most microputers requires only simple timing pon
22、ents. 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 ponents or can be driven from an external source. This latter arrangement is useful is
23、external synchronization of the microputer is required. WORDS AND TERMSculmination n.顶点 volatile n. 易变的socket n. 插座B:PLC1PLCs （programmable logical controller） face ever more plex challenges these days . Where once they quietly replaced relays and gave an occasional report to a corporate mainframe,
24、they are now grouped into cells, given new job and new languages, and are forced to pete against a growing array of control products. For this years annual PLC technology update ,we queried PLC makers on these topics and more .Programming languages Higher level PLC programming languages have been ar
25、ound 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 bee more practical, efficient, a
26、nd 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.
27、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 in
28、dustry 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, mercial marketing, Allen-bradley Programmable Controls Div., cites PLCss increasing use such industries as food
29、,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
30、, 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 tha
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