1、1 Microcomputer Systems Electronic 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 in
2、formation. 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 processi
3、ng 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. Thi
4、s partitioning 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
5、the 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 i
6、mpossible 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 section
7、s that 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 s
8、imilar problems of construction and maintenance which occur in conventional engineering are encountered when producing software. 2 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 co
9、ntrolled 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 Microcomputer Although only
10、 one storage 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 refer
11、enced 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 wr
12、itten, 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 behavio
13、r of a 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. T
14、he widely 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 P
15、rogrammable Read Only Memories, 3 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 hardwa
16、re such 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 withou
17、t directly 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 contro
18、llers 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 incorp
19、orated 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 ar
20、e required. 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
21、the benefits 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 integr
22、ated circuits. 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 the system would
