单片机外文文献翻译

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 company s accounts but in each case the same main type of operation are involved the processing storage and transmission of information 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 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 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 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 impossible 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 system s behavior would contain sections 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 similar 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 controlled 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 2 Fig 1 1 Three Sections of a Typical Microcomputer Although only 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 referenced 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 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 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 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 Programmable 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 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 directly involving the main microprocessor The microprocessor processes data under the control of the program controlling the flow of 3 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 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 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 controllers 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 incorporated 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 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 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 integrated 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 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 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 virtually become part of the electronic hardware and are often referred to as firmware More sophisticated process controllers 4 require minicomputers for their implementation although the use lf large scale integrated circuits the distinction between mini and microcomputers Products and process controllers of various kinds represent the majority of present day microcomputer applications the exact figures depending on one s 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 study 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 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 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 pressure 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 impractical 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 hundred 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 minimum 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 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 converted to electrical signal that 5 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 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 range of 0 600kPa The gain is defined as the change in output divided by the change in input In this case the output is electrical signal 4 20mA DC and the input is process pressure 0 600kPa Thus the gain Beside we must measure Temperature Temperature measurement is important in industrial control 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 gases 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 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 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 Description The AT89C51 is a low power high performance CMOS 8 bit microcomputer with 4K bytes of Flash programmable and erasable read only memory PEROM The device is manufactured using Atmel s high density nonvolatile memory technology and is compatible with the industry standard MCS 51 instruction set and pinout The on chip Flash allows the program memory to be reprogrammed in system or by a conventional nonvolatile memory programmer By combining a versatile 8 bit CPU with kPa mA kPa mA kPakPa mAmA Kr027 0 600 16 0600 420 6 Flash on a monolithic chip the Atmel AT89C51 is a powerful microcomputer which provides a highly flexible and cost effective solution to many embedded control applications Function characteristic The AT89C51 provides the following standard features 4K bytes of Flash 128 bytes of RAM 32 I O lines two 16 bit timer counters a five vector two level interrupt architecture a full duplex serial port on chip oscillator and clock circuitry In addition the AT89C51 is designed with static logic for operation down to zero frequency and supports two software selectable power saving modes The Idle Mode stops the CPU while allowing the RAM timer counters serial port and interrupt system to continue functioning The Power down Mode saves the RAM contents but freezes the oscillator disabling all other chip functions until the next hardware reset Pin Description VCC Supply voltage GND Ground Port 0 Port 0 is an 8 bit open drain bi directional I O port As an output port each pin can sink eight TTL inputs When 1s are written to port 0 pins the pins can be used as highimpedance inputs Port 0 may also be configured to be the multiplexed loworder address data bus during accesses to external program and data memory In this mode P0 has internal pullups Port 0 also receives the code bytes during Flash programming and outputs the code bytes during programverification External pullups are required during programverification Port 1 Port 1 is an 8 bit bi directional I O port with internal pullups The Port 1 output buffers can sink source four TTL inputs When 1s are written to Port 1 pins they are pulled high by the internal pullups and can be used as inputs As inputs Port 1 pins that are externally being pulled low will source current IIL because of the internal pullups Port 1 also receives the low order address bytes during Flash programming and verification Port 2 Port 2 is an 8 bit bi directional I O port with internal pullups The Port 2 output buffers can sink source four TTL inputs When 1s are written to Port 2 pins they are pulled high by the internal pullups and can 7 be used as inputs As inputs Port 2 pins that are externally being pulled low will source current because of the internal pullups Port 2 emits the high order address byte during fetches from external program memory and during accesses to external data memory that use 16 bit addresses In this application it uses strong internal pullupswhen emitting 1s During accesses to external data memory that use 8 bit addresses Port 2 emits the contents of the P2 Special Function Register Port 2 also receives the high order address bits and some control signals during Flash programming and verification Port 3 Port 3 is an 8 bit bi directional I O port with internal pullups The Port 3 output buffers can sink source four TTL inputs When 1s are written to Port 3 pins they are pulled high by the internal pullups and can be used as inputs As inputs Port 3 pins that are externally being pulled low will source current IIL because of the pullups Port 3 also serves the functions of various special features of the AT89C51 as listed below Port 3 also receives some control signals for Flash programming and verification RST Reset input A high on this pin for two machine cycles while the oscillator is running resets the device ALE PROG Address Latch Enable output pulse for latching the low byte of the address during accesses to external memory This pin is also the program pulse input PROG during Flash programming In normal operation ALE is emitted at a constant rate of 1 6 the oscillator frequency and may be used for external timing or clocking purposes Note however that one ALE pulse is skipped during each access to external Data Memory If desired ALE operation can be disabled by setting bit 0 of SFR location 8EH With the bit set ALE is active only during a MOVX or MOVC instruction Otherwise the pin is weakly pulled high Setting the ALE disable bit has no effect if the microcontroller is in external execution mode PSEN Program Store Enable is the read strobe to external program memory When the AT89C51 is executing code from external program memory PSEN is activated twice each machine cycle except that two PSEN activations are skipped during each access to external data memory 8 EA VPP External Access Enable EA must be strapped to GND in order to enable the device to fetch code from external program memory locations starting at 0000H up to FFFFH Note however that if lock bit 1 is programmed EA will be internally latched on reset EA should be strapped to VCC for internal program executions This pin also receives the 12 volt programming enable voltage VPP during Flash programming for parts that require12 volt VPP XTAL1 Input to the inverting oscillator amplifier and input to the internal clock operating circuit XTAL2 Output from the inverti