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Choosing a microcontroller
Translation
Company "Tornado Modular Systems",
Russia, Novosibirsk, 1995
- Introduction
- Appointment
- The selection process
- Criteria for selection
- System Requirements
- The main features of the microcontroller
- Features of the microcontroller
- The command set of the microcontroller
- Interrupts the microcontroller
- Feature your company
- Characteristics of the provider
- Hharakteristika manufacturer
- Potderzhka manufacturer
- Literary potderzhka
- Finishing choices
- Work team
- Conclusion
INTRODUCTION
Perhaps, the choice of microcontroller is one of the most important decisions that affect the success or failure of the planned project. When selecting a microcontroller is necessary to consider and evaluate a large number of factors. The basis of the sequence of deliberate actions leading to a final decision can be taken under consideration in this paper plan. By combining their own knowledge and requirements to the information provided in this article, the reader should appreciate everything in general, to make the right decision.
APPOINTMENT
The main goal is to select the least expensive microcontroller (to reduce total system cost), but at the same time satisfying the specifications of the system, ie requirements for performance, reliability, application conditions, etc. The total cost of the system includes all: engineering research and development, production (parts and labor) warranty repair, a further improvement, maintenance, compatibility, ease of use, etc.
SELECTION PROCESS
Getting to the selection, the developer must first ask the question: "What should the microcontroller in my system?" The answer to this simple question defines the required characteristics of the system being developed for the microcontroller and, thus, is a determining factor in the selection process.
The second step of conducting the search of microcontrollers that meet all system requirements. It usually involves the selection of literature, technical manuals and technical journals, as well as counseling. At the present time has become quite affordable information offered as a traditional, non-industry-standard microcontrollers and advanced microcontrollers. Well, if you meet the system requirements will be well known to the microcontroller, otherwise it should be conducted a secondary search to find a microcontroller that best meets the requirements, a minimum of external components mounted and is suitable for cost and size. Clearly, the single chip is preferable because of price and reliability.
The final stage of selection consists of several steps aimed to narrow the list of acceptable up to a microcontroller. These steps include the analysis of price, availability, development tools, support products, stability of production and the availability of specific microcontrollers from other manufacturers or suppliers. To arrive at an optimal solution may have to repeat the whole process several times.
SELECTION CRITERIA
The main criteria for selection of the microcontroller are presented below in order of importance. Each criterion is explained in detail in the future.
- Suitability for the application of the system. Can it be done on single-chip microcontroller, or it can be implemented on the basis of any ASIC?
- Does the required number of microcontroller pins / I / O ports, as in case of shortage he will not do the job, and if the excess price will be too high?
- Does it have all the necessary peripherals such as serial I / O ports, RAM, ROM, A / D, D / A, etc.?
- Does it have other peripherals that are not needed in the system?
- Does the processor core the performance, ie computing power, allowing the system to handle requests throughout the life of the selected application language? Too much is wasteful, too little will not work.
- Emphasis is in the project budget enough money to afford to use the microcontroller. To answer this question, it usually requires vendor fees. If the microcontroller is not acceptable for the project, all other issues become irrelevant, and you should start looking for another microcontroller.
- Availability.
- Is there a device in sufficient quantities?
- Does it now?
- What is expected in the future?
- Support for the developer.
- Assemblers.
- Compilers.
- Debugging Tools.
- Evaluation module (EVM).
- In-circuit emulators.
- Nozzles for logic analyzers.
- Debugging Monitor.
- Debugging software in source code.
- Information Support
- Examples of the application.
- Error Messages.
- Utilities, including "free" assemblers.
- Sample source code.
- Support for applications from the vendor.
- Is there a special group, which deals only with the support of applications?
- Are there any engineers, technicians or vendors?
- As far as skilled support personnel, whether or not he is interested in helping you to solve your problem?
- Is there a telephone and / or FAXovaya connection?
- The reliability of the manufacturer.
- Competence, confirmed by developments.
- Reliability of production, ie product quality.
- While working in this field.
SYSTEM REQUIREMENTS
Conducting systematic analysis of your project will identify and requirements for the microcontroller. What peripherals are required? Are bitwise operators, or only numeric? What is required for data manipulation? Should the system is controlled by an interrupt, the willingness or the commands of man? How many units (bits of input / output) needs to be managed? Which devices are among the many possible types of I / O devices should be monitored controlled: terminals, switches, relays, buttons, sensors (temperature, light, voltage, etc.), sound devices, visual indicators (LCDdisplei, LED), analog-( A / D), digital to analog (D / A) converters? One or more supply voltages required for the system? How otkazoustoychiv power supply? Will the device at a voltage of your power supply? Should the voltage be kept within a narrow range of fixed changes, or the system can operate at high volatility? What is the operating current of? The product shall operate on AC or battery? If the battery, should be used in rechargeable batteries, and if so, what is the time of operation without recharging, and what it requires time? Are there any restrictions on the size, weight, aesthetic parameters, such as the shape and / or color? Are there any specific requirements for environmental conditions such as war conditions, temperature, humidity, atmosphere (explosive, corrosive, etc.), pressure / height? User software should be based on a disk or ROM? The product works in real time, and if so, are you going to build or acquire the core real-time programs, or it may be quite widely used standard version? Is there enough staff and time to develop your own core programs? As will be paid to copyright and software? To solve the problems of real time takes a lot of research work to meet their specific requirements.
KEY FEATURES OF MICROCONTROLLER
Microcontrollers can generally be divided into groups of 8, 16 and 32razryadnyh the size of arithmetic and index registers, although some developers believe that 8/16/32razryadnuyu architecture defines the bus width. Can a cheap microcontroller to meet the requirements of the system or require an expensive 16 or 32razryadny? Can 8razryadnaya software emulation of the microcontroller features 16/32razryadnogo allow the use of cheap 8razryadnogo, sacrificing the size of the executable code and speed? For example, can be used 8razryadny microcontroller to program a macro to simulate 16razryadny battery and operation of indexing? The choice of application language (instead of high-level assembler) can greatly affect system performance, which can then dictate the choice 8/16/32razryadnoy architecture, but at a price limit may reject this choice.
Clock frequency or, more precisely, the bus speed determines how much computation can be performed per unit time. Some microcontrollers, mainly in the early development have a narrow range of permissible clock frequency, while others can operate down to zero frequency. Sometimes a specific clock frequency is selected to generate another clock frequency required in the system, for example, to set the speed sequential transmission. In general, computing power, power consumption and system cost increases with an increase in clock speed. Price of the system increases with increasing frequency because of the cost not only of the microcontroller, but also all required additional circuits such as RAM, ROM, PLD, and bus controllers.
Consider a technology, which is made using a microprocessor: Nkanalnuyu metallokiselpoluprovodnik (NMOS), which was used in microcontrollers early development, comparable to modern CMOS technology with a high level of integration (HCMOS). In contrast to earlier NMOSprotsessorov in HCMOS signal levels vary in the range from 0 to the level of the supply voltage. In connection with this circumstance is preferred HCMOS processors. In addition, HCMOS uses less power and therefore less heat. The geometrical dimensions of the elements in the HCMOS less, which allows for denser circuits and thus operate at higher speeds. More compact design also reduces the cost of a single microcontroller, since the on a silicon wafer of the same size you can get more chips. For these reasons, today the vast majority of microcontrollers manufactured using HCMOStehnologii.
Capabilities on the board
At the expense of achieving higher levels of integration and reliability while maintaining low prices, all microcontrollers have built-in optional devices. These devices are under the control of microprocessor core microcontroller perform certain functions. Embedded devices have high reliability because they do not require any external electrical circuits. The most well-known devices include built-in memory and I / O ports (I / O), timer, system clock / oscillator. Memory devices include memory (RAM), read-only memories (ROM), flash ROM (EPROM), electrically reprogrammable ROM (EEPROM). Timers and include real-time clock and timer interrupts. It should take into account the range and resolution of the timer, as well as other sub-functions, such as a comparison function and / or capture input lines for measuring the duration of the signal. Tools I / O includes serial communication ports, parallel ports (I / O lines), ADC (A / D), digital to analog converter (D / A), LCD driver display (LCD) drivers, or vacuum fluorescent display (VFD).
Other, less frequently used, resources are integrated internal / external bus, the timer tracking the normal functioning of the system watchdog, the system clock failure detection, the choice of memory configurations and system integration module (SIM). SIM is usually replaces the external "glue" logic required for interaction with microcontroller peripherals at specific contacts of the chip.
In most of the microcontroller in-circuit power resources include the configuration registers for the management of these resources. Sometimes the block itself can be reflected in the various places the memory card. Sometimes a user and / or factory test case, indicating the importance attached to the quality of the manufacturer. The presence of the configuration registers leads to the problem of random changes in the desired configuration, "wandering" code. To prevent the possibility of such a random mechanism is used "blocking", ie before the configuration register can be changed, the other bits in the register must be changed in a certain sequence. Although the configuration registers may at first scare of their complexity, they are extremely valuable because they provide greater flexibility at a low cost configuration, so that one can find a microcontroller to a variety of applications.
SET COMMAND MICROCONTROLLER
You should carefully study the instruction set and registers of each of the microcontroller, as they play a crucial role in determining the capacity of the system as a whole. Are your programmers have learned the index addressing modes in connection with the alleged needs of your system? Is there kakielibo special teams that will be used in your system, such as multiplication, division and interpolation of the table? Is there kakielibo power-saving modes to conserve battery power, such as stop, stop with the low power consumption and / or with the expectation? Is there a command kakielibo bit manipulation (bit setting, clearing bits, the test bit, the change bit transition team to set / clear bit) to facilitate the use of a microcontroller, or a command manipulation with bit fields?
Be careful with wonderful teams who make a lot of work as a team. The real measure of performance is the number of clock cycles required to complete a task, not the number of executed instructions. For a fair comparison is better to encode the same program and compare the total number of clock cycles executed and used bytes. Is there a map of the operating instructions and codes unrealized what happens if they accidentally executed? Does the system handle this situation correctly handler "exceptional" events, or it will lead to a corrupted system?
INTERRUPTION OF MICROCONTROLLER
Checking the structure of the interrupt is required whenever a system of real-time. How many lines or interrupt levels available and how much you want for your system? Is there a level interrupt mask? When the interrupt level is confirmed, whether individual vectors programmyobrabotchika interrupt or polled to interrupt all possible sources to determine the source? At the critical speed applications such as printer control, the criterion for selecting a suitable microcontroller can be the response time of interrupt, ie, time from start of the interrupt (in the worst case, the relative phased clock of the microcontroller) to run the first command corresponding to the interrupt handler.
DESCRIPTION OF YOUR COMPANY
Critically analyze the property status of your company. Does your company have sufficient funds for staff training intricacies of production systems based on microcontrollers and the use of funds for their development? Does your company have adequate resources development, or are you going to buy or rent them? If we consider a new microcontroller, are there any available development tools, such as high-level language compilers, assemblers / linkers, debuggers, and prototype modules / emulators? Is easy to expand your existing development tools for new microcontrollers? Do I need to hire and train additional staff for this project? Can you bring an expert to teach the other members of your team? Does your budget for hiring additional permanent staff and / or contract workers? Is your company satisfied with the microcontroller, available now vremyana market, as well as service?
DESCRIPTION OF THE PROVIDER
The third step in reducing the list of technically acceptable verification of microcontroller manufacturers and suppliers of microcontrollers, ie companies with whom you plan to enter into long-term relationships for mutual benefit. The supplier may be the manufacturer of microcontrollers, or it could be a dealer, who is the authorized representative of several manufacturers. The best way to satisfy your needs with the provider of a wide range of products and reputation for quality, reliability, service and timely delivery at a fair price. In addition, the more products you buy from one supplier, the more benefits you get in terms of price, service and support. Always keep in mind that although the dollar amount of your purchase you may seem high, it is always a relative value to the total sales of the supplier. Vendors that supply not only the microcontroller, but the memory (RAM, ROM), discrete devices (transistors, diodes, etc.), standard digital logic devices (7400, 74HC00, etc.), special chips, custom-made devices (CSIC), ASIC (ASIC) and programmable logic devices (PLD), will be better able to meet your growing needs. Does the manufacturer and / or supplier kakielibo awards for quality, reliability, service and / or delivery? It should not be too trusting samoprisuzhdaemym awards.
Manufacturers' specifications
Other criteria in selecting a manufacturer / supplier of microcontrollers are the stability of its monopolistic position, data from the literature and support. Stability can be reliably verified by establishing seniority manufacturer in this field and his achievements. Supply Division and the credit department of your company can help you in these matters. The monopoly position of the supplier, unfortunately, is usually the norm, as most manufacturers of microcontrollers are rarely intersect in the production of other producers. If the manufacturer has a good record in the supply, delivery and price, its monopoly position should not be an obstacle.
MANUFACTURER SUPPORT
Direct Marketing includes the support for producer / sales and application engineering support. When you call, asking for help if you can directly contact those who you want, or do you have to play "deaf telephone"? Whether calls are transferred immediately? Is there a fax number? How many phone lines are available? Phone lines are always busy? Do they have a switching system or the secretary sends your messages responsible for support? At what time working support staff? Do they have other responsibilities besides support? What is the number of members of staff? With a willingness to help him plant personnel, namely, specialists in the finished product, production, quality, engineers, computer programmers? Does the factory engineers are friendly with the staff support? Knowing whether the support staff, whether the necessary skills, and whether they fulfill in a timely manner that was promised, for example, to solve your problem or send you anything should? Whether it comes by mail, you pay for fast delivery? Does the manufacturer bulletin board (BBS) or the page on the Internet, where you can get information such as application software, product news, latest software, source code, error messages, electronic mail, conferences? What are the supported transmission rate? How many telephone lines available? What are the hours? Do you need a specific brand of computer and / or a modem to access? Is there a system operator (sysop)?
LITERARY SUPPORT
Literature covers a wide range of printed materials that can help you make the right choice. It includes the issues of the manufacturer, such as technical specifications and recommendations for use, and publications are available at the local bookstore and / or library. Publications from the local shop and / or library is not only point to the popularity of the manufacturer / microcontroller, but also offer unbiased opinions, if they are made independent of the manufacturer of the authors.
To selection
For the final step in the selection process, build a table containing microcontrollers discussed in one column, and their important characteristics in the other. Then attach the forms of technical descriptions of producers to get a fair visual comparison. Some manufacturers have pre-made comparative description of microcontrollers, which will simplify your task, but check the Data Sheet, whether all the latest products are presented. Among the possible characteristics of the price (the expected volume of production, including the prediction of future prices, ie, decrease the price, if you will be enrolled into production?), RAM, ROM, EPROM, EEPROM, timer (s), A / D, D / A, serial ports, parallel ports, bus speed (minimum / maximum), special teams (multiplication, division, etc.), the number of available interrupts, interrupt response time (time from start of the interrupt before the first command-driven interrupt) body size / type (ceramic DIP or LCC, a plastic 0.3 "DIP or 0.6" DIP, compressed DIP (the distance between the contacts .071 "), PLCC, PQFP, EIAJQFP, SOIC; some of them use surface mount technology), the power requirements and other details that are important for devices on your system.
If after all this you are still in the list of more than one microcontroller, consider the possibility of expanding the system and cost. What extensions do you think you may need in future versions of this product? Finally, consider the price, because If two of the microcontroller are the same, but one offers a bit more features that are not required today, but would make future extensions available without additional costs, choose this microcontroller.
TEAM WORK
As the leader of the project you can do all the research work alone, or you can begin to bring your team, setting its members research tasks, such as the assessment of the programmers considered a set of commands for each microcontroller. Involving your team in the selection process in the early stages, you will not only create a team spirit, but also get a personal commitment to the project through active participation in it. This approach will undoubtedly lead to some conflicts, because everyone has their own opinion, but your job as a project leader to be a mediator. After listening to all opinions, yet you choose. As in political elections as soon as the winner in the primaries, all party members are ready to fully support the leader and the project team must support the decisions of a leader to succeed in carrying out the project.
CONCLUSION
The final selection of a suitable microcontroller for your project, not an easy decision. Microcontrollers have become more complex devices as long as resources are added in-circuit. And since then, as the process moves towards increasingly-chip integration of external resources to reduce system cost, the decision becomes more complicated. This article does not force the developer to kakoylibo choice, its purpose is to specify all possible selection criteria, which must be taken into account in the decision making process.
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