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Let's share :. And the danger of unsafe operation makes security problems even more important. Security was originally applied to information processing systems such as banking systemsdin these cases, we are concerned with the data stored in the computer system. But Stuxnet, which we will discuss in Section 7. Like dependability, security has several related concepts. Privacy refers to the unauthorized release of data. Safety relates to the way in which energy is released or controlled [Koo10]. Breaches in security can cause embedded computers to operate a physical machine improperly, causing a safety problem.
Poor system design can also cause similar safety problems. Safety is a vital concern for any computer connected to physical devices, whether the threats come from malicious external activity, poor design, or improper use of the equipment. Safety and security are related but distinct concepts. An insecure system may not necessarily present a safety problem. But the combination of these two dangers 1. Security is traditionally associated with information technology IT systems. Unauthorized disclosure of private information may, for example, allow the victim of the breach to be threatened, but a typical credit card database breach does not directly cause safety problems.
Similarly, the safety of a traditional mechanical system was not directly related to any information about that system. Today, the situation is profoundly differentdwe need safe and secure systems. A poorly designed car can allow an attacker to install software in the car and take over operation of that car, perhaps even cause the car to drive to a location at which the driver can be assaulted. Security and safety cannot be bolted ondthey must be baked in. A basic understanding of security and safety issues is important for every embedded system designer. We will make use of a few basic concepts in cryptography throughout the book [Sch96]: cryptography, public-key cryptography, hashing, and digital signatures. Cryptography is designed to encode a message so that it cannot be directly read by someone who intercepts the message; the code should also be difficult to break.
Traditional techniques are known as secret-key cryptography because it relies on the secrecy of the key used to encrypt the message. AES is a widely used encryption algorithm [ISO10]. It encrypts data in blocks of bits and can use keys of three different sizes: , , or bits. The SIMON block cipher [Bea13] has been developed as a lightweight cipher. It operates on blocks of several sizes ranging from 32 to bits and with keys ranging from 64 to bits. Public-key cryptography splits the key into two pieces: a private key and a public key. The two are related such that a message encrypted with the private key can be decrypted using the pubic key, but the private key cannot be inferred from the public key.
Since the public key does not disclose information about how the message is encoded, it can be kept in a public place for anyone to use. RSA is one widely used public-key algorithm. A cryptographic hash function has a somewhat different purposedit is used to generate a message digest from a message. The message digest is generally shorter than the message itself and does not directly reveal the message contents. The hash function is designed to minimize collisionsdtwo different messages should be very unlikely to generate the same message digest. As a result, the hash function can be used to generate short versions of more lengthy messages. SHA-3 [Dwo15] is the latest in a series of SHA hash function standards. We can use a combination of public-key cryptography and hash functions to create a digital signatureda message that authenticates a message as coming from a particular sender. A digital signature ensures the identity of the person who encrypts the message; the signature is unforgeable and unalterable.
We can also combine digital signatures with encryption of the message. In this case, both 10 CHAPTER 1 Embedded Computing the sender and receiver have private and public keys. Cryptographic functions may be implemented in software or hardware. The next example describes an embedded processor with hardware security accelerators. Application Example 1. The CPU is an ARM Cortex-M4. It includes accelerator for AES, DES, SHA, MD5, and cyclic redundancy check CRC. Let us consider some important problems that must be taken into account in embedded system design. We have a great deal of control over the amount of computing power we apply to our problem. Not only can we select the type of microprocessor used, but also we can select the amount of memory, the peripheral devices, and more. Because we often must meet both performance deadlines and manufacturing cost constraints, the choice of hardware is importantdfor too little hardware, the system fails to meet its deadlines; for too much hardware, it becomes too expensive.
How do we meet deadlines? The brute force way of meeting a deadline is to speed up the hardware so that the program runs faster. Of course, that makes the system more expensive. How do we minimize power consumption? In battery-powered applications, power consumption is extremely important. Even in nonbattery applications, excessive power consumption can increase heat dissipation. One way to make a digital system consume less power is to make it run more slowly, but naively slowing down the system can obviously lead to missed deadlines. Careful design is required to slow down the noncritical parts of the machine for power consumption while still meeting necessary performance goals. How do we design for upgradeability? The hardware platform may be used over several product generations, or for several different versions of a product in the same generation, with few or no changes.
However, we want to be able to add features by changing software. How can we design a machine that will provide the required performance for software that we have not yet written? Reliability is always important when selling productsd customers rightly expect that products they buy will work. Reliability is especially important in some applications, such as safety-critical systems. If we wait until we have a running system and try to eliminate the bugs, we will be too latedwe will not find enough bugs, it will be too expensive to fix them, and it will take too long as well. Another set of challenges comes from the characteristics of the components and systems themselves. If workstation programming is like assembling a machine on a bench, then embedded system design is often more like working on a cardcramped, delicate, and difficult.
Is it secure? Security is a key concern for modern embedded system design and is an even bigger challenge than security in traditional information technology systems. Attacks on embedded systems can not only gain access to personal data but also cause the physical systems controlled by those embedded computers to perform dangerous acts. Let us consider some ways in which the nature of embedded computing machines makes their design more difficult. We may have to run a real machine to generate the proper data. The timing of data is often important, meaning that we cannot separate the testing of an embedded computer from the machine in which it is embedded. Limited observability and controllability: Embedded computing systems usually do not come with keyboards and screens. We may be forced to watch the values of electrical signals on the microprocessor bus, for example, to know what is going on inside the system. Moreover, in real-time applications we may not be able to easily stop the system to see what is going on inside.
Restricted development environments: The development environments for embedded systems the tools used to develop software and hardware are often much more limited than those available for PCs and workstations. We generally compile code on one type of machine, such as a PC, and download it onto the embedded system. To debug the code, we must usually rely on programs that run on the PC or workstation and then look inside the embedded system. Most general-purpose programmers use no tools that are designed to help them improve the performance of their programs. Embedded system designers, in contrast, have a very clear performance goal in minddtheir program must meet its deadline.
At the heart of embedded computing is real-time computing, which is the science and art of programming to deadlines. If the program does not produce the required output by the deadline, then the program does not work, even if the output that it eventually produces is functionally correct. This notion of deadline-driven programming is at once simple and demanding. It is not easy to determine whether a large, complex program running on a sophisticated microprocessor will meet its deadline. We need tools to help us analyze the realtime performance of embedded systems; we also need to adopt programming disciplines and styles that make it possible to analyze these programs.
To understand the real-time behavior of an embedded computing system, we have to analyze the system at several different levels of abstraction. As we move through this book, we will work our way up from the lowest layers that describe components of the system up through the highest layers that describe the complete system. The platform components that surround the CPU are responsible for feeding the CPU and can dramatically affect its performance. Program: Programs are very large and the CPU sees only a small window of the program at a time. We must consider the structure of the entire program to determine its overall behavior. Task: We generally run several programs simultaneously on a CPU, creating a multitasking system.
The tasks interact with each other in ways that have profound implications for performance. Multiprocessor: Many embedded systems have more than one processordthey may include multiple programmable CPUs as well as accelerators. Once again, the interaction between these processors adds yet more complexity to the analysis of overall system performance. First, it will give us an introduction to the various steps in embedded system design before we delve into them in more detail. Second, it will allow us to consider the design methodology itself. A design methodology is important for three reasons.
First, it allows us to keep a scorecard on a design to ensure that we have done everything we need to do, such as optimizing performance or performing functional tests. Second, it allows us to develop computer-aided design tools. Developing a single program that takes in a concept for an embedded system and emits a completed design would be a daunting task, but by first breaking the process into manageable steps, we can work on automating or at least semiautomating the steps one at a 1. Third, a design methodology makes it much easier for members of a design team to communicate. By defining the overall process, team members can more easily understand what they are supposed to do, what they should receive from other team members at certain times, and what they are to hand off when they complete their assigned steps. Because most embedded systems are designed by teams, coordination is perhaps the most important role of a well-defined design methodology.
In this top-down view, we start with the system requirements. In the next step, specification, we create a more detailed description of what we want. But the specification states only how the system behaves, not how it is built. Once we know the components we need, we can design those components, including both software modules and any specialized hardware we need. Based on those components, we can finally build a complete system. In this section we will consider design from the top-downdwe will begin with the most abstract description of the system and conclude with concrete details.
The alternative is a bottom-up view in which we start with components to build a system. Bottom-up design steps are shown in the figure as dashed-line arrows. We need bottom-up design because we do not have perfect insight into how later stages of the design process will turn out. Decisions at one stage of design are based upon estimates of what will happen later: How fast can we make a particular function run? How much memory will we need? How much system bus capacity do we need? If our estimates are inadequate, we may have to backtrack and amend our original decisions to take the new facts into account. In general, the less experience we have Requirements Top-down design Bottom-up design Specification Architecture Components System integration FIGURE 1. But the steps in the design process are only one axis along which we can view embedded system design. We must also consider the tasks we need to perform at every step in the design process. We must then refine the design to add detail.
And we must verify the design to ensure that it still meets all system goals, such as cost, speed, and so on. The initial stages of the design process capture this information for use in creating the architecture and components. We generally proceed in two phases: first, we gather an informal description from the customers known as requirements, and we refine the requirements into a specification that contains enough information to begin designing the system architecture. Separating out requirements analysis and specification is often necessary because of the large gap between what the customers can describe about the system they want and what the architects need to design the system.
Consumers of embedded systems are usually not themselves embedded system designers or even product designers. Requirements may be functional or nonfunctional. We must of course capture the basic functions of the embedded system, but functional description is often not sufficient. As we have noted, performance may be a combination of soft performance metrics such as approximate time to perform a user-level function and hard deadlines by which a particular operation must be completed. Cost: The target cost or purchase price for the system is almost always a consideration.
Cost typically has two major components: manufacturing cost 1. Physical size and weight: The physical aspects of the final system can vary greatly depending upon the application. An industrial control system for an assembly line may be designed to fit into a standard-size rack with no strict limitations on weight. A handheld device typically has tight requirements on both size and weight that can ripple through the entire system design. Power consumption: Power, of course, is important in battery-powered systems and is often important in other applications as well. Power can be specified in the requirements stage in terms of battery lifedthe customer is unlikely to be able to describe the allowable wattage. Validating a set of requirements is ultimately a psychological task because it requires understanding both what people want and how they communicate those needs.
The mock-up may use canned data to simulate functionality in a restricted demonstration, and it may be executed on a PC or a workstation. But it should give the customer a good idea of how the system will be used and how the user can react to it. Physical, nonfunctional models of devices can also give customers a better idea of characteristics such as size and weight. Requirements analysis for big systems can be complex and time consuming. However, capturing a relatively small amount of information in a clear, simple format is a good start toward understanding system requirements. To introduce the discipline of requirements analysis as part of system design, we will use a simple requirements methodology.
We can use the form as a checklist in considering the basic characteristics of the system. Giving a name to the project not only simplifies talking about it to other people but can also crystallize the purpose of the machine. Name Purpose Inputs Outputs Functions Performance Manufacturing cost Power Physical size and weight FIGURE 1. If you cannot describe the essence of your system in one or two lines, chances are that you do not understand it well enough. Inputs and outputs: These two entries are more complex than they seem. Digital data? Mechanical inputs? Occasional user inputs? How many bits per data element? Video displays? A good way to approach this is to work from the inputs to the outputs: When the system receives an input, what does it do?
How do user interface inputs affect these functions? How do different functions interact? Performance: Many embedded computing systems spend at least some time controlling physical devices or processing data coming from the physical world. In most of these cases, the computations must be performed within a certain time frame. It is essential that the performance requirements be identified early because they must be carefully measured during implementation to ensure that the system works properly. Manufacturing cost: This includes primarily the cost of the hardware components. Even if you do not know exactly how much you can afford to spend on system components, you should have some idea of the eventual cost range. Power: Similarly, you may have only a rough idea of how much power the system can consume, but a little information can go a long way.
Typically, the most important decision is whether the machine will be battery powered or plugged into the wall. Battery-powered machines must be much more careful about how they spend energy. Physical size and weight: You should give some indication of the physical size of the system to help guide certain architectural decisions. A desktop machine has much more flexibility in the components used than, for example, a lapel-mounted voice recorder. A more thorough requirements analysis for a large system might use a form similar to Fig.
After an introductory section containing this form, a longer requirements document could include details on each of the items mentioned in the introduction. For example, each individual feature described in the introduction in a single sentence may be described in detail in a section of the specification. Did you consider all the modes in which you want the system to operate? Did you place an unrealistic number of features into a battery-powered, low-cost machine? To practice the capture of system requirements, Example 1. Example 1. The moving map obtains its position from the GPS, a satellite-based navigation system. The system should show major roads and other landmarks available in standard topographic databases.
The device should be controlled by no more than three buttons. A menu system should pop up on the screen when buttons are pressed to allow the user to make selections to control the system. Upon power-up, a display should take no more than 1 s to appear, and the system should be able to verify its position and display the current map within 15 s. Notice that many of these requirements are not specified in engineering unitsdfor example, physical size is measured relative to a hand, not in centimeters. Although these requirements must ultimately be translated into something that can be used by the designers, keeping a record of what the customer wants can help to resolve questions about the specification that may crop up later during design. For example, it provides actual dimensions of the device. The manufacturing cost was derived from the selling price by using a simple rule of thumb: the selling price is four to five times the cost of goods sold the total of all the component costs.
A use case describes the actions of human users or other machines as they interact with the system. A set of use cases that describe typical usage scenarios often helps to clarify what the system needs to do. Specification is probably the least familiar phase of this methodology for neophyte designers, but it is essential to creating working systems with a minimum of designer effort. Designers who lack a clear idea of what they want to build when they begin typically make faulty assumptions early in the process that are not obvious until they have a working system. At that point, the only solution is to take the machine apart, throw away some of it, and start again. Not only does this take a lot of extra time, but the resulting system is also very likely to be inelegant, kludgy, and bugridden. It should also be unambiguous enough that designers know what they need to build. Designers can run into several different types of problems caused by unclear specifications.
If the behavior of some feature in a particular situation is unclear from the specification, the designer may implement the wrong functionality. If global characteristics of the specification are wrong or incomplete, the overall system architecture derived from the specification may be inadequate to meet the needs of implementation. UML, a language for describing specifications, will be introduced in the next section. We will practice writing specifications in each chapter as we work through example system designs. We will also study specification techniques in more detail in Chapter 7. Describing how the system implements those functions is the purpose of the architecture. The architecture is a plan for the overall structure of the system that will be used later to design the components that make up the architecture. The creation of the architecture is the first phase of what many designers think of as design. To understand what an architectural description is, let us look at a sample architecture for the moving map of Example 1.
Search engine Renderer User interface Display 20 CHAPTER 1 Embedded Computing architecture in the form of a block diagram that shows major operations and data flows among them. This block diagram is still quite abstractdwe have not yet specified which operations will be performed by software running on a CPU, what will be done by special-purpose hardware, and so on. The diagram does, however, go a long way toward describing how to implement the functions described in the specification. We clearly see, for example, that we need to search the topographic database and to render ie, draw the results for the display.
We have chosen to separate those functions so that we can potentially do them in paralleldperforming rendering separately from searching the database may help us update the screen more fluidly. Only after we have designed an initial architecture that is not biased toward too many implementation details should we refine that system block diagram into two block diagrams: one for hardware and another for software. These two more refined block diagrams are shown in Fig. The software block diagram fairly closely follows the system block diagram, but we have added a timer to control when we read the buttons on the user interface and render data onto the screen. Pixels 1. Architectural descriptions must be designed to satisfy both functional and nonfunctional requirements. Not only must all the required functions be present, but also we must meet cost, speed, power, and other nonfunctional constraints. Starting out with a system architecture and refining that to hardware and software architectures is one good way to ensure that we meet all specifications: We can concentrate on the functional elements in the system block diagram and then consider the nonfunctional constraints when creating the hardware and software architectures.
How do we know that our hardware and software architectures in fact meet constraints on speed, cost, and so on? We must somehow be able to estimate the properties of the components of the block diagrams, such as the search and rendering functions in the moving map system. Accurate estimation derives in part from experience, both general design experience and particular experience with similar systems. However, we can sometimes create simplified models to help us make more accurate estimates. Sound estimates of all nonfunctional constraints during the architecture phase are crucial, because decisions based on bad data will show up during the final phases of design, indicating that we did not, in fact, meet the specification. The component design effort builds those components in conformance to the architecture and specification. The components will in general include both hardwaredFPGAs, boards, and so ondand software modules.
Some of the components will be ready-made. The CPU, for example, will be a standard component in almost all cases, as will memory chips and many other components. In the moving map, the GPS receiver is a good example of a specialized component that will nonetheless be a predesigned, standard component. We can also make use of standard software modules. One good example is the topographic database. Standard topographic databases exist, and you probably want to use standard routines to access the databasednot only are the data in a predefined format, but also they are highly compressed to save storage. Using standard software for these access functions not only saves us design time, but it may also give us a faster implementation for specialized functions such as the data decompression phase.
You will have to design some components yourself. Even if you are using only standard integrated circuits, you may have to design the printed circuit board that connects them. You will probably have to do a lot of custom programming as well. When creating these embedded software modules, you must of course make use of your expertise to ensure that the system runs properly in real time and that it does not take up more memory space than is allowed. The power consumption of the moving 22 CHAPTER 1 Embedded Computing map software example is particularly important. You may need to be very careful about how you read and write memory to minimize powerdfor example, because memory accesses are a major source of power consumption, memory transactions must be carefully planned to avoid reading the same data several times. Of course, this phase usually consists of a lot more than just plugging everything together and standing back. Bugs are typically found during system integration, and good planning can help us find the bugs quickly.
By building up the system in phases and running properly chosen tests, we can often find bugs more easily. If we debug only a few modules at a time, we are more likely to uncover the simple bugs and be able to easily recognize them. Only by fixing the simple bugs early will we be able to uncover the more complex or obscure bugs that can be identified only by giving the system a hard workout. We need to ensure during the architectural and component design phases that we make it as easy as possible to assemble the system in phases and test functions relatively independently.
System integration is difficult because it usually uncovers problems. It is often hard to observe the system in sufficient detail to determine exactly what is wrongdthe debugging facilities for embedded systems are usually much more limited than what you would find on desktop systems. As a result, determining why things do not work correctly and how they can be fixed is a challenge in itself. Careful attention to inserting appropriate debugging facilities during design can help ease system integration problems, but the nature of embedded computing means that this phase will always be a challenge. It is often helpful to conceptualize these tasks in diagrams. Luckily, there is a visual language that can be used to capture all these design tasks: the Unified Modeling Language UML [Boo99; Pil05].
UML was designed to be useful at many levels of abstraction in the design process. UML is useful because it encourages design by successive refinement and progressively adding detail to the design, rather than rethinking the design at each new level of abstraction. UML is an object-oriented modeling language. We can also use UML to model the outside world that interacts with our system, in which case the objects may correspond to people or other machines. It is sometimes important to implement something we think of at a high level as a single object using several distinct pieces of code or to otherwise break up the object correspondence in the implementation. However, thinking of the design in terms of actual objects helps us understand the natural structure of the system.
Both views are useful. At a minimum, object-oriented specification is a set of linguistic mechanisms. In many cases, it is useful to describe a system in terms of real-world analogs. However, performance, cost, and so on may dictate that we change the specification to be different in some ways from the real-world elements we are trying to model and implement. In this case, the object-oriented specification mechanisms are still useful. What is the relationship between an object-oriented specification and an objectoriented programming language such as Cþþ [Str97]? A specification language may not be executable.
But both object-oriented specification and programming languages provide similar basic methods for structuring large systems. UML is a large language, and covering all of it is beyond the scope of this book. In this section, we introduce only a few basic concepts. In later chapters, as we need a few more UML concepts, we introduce them to the basic modeling elements introduced here. Because UML is so rich, there are many graphical elements in a UML diagram. It is important to be careful to use the correct drawing to describe somethingdfor instance, UML distinguishes between arrows with open and filled-in arrowheads, and solid and broken lines.
As you become more familiar with the language, uses of the graphical primitives will become more natural to you. We also will not take a strict object-oriented approach. We may not always use objects for certain elements of a designdin some cases, such as when taking particular aspects of the implementation into account, it may make sense to use another design style. However, object-oriented design is widely applicable, and no designer can consider himself or herself design literate without understanding it. Classes as types component of an object-oriented design is, naturally enough, the object. An object includes a set of attributes that define its internal state. When implemented in a programming language, these attributes usually become variables or constants held in a data structure. In some cases, we will add the type of the attribute after the attribute name for clarity, but we do not always have to specify a type for an attribute.
An object describing a display such as a CRT screen is shown in UML notation in Fig. The text in the folded-corner-page icon is a note; it does not correspond to an object in the system and only serves as a comment. The attribute is, in this case, an array of pixels that holds the contents of the display. The object is identified in two ways: it has a unique name, and it is a member of a class. The name is underlined to show that this is a description of an object and not of a class. A class is a form of type definitiondall objects derived from the same class have the same characteristics, although their attributes may have different values. A class defines the attributes that an object may have.
It also defines the operations that determine how the object interacts with the rest of the world. In a programming language, the operations would become pieces of code used to manipulate the object. The UML description of the Display class is shown in Fig. The class has the name that we saw used in the d1 object because d1 is an instance of class Display. The Display class defines the pixels attribute seen in the object; remember that when we instantiate the class an object, that object will have its own memory so that different objects of the Display Pixels is a 2-D array FIGURE 1. Other classes can examine and modify class attributes; if we have to do something more complex than use the attribute directly, we define a behavior to perform that function.
The implementation includes both the attributes and whatever code is used to implement the operations. As long as we do not change the behavior of the object seen at the interface, we can change the implementation as much as we want. This lets us improve the system by, for example, speeding up an operation or reducing the amount of memory required without requiring changes to anything else that uses the object. Clearly, the choice of an interface is a very important decision in object-oriented design. However, excessive generality often makes the object large and slow. Big, complex interfaces also make the class definition difficult for designers to understand and use properly. Aggregation describes a complex object made of smaller objects. Composition is a type of aggregation in which the owner does not allow access to the component objects.
Generalization allows us to define one class in terms of another. The elements of a UML class or object do not necessarily directly correspond to statements in a programming languagedif the UML is intended to describe something more abstract than a program, there may be a significant gap between the contents of the UML and a program implementing it. The attributes of an object do not necessarily reflect variables in the object. An attribute is some value that reflects the current state of the object. In the program implementation, that value could be computed from some other internal variables. The behaviors of the object would, in a higher-level specification, reflect the basic things that can be done with an object. Implementing all these features may require breaking up a behavior into several smaller behaviorsdfor example, initialize the object before you start to change its internal state.
UML, like most object-oriented languages, allows us to define one class in terms of another. An example is shown in Fig. This does not require us to add new attributes or operations, but we can specialize both to work on one-bit pixels. Generalization and inheritance even with a small number of bits per pixel. A derived class inherits all the attributes and operations from its base class. In this class, Display is the base class for the two derived classes. A derived class is defined to include all the attributes of its base class. Inheritance has two purposes. It of course allows us to succinctly describe one class that shares some characteristics with another class. Even more important, it captures those relationships between classes and documents them. UML considers inheritance to be one form of generalization.
A generalization relationship is shown in a UML diagram as an arrow with an open unfilled arrowhead. UML also allows us to define multiple 1. inheritance, in which a class is derived from more than one base class. Most object-oriented programming languages support multiple inheritance as well. An example of multiple inheritance is shown in Fig. The derived class inherits all the attributes and operations of both its base classes, Display and Speaker. Because multiple inheritance causes the sizes of the attribute set and operations to expand so quickly, it should be used with care. A link describes a relationship between objects; association is to link as class is to object. We need links because objects often do not stand alone; associations let us capture type information about these links.
When we consider the actual objects in the system, there is a set of messages that keeps track of the current number of active messages two in this example and points to the active messages. In this case, the link defines the contains relation. When generalized into classes, we define an association between the message set class and the message class. The association is drawn as a line between the two labeled with the name of the association, namely, contains. The ball and the number at the message class end indicate that the message set may include zero or more message objects. Typically, we find that we use a certain combination of elements in an object or class many times. in UML.
One way to specify the behavior of an operation is a state machine. These state machines will not rely on the operation of a clock, as in hardware; rather, changes from one state to another are triggered by the occurrence of events. An event is some type of action. The event may originate outside the system, such as the button press. It may also originate inside, such as when one routine finishes its computation and passes the result on to another routine. UML defines several special types of events as illustrated in Fig. A call event follows the model of a procedure call in a programming language. button1 S2 S1 button2 S3 FIGURE 1. The label tm time-value on the edge gives the amount of time after which the transition occurs. A time-out is generally implemented with an external timer.
This notation simplifies the specification and allows us to defer implementation details about the time-out mechanism. We show the occurrence of all types of signals in a UML diagram in the same waydas a label on a transition. A state machine for an operation of the display is shown in Fig. The start and stop states are special states that help us organize the flow of the state machine. The states in the state machine represent different conceptual operations. In some cases, we take conditional transitions out of states based on inputs or the results of some computation done in the state. In other cases, we make an unconditional transition to the next state. Both the unconditional and conditional transitions make use of the call event.
Splitting a complex operation into several states helps document the required steps, much as subroutines can be used to structure code. It is sometimes useful to show the sequence of operations over time, particularly when several objects are involved. In this case, we can create a sequence diagram, which is often used to describe use cases. A sequence diagram is somewhat similar to a hardware timing diagram, although the time flows vertically in a sequence diagram, whereas time typically flows horizontally in a timing diagram. The sequence diagram is designed to show a particular scenario or choice of eventsdit is not convenient for showing a number of mutually exclusive possibilities. An example of a mouse click and its associated actions is shown in Fig.
The mouse click occurs on the menu region. Processing includes three objects shown at the 1. top of the diagram. Extending below each object is its lifeline, a dashed line that shows how long the object is alive. In this case, all the objects remain alive for the entire sequence, but in other cases objects may be created or destroyed during processing. The boxes along the lifelines show the focus of control in the sequence, that is, when the object is actively processing. The display object remains in play longer; it in turn uses call events to invoke the menu object twice: once to determine which menu item was selected and again to actually execute the menu call.
The user sends messages to the train with a control box attached to the tracks. The control box may have familiar controls such as a throttle, emergency stop button, and so on. Because the train receives its electrical power from the two rails of the track, the control box can send signals to the train over the tracks by modulating the power supply voltage. As shown in the figure, the control panel sends packets over the tracks to the receiver on the train. Each packet includes an address so that the console can control several trains on the same track; the packet also includes an error correction code ECC to guard against transmission errors.
This is a one-way communication systemdthe model train cannot send commands back to the user. We start by analyzing the requirements for the train control system. We will base our system on a real standard developed for model trains. We then develop two specifications: a simple, high-level specification and then a more detailed specification. The speed of each train shall be controllable by a throttle to at least 63 different levels in each direction forward and reverse. There shall be an inertia control that shall allow the user to adjust the responsiveness of the train to commanded changes in speed. Higher inertia means that the train responds more slowly to a change in the throttle, simulating the inertia of a large train. The inertia control will provide at least eight different levels. There shall be an emergency stop button. An error detection scheme will be used to transmit messages. We could develop our own train control system from scratch, but basing our system upon a standard has several advantages in this case: It reduces the amount of work we have to do, and it allows us to use a wide variety of existing trains and other pieces of equipment.
Hobbyists started building homebrew digital control systems in the s, and Marklin developed its own digital control system in the s. DCC was created to provide a standard that could be built by any manufacturer so that hobbyists could mix and match components from multiple vendors. Standard S Any DCC-conforming device must meet these specifications. DCC also provides several recommended practices. These are not strictly required but they provide some hints to manufacturers and users as to how to best use DCC. The DCC standard does not specify many aspects of a DCC train system.
It does not define the control panel, the type of microprocessor used, the programming language to be used, or many other aspects of a real model train system. The standard concentrates on those aspects of system design that are necessary for interoperability. The Electrical Standard deals with voltages and currents on the track. Although the electrical engineering aspects of this part of the specification are beyond the scope of the book, we will briefly discuss the data encoding here. The standard must be carefully designed because the main function of the track is to carry power to the locomotives. The signal encoding system should not interfere with power transmission either to DCC or non-DCC locomotives. A key requirement is that the data signal should not change the DC value of the rails.
The data signal swings between two voltages around the power supply voltage. As shown in Fig. A 0 is at least ms while a 1 is nominally 58 ms. The durations of the high above nominal voltage and low below nominal voltage parts of a bit are equal to keep the DC value constant. The specification also gives the allowable variations in bit times that a conforming DCC receiver must be able to tolerate. The standard also describes other electrical properties of the system, such as allowable transition times for signals. The DCC Communication Standard describes how bits are combined into packets and the meaning of some important packets. Some packet types are left undefined in the standard, but typical uses are given in Recommended Practices documents. We can write the basic packet format as a regular expression: PSAðsDÞ þ E 1. The command station should send at least 14 of these 1 bits, some of which may be corrupted during transmission. S is the packet start bit.
A is an address data byte that gives the address of the unit, with the most significant bit of the address transmitted first. An address is 8 bits long. The addresses , , and are reserved. D is the data byte, which includes 8 bits. A data byte may contain an address, instruction, data, or error correction information. E is a packet end bit, which is a 1 bit. Note that the address data byte is a specific type of data byte. A baseline packet is the minimum packet that must be accepted by all DCC implementations. More complex packets are given in a Recommended Practice document. A baseline packet has three data bytes: An address data byte gives the intended receiver of the packet; the instruction data byte provides a basic instruction; and an error correction data byte is used to detect and correct transmission errors.
The instruction data byte carries several pieces of information. Bits 0e3 provide a 4-bit speed value. Bit 4 has an additional speed bit, which is interpreted as the least significant speed bit. Bit 5 gives direction, with 1 for forward and 0 for reverse. Bits 7e8 are set at 01 to indicate that this instruction provides speed and direction. The error correction data byte is the bitwise exclusive OR of the address and instruction data bytes. The standard says that the command unit should send packets frequently because a packet may be corrupted. Packets should be separated by at least 5 ms. But DCC deliberately does not specify everything about a model train control system. We need to round out our specification with details that complement the DCC specification. A conceptual specification allows us to understand the system a little better.
We will use the experience gained by writing the conceptual specification to help us write a detailed specification to be given to a system architect. This specification does not correspond to what any commercial DCC controllers do, but it is simple enough to allow us to cover some basic concepts in system design. A train control system turns commands into packets. A command comes from the command unit while a packet is transmitted over the rails. Commands and packets may not be generated in a 1-to-1 ratio. In fact, the DCC standard says that command units should resend packets in case a packet is dropped during transmission.
Estop emergency stop requires no parameters whereas Set-speed and Set-inertia do. We now need to model the train control system itself. There are clearly two major subsystems: the command unit and the train-board component. Each of these subsystems has its own internal structure. The basic relationship between them is illustrated in Fig. The notation on the arrow 36 CHAPTER 1 Embedded Computing Command Set-speed Set-inertia value: integer value: unsigned-integer Estop FIGURE 1. n: command :console :receiver FIGURE 1. Those messages are of course carried over the track. Because the track is not a computer component and is purely passive, it does not appear in the diagram.
However, it would be perfectly legitimate to model the track in the collaboration diagram, and in some situations it may be wise to model such nontraditional components in the specification diagrams. For example, if we are worried about what happens when the track breaks, modeling the tracks would help us identify failure modes and possible recovery mechanisms. The console needs to perform three functions: read the state of the front panel on the command unit, format messages, and transmit messages. The train receiver must also perform three major functions: receive the message, interpret the message taking into account the current speed, inertia setting, etc. In this case, let us use a class diagram to represent the design; we could also use an object diagram if we wished.
The UML class diagram is shown in Fig. It shows the console class using three classes, one for each of its major components. The Transmitter class interfaces to analog electronics to send the message along the track. There will be one instance of the Console class and one instance of each of the component classes, as shown by the numeric values at each end of the relationship links. Likewise, the Train makes use of three other classes that define its components: The Receiver class knows how to turn the analog signals on the track into digital form. The Controller class includes behaviors that interpret the commands and figures out how to control the motor. The Motor interface class defines how to generate the analog signals required to control the motor. We have also defined a special class, Train set, to help us remember that the system can handle multiple trains. The values on the relationship edge show that one train set can have t trains. We would not actually implement the train set class, but it does serve as useful documentation of the existence of multiple receivers.
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We then develop two specifications: a simple, high-level specification and then a more detailed specification. Physical size and weight: The physical aspects of the final system can vary greatly depending upon the application. In many cases, we can make very high-level decisions about the structure of our programs to greatly improve their real-time performance and power consumption. Summary What we learned Further reading Chapter 8. E is a packet end bit, which is a 1 bit. Careful design is required to slow down the noncritical parts of the machine for power consumption while still meeting necessary performance goals. It is tempting to think that the overhead of fetching, decoding, and executing instructions is so high that it cannot be recouped.
In the early days of microprocessors, when all the components were relatively small and simple, it was necessary and desirable to concentrate on individual instructions and logic gates. It is now up to the architects and designers of embedded systems to make these possibilities a reality. The Motor interface class computers as components 4th edition pdf download how to generate the analog signals required to control the motor. First, standards are important. But because these messages must work with a variety of trains and we may want to add more commands in a later version of the system, we need to specify the basic features of messages for compatibility. English Pages [] Year Cathy Wicks and Naser Salameh of Texas Instruments gave me invaluable help in figuring out the C55x.
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