state machine model in embedded system
What is even better than switch statements is using labels as values; the same direct dispatch as function pointers but without the stack overhead. Protothreads are more like “keep these 5 plates spinning” and state machines are “first do this, then do this”. Honestly, a transition table is a pretty nice way to record what your machine should be doing, and doesn’t require any graphics at all. Make the small changes needed to pass the test. To turn the popper into a coffee roaster, the fan and heater circuits were cut apart and made separately controllable. Let’s use the state machine in Figure 2 to explore this concept. -Switch statements neatly hold all the logic within 1 clause and even allow fall through for state transitions The graphics code will still have control of the SPI bus. From the roast profile, we get the states “preheat”, “soak”, “ramp”, and “cooldown” corresponding to the obvious phases of the process. Times India, EE And that’s exactly what function pointers do in C. Here’s a quick rundown on pointers in general, and function pointers in particular. Just writing this down gets us a lot of the way toward defining our state machine. -Amateur programmers quickly create spaghetti code when they slicing up their code into little functions and disperse these functions throughout their files. It lets you keep the state function pointer inside main() and makes it very obvious (in main) where the state is getting updated. In this article, we will explore the use of FSM in building complex React components. That said, I might be tempted to try it next time just for the hell of it; practically everything I write ends up using one or more state machines anyway. QM™ (QP Modeler) is a freeware model-based design (MBD) and automatic code generation tool for designing software based on modern finite state machines (UML statecharts) and the QP™ Real-Time Embedded Frameworks.QM™ is available for Windows, Linux and macOS hosts. Our modern QP™ real-time embedded frameworks (RTEFs) and a suite of complementary host-based tools enable event-driven architecture, finite state machines, model-based design and automatic code generation. Languages ¥State Machine Model ÐFSM/FSMD ÐHCFSM and Statecharts Language ÐProgram-State Machine (PSM) Model You can pass the struct by value, which is great b/c it prevents modification of the overall variables from within the handler functions, but can be tough on stack space in small-RAM systems. For example, when it needs to draw a line, it it pushes a structure to the stack containing all the parameters and variables required to draw a line, and then proceeds through the necessary states. The key is maintaining your own stack to simulate function calls. Enter your email below, and we'll send you another email. writeOutputs(outputs); Also, I’m not sure why the model checker wouldn’t be able to handle function pointers correctly. We have sent a confirmation email to {* emailAddressData *}. For instance, when you fire up your coffee roaster, it may enter an initialization state, take some actions, and then go straight into a waiting-for-input state. Tests are small. And it will make it very easy to port to other graphics devices, as well. Enter your email below, and we'll send you another email. A state machine is a behavior model. It consists of a finite number of states and is therefore also called finite-state machine (FSM). Another excellent article in your series for beginners. However, in some cases more than a single transition is needed to check functionality. Many applications from simple home appliances to complex communication systems implement event based state machines. It also had a thermal fuse to keep it from burning up, and an on-off switch. Raindrops on roses, and whiskers on kittens. But in general you can't assume this. That’s all, and that’s all that’s necessary for the job. It is helpful to create a test list before developing new functionality. This, along with my protothreaded Ethernet, and mesh RF network, mean I’ll very rarely require more than one SPI bus. A program that must sequence a series of actions, or handle inputs differently depending on what state it's in, is often best implemented as a state machine. A state machine model is a mathematical model that groups all possible system occurrences, called states. I recently got into writing VHDL for Xilinx FPGAs. There must be other stuff out there that’s more specialized to the task, though. Please confirm the information below before signing in. If the sensor code forcibly takes control of the bus, the graphics code won’t have a clue what happened, and when returned to will write corrupted data. Build your own Finite State Machine and run it on your embedded platform. Daniel Söderholm started following Embedded Rust Hack Chat. First of all, TDD is a technique for building software incrementally. Along the way, we’ll Goldilocks solution for a particular application I had, controlling a popcorn popper that had been hacked into a coffee roaster. This keeps everything encapsulated in a class, and easily allows to make more instances of the same FSM. If you’d like more state machine background, go give his excellent article a read. Now I just try to create and maintain good habits, the top habit trying to make things homogeneous. The global_state variable that gives a name to each of the states, that we thought of as being the central mechanism, is actually superfluous. Use them with care, but don’t run. For instance, we know what events are going to be driving most of our state transitions: four buttons and a temperature sensor. The flip side of that coin is that doing it “correctly” tends to take so long to implement it might never have been made in the first place. Protothreads are typically used on systems that are too resource limited to allow for a preemptive multitasking kernel to be used. Each state also has a list of events which can trigger a transition to another state. Satellite connectivity bridges gap to underserved IoT markets, Embedded design with FPGAs: Development process, Samsung I-Cube4 puts 4 HBMs & logic die on paper-thin silicon interposer, Reduced operation set computing (ROSC) for NNA functional coverage, EE Times A state machine model is a mathematical model that groups all possible system occurrences, called states. You’re a plant! This is because the effect only gets visible due to a chain of actions of subsequent transitions, which means that functionality involves a set of states, events and transitions of the SMUT. Run all the tests and prove if the new one passes. OTOH, I left it with the global b/c it was more parallel with the switch…case implementation as I wrote it. Just thinking your project through this way is half the battle. Though, both these methods (returning states in one way or another) seems like bad coding practice to me, as it increases the coupling between the different modules (states) in the program. The initial state of a state machine diagram, known as an initial pseudo-state, is indicated with a solid circle. As you can see, all of the per-state code is handled in its own functions, just as in the streamlined switch...case version, but we’ve gotten rid of the cumbersome switch...case construct and the global_state variable, replacing it with a global function pointer, do_state. When the state machine is initialized, it starts from the StateA state. The concept really comes in to its own here when the state machine is described solely by a table, be it an array of structures, parallel arrays, or whatever, and the function pointers are part of the table data. Protothreading is a method of cooperative multitasking. Every possible state of a system is evaluated, showing all possible interactions between subjects and objects. Dominik Meffert has updated the log for Continuous Inkjet Printing. }. Because the state handler functions must all have the same prototype (b/c they need to fit the same pointer) you can’t pass different arguments to different states, so you end up passing all the arguments to all the functions. And you’ve just decided to add a sensor, which only takes a couple of us to read, but it must be read within 20us of a particular event occurring or it will be too late. I do enjoy them though, the basics are explained in a clear way. Pointers are variables that essentially contain memory locations, and function pointers are variables that contain the address of code in memory. Putting them in separate functions enforces that. This is checked by verifying that the value of the out attribute is equal to the number of executed iterations (line 101). Pointers do a little more than just store a memory location, though. It seems to me that do_state() never gets updated in the while loop. Start out with some simple scaffolding, either the switch...case variety or the function pointer version, and expand out your own code on it. A program that must sequence a series of actions, or handle inputs differently depending on what mode it's in, is often best implemented as a state machine. I use this example because I’m in the process of writing a protothreaded graphics engine. Or maybe you’re out of pins or SPI busses, in which case you’ll have to do some more drastic redesign, possibly switching to a different MCU. Already have an account? Among these rules there is a big NO function pointers. Run all the tests and if the new one fails, it may not even compile. state machine module built using the proposed model, a C compiler [3] for the ATMega microcontrollers – IAR Embedded Workbench, and the simulator [4] integrated in the IAR IDE. You’ll be surprised how quickly the code can flow once you’ve got the right structure, and state machines can provide the right structure for a reasonably broad variety of tasks. Also they can have multiple memory space for return stacks (stack pointer is only 8-bit and can only point to internal RAM), but could have variables in internal or external RAM. Behind the scenes, the statement function_pointer = add_one takes the location in memory at which the function add_one is stored, and saves it in the function pointer. Time is a critical parameter in an embedded system. State machines are a method of modeling systems whose output depends on the entire history of their inputs, and not just on the most recent input. EE6602 – EMBEDDED SYSTEM Syllabus: UNIT III EMBEDDED FIRMWARE DEVELOPMENT ENVIRONMENT Embedded Product Development Life Cycle- objectives, different phases of EDLC, Modelling of EDLC; issues in Hardware-software Co-design, Data Flow Graph, state machine model, Sequential Program Model, concurrent Model, object oriented Model. Dr. Cockroach liked Nixie Pendulum Clock - Concept. And why can’t you pass the temperature as a function parameter? These files actually represent a demonstrative implementation of DoWhile using ‘switch-case’ sentences in C. Code Listing 4: Fragment of DoWhile implementation (Source: VortexMakes), Code Listing 5: Fragment of DoWhile specification (Source: VortexMakes). The transitions between states are taken care of inside the handler functions, and are as simple as setting the state variable to the next state name. You might want to split the state handlers off into a separate module, and that’s particularly easy given the above framework. Except compilers don’t inline calls made through function pointers. 1. Assuming you have or can add a preemptive multitasking kernel, you can run the graphics and sensor code in separate threads, and the sensor thread can preempt the graphics thread. FINITE STATE MACHINES Finite state machines are used to model system behavior in many types of engineering and scientific applications. Here are 2 articles on the subject of state machines via function pointers, both by the same author: This type of sequencial system is an abstraction for many Ask Hackaday: If Aliens Came By, Would We Even Notice? Here, we’re just interested in making your embedded coding life easier by presenting a few possible design patterns that have stood the test of time. Often implemented as one instruction (one or a few clock cycles), compare that to the jump/branch used in infinite loops — and, compare it to the rest of the instructions the program exectues. struct inputStruct inputs; Analog, Electronics PixJuan wrote a comment on Gameboy Clock. In each of the states the code controls the heater, the fan, and the LEDs, and potentially logs output over the serial port. Specifically, you try to break your problem down to a bunch of states. When I develop embedded code (most of my time) I follow these rules developed by NASA/JPL: http://spinroot.com/gerard/pdf/P10.pdf Since state machine models are widely used in embedded systems, this article explores several strategies to develop state machine (SM) software under the Test-Driven Development (TDD) approach. It sets the iteration number for zero to be executed by DoWhile loop. It’s a state engine that’s 1,500 lines long, and it will probably be 2,000 before I’m done. The number of loop iterations must be set through ‘x++’ or ‘x = (x > 0) ? State machine diagram is a UML diagram used to model the dynamic nature of a system. At the core of the TDD process, there is a repeating cycle composed of short steps known as “TDD microcycles”. A FINITE STATE MACHINE MODEL USED IN EMBEDDED SYSTEMS SOFTWARE DEVELOPMENT BY ANDREI STAN, NICOLAE BOTEZATU, LUCIAN PANDURU and ROBERT GABRIEL LUPU Abstract. Both of the strategies introduced above provide simple and ordered methods to develop state machine software using TDD — one of the most important approaches to increase software quality. I have recently implemented a stateful, multi level menu system using C++ object inheritance. Finally, the temperature updating routine is called outside of the state machine. Re: arguments. The first strategy consists mainly of verifying the state transition table of the SMUT. If the panic button is pressed, the soak temperature is reached, or the thermocouple isn’t working, the machine moves on to another state. {* signInEmailAddress *} We will activate the phase-lock-loop (PLL) for two reasons. Dominik Meffert has updated the project titled Continuous Inkjet Printing. Or the sensor thread/interrupt can wait for the release of the SPI bus, but then it will often run too late. So you must put the sensor on a separate SPI bus. “If in state A, do something, else if in state B, do something else”, etc. Poor design using function pointers is deadly to debug, but that goes for ANY poor design, and isn’t restricted to those using func pointers. Calling the function-pointer’s function happens _a lot_ in this style of code, and minimizing that overhead is probably worthwhile if you’re going for speed and minimal RAM/stack footprint. One is a LCD graphics display, which is rather slow, and operations like a full-screen scroll take up to 160ms. Similar idea to the function pointers, but with fewer globals. A transition from this state will show the first real state The final state of a state machine diagram is shown as concentric circles. Sounds scary, but it’s actually very well organized and readable. Hence, global variables and avoiding unnecessary if statements. I have always ran away from the VHDL and state machine diagrams but once I got used to drawing out the diagrams before trying to implement any code it was very satisfying to get something working. The DoWhile state machine (Source: VortexMakes). I have multiple subclasses of Menu, one for each menu screen. This makes a test case per state, in which the SMUT is stimulated by the events of interest to verify which transitions are triggered. Or you can pass a pointer to the struct in memory, but then the values inside the struct aren’t protected against abuse from inside a handler function any more, so they might as well be global, IMO. After defining two trivial functions and a variable to store the result in, we can finally point the function pointer at a function. i.e. Programming finite-state machines manually can become an overwhelming task and produce results that are convoluted and hard to maintain. Many applications from simple home appliances to complex communication systems implement event based state machines. If you don’t need too many frills you can do it easily in Graphviz. -There is more stack manipulation overhead to using function pointers State machines are an ideal paradigm for developing reactive systems. Please check your email and click on the link to verify your email address. handle_state_WAIT_FOR_INPUT() in the loop. At the very lowest level, everything is decomposed into a series of SPI pixel transfers, short enough that it can always exit within the required time, and handled by just a few key states. Because we’ve written the exact same code, a = function_pointer(7), and gotten different answers depending on which function is being pointed to. I won’t argue against the return-the-next-state version at all! It also helped to go to a single process clocked state machine. We didn't recognize that password reset code. A concrete example is worth a thousand words, so let’s start off with a perfect state machine application: controlling a small home appliance. But either only gives the sensor code control of the CPU. The test list derives from the specification and it defines the best vision of what it should be done. I discovered this beautiful piece of code recently. Most of it’s in a single C function, which is typical of protothreads; as they can’t exit quickly if they have to call any long-running functions. A nice thing about state-machine code is that you know exactly which sections of the code you have to modify when you’re adding a new state, and more importantly, which sections of code you don’t have to touch. Useful for NASA (don’t lose a rocket, please) and when developing concurrent systems, which often behave non-deterministically, making them hard to test. Without parameters to the function, it usually boils down to pushing the return address (a constant, or in this case something that propably already is in a register from the last return call) to the stack and jumping. Hope you enjoy. Since it does not need to be perfect, the former list is just a temporary document that could be modified later on. (That was fun to type!) Code Listing 1 shows a fragment of test_DoWhile.c file, which implements the test list above by applying the previously mentioned strategy. Should there be a call to Another C++ implementation, using member function pointers. While a state is a condition of a system or an element, a transition is a path from one state to another, usually initiated by an event of interest that connects a predecessor (source) state with a subsequent (target) state. If all the code looks and behaves the same way everywhere, it’s easy to find/fix it because its of the same basic form. (Comment Policy). If it is true, the loop body is evaluated again, and so on. When the termination condition becomes false, the loop terminates executing the ‘output = i’ action. I’m, however, not that sure about how useful it is for the quick hacks I assume most readers will use this knowledge for. Hard to understand but I think it is as minimal as it gets, for the comfort it provides. Simply put, no production code is written without first writing a failing unit test. This second strategy makes the test more functional and less abstract than the first one. A little more disconcerting is that the temperature variable, and every other argument to a state handler, will need to be declared global as it stands now, but let’s just roll with it for a moment. This is all tied together by a microcontroller that’s got four user pushbuttons plus reset, two LEDs, a TTL serial in/out, and a potentiometer connected to an ADC channel just for good measure. A finite state machine is one of the most popular design patterns in embedded systems. Then the state is contained on the stack and not in the global space. In other words, the handler function really _is_ the state. Code Listing 2: Multiple iteration test (Source: VortexMakes). At the same time, it checks that the arguments and return value of the add_one function match those of the function pointer, to keep us honest. Two files represent this state machine, DoWhile.h and DoWhile.c, so they are the source code under test. Figure 1. Yet, state machines are perhaps the most effective method for developing robust event-driven code because they give us the map to the software maze. This publication starts by explaining basic state machine concepts and the TDD technique. This test verifies that DoWhile can execute just one iteration correctly. After that, the DoWhile is ready to process events by calling DoWhile_dispatch(). Or you can put the sensor code in an interrupt. Sequential programs vs. C. 7. So a function pointer contains the memory address of some code to run, and also must be prototyped the same way as the functions it can point to. The other strategy proposes to realize a test case for a complete and functional scenario, which frequently involves a set of states, events and actions of the SMUT. Of course depends on examples but if you use descriptive names and don’t use it to excess it makes it more readable, even if you do or don’t know the codebase. Dr. Cockroach wrote a comment on Nixie Pendulum Clock - Concept. Not having to fool around with the state enum is one less thing to worry about. Once it receives the Alpha event, the state machine transitions to the StateB state by executing the xStateA(), effect() and nStateB() actions in that order. I completely agree with pelrun. This method makes a test case per state. If the overall topic/idea is “alternatives to OSes for embedded systems”, I see protothreads and state machines as two entirely different sub-topics. int (*currentState)(struct inputStruct, struct* outputStruct, struct* stateVector); while (1) { Japan. One of the big fears with globals is that you will make confusing spaghetti code by sneaking data from one function to another. … ‘ work then, given we’re passing global state around? A great solution for that case is to pass structs to the handlers that contain all of the relevant arguments, but that’s a tale for another time. It turns out that roasting coffee is a lot like doing reflow soldering: you warm everything up, then ramp up the temperature at a controlled rate to a desired maximum, and then cool it back down again. The EMERGENCY_SHUTDOWN state was added late in the firmware’s life, and it’s worth looking at where you need to touch the code to add or remove a state. Thank you for verifiying your email address. over-?) There are probably as many ways to implement a state machine a… Jonathan Valvano and Ramesh Yerraballi . If an action is sufficiently complex, it is more suitable to make a specific test case for that. Sign In. Hackaday’s own Al Williams recently wrote an article covering state machines in these terms, and his article is a great complement to this one. model, state machine model • Certain languages better at capturing certain models Models Languages Recipe English Spanish Japanese Poetry Story Sequent. This is a fantastic document that I would recommend to everyone working on embedded systems. Whoah! Thanks again for the series. Almost all computer systems in general, and embedded systems in particular, are driven by events. The concept of state machines is very simple and powerful. Figure 2 shows a state machine called DoWhile, which models an execution loop similar to ‘do-while’ one. In fact, you are already using them when you use an interrupt handle table. Code Listing 3: Non iteration test (Source: VortexMakes). Their rationale is that they want to run a model checker against it, effectively trying every combination of scenarios that the program may execute. Advisor, EE Times If every state is proven to be secure, the system is proven to be secure. The other is that, for applications like automatic code generation and automatic FPGA synthesis, there are sophisticated tools that transform your description of the state machine into a finished product without you getting your hands dirty with any of the actual coding. The INIT state simply does its work and passes on to the next state. It’s not the be-all, end-all approach to state machines, but it is another (somewhat more object oriented) model which can work in some cases. I *GREATLY* prefer to have something like do_state = do_state(), since that makes the update explicit. Explanation: The State machine model is used for modeling reactive or event-driven embedded systems whose processing behavior depended on the state transition. program C C++ Java State machine ... – Support for all concepts found in embedded systems • Orthogonality – Orthogonal constructs for orthogonal concepts – Minimality • Simplicity Since Eliot doesn’t seem to care about stack space, what is even worse is he is using a global variable to hold the state. When the CPU and SPI bus are idle, the engine runs, and processes the queue. {| create_button |}, Developing state machines with test-driven development, https://github.com/leanfrancucci/sm-tdd.git. Compared to purely functional systems, in which the output is purely determined by the input, state machines have a performance that is determined by its history. A little more disconcerting is that the temperature variable, and every other argument to a state handler, will need to be declared global as it stands now, but let’s just roll with it for a moment.”. Waiting for the graphics code to complete its current operation is therefore not an option. It’s the same as any other program where you have data that lots of parts of the program need to use. WAIT_FOR_INPUT doesn’t change the state variable unless a button is pressed, so the microcontroller spins around here waiting. Style-wise, it’s nice to prototype the functions that get pointed to and the function pointer that will point to them all in the same place because that makes it easy to spot any inconsistencies, which the compiler will thankfully also warn you about. State machine diagrams can also show how an entity responds to various events by changing from one state to another. Now, instead of storing the state’s name in a variable and then calling different functions based on the name, we can just store the functions themselves in the state variable. Certainly worth checking http://www.beningo.com/tools-open-source-circular-buffers/, Please be kind and respectful to help make the comments section excellent. They’re ok, but state machines are absolutely on our short list of favorite things. This is completely unnecessary as the function could simply return a pointer to the next state! Is it? Specifically, you try to break your problem down to …