Advanced Memory Optimization Techniques for Low-Power Embedded Processors

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CS 8803 Special Topics: Embedded Software
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These modules can be manufactured in high volume, by organizations familiar with their specialized testing issues, and combined with much lower volume custom mainboards with application-specific external peripherals. Implementation of embedded systems has advanced so that they can easily be implemented with already-made boards that are based on worldwide accepted platforms. These platforms include, but are not limited to, Arduino and Raspberry Pi. A common array for very-high-volume embedded systems is the system on a chip SoC that contains a complete system consisting of multiple processors, multipliers, caches and interfaces on a single chip.

Embedded systems talk with the outside world via peripherals , such as:. As with other software, embedded system designers use compilers , assemblers , and debuggers to develop embedded system software. However, they may also use some more specific tools:. As the complexity of embedded systems grows, higher level tools and operating systems are migrating into machinery where it makes sense. For example, cellphones , personal digital assistants and other consumer computers often need significant software that is purchased or provided by a person other than the manufacturer of the electronics.

Embedded systems are commonly found in consumer, cooking, industrial, automotive, medical applications. Household appliances, such as microwave ovens, washing machines and dishwashers, include embedded systems to provide flexibility and efficiency. Embedded debugging may be performed at different levels, depending on the facilities available.

The different metrics that characterize the different forms of embedded debugging are: does it slow down the main application, how close is the debugged system or application to the actual system or application, how expressive are the triggers that can be set for debugging e. Unless restricted to external debugging, the programmer can typically load and run software through the tools, view the code running in the processor, and start or stop its operation.

The view of the code may be as HLL source-code , assembly code or mixture of both. Because an embedded system is often composed of a wide variety of elements, the debugging strategy may vary. For instance, debugging a software- and microprocessor- centric embedded system is different from debugging an embedded system where most of the processing is performed by peripherals DSP, FPGA, and co-processor.

An increasing number of embedded systems today use more than one single processor core. A common problem with multi-core development is the proper synchronization of software execution. Real-time operating systems RTOS often supports tracing of operating system events. A graphical view is presented by a host PC tool, based on a recording of the system behavior. The trace recording can be performed in software, by the RTOS, or by special tracing hardware. RTOS tracing allows developers to understand timing and performance issues of the software system and gives a good understanding of the high-level system behaviors.

Embedded systems often reside in machines that are expected to run continuously for years without errors, and in some cases recover by themselves if an error occurs. Therefore, the software is usually developed and tested more carefully than that for personal computers, and unreliable mechanical moving parts such as disk drives, switches or buttons are avoided.

A variety of techniques are used, sometimes in combination, to recover from errors—both software bugs such as memory leaks , and also soft errors in the hardware:. For high volume systems such as portable music players or mobile phones , minimizing cost is usually the primary design consideration. For low-volume or prototype embedded systems, general purpose computers may be adapted by limiting the programs or by replacing the operating system with a real-time operating system.

In this design, the software simply has a loop. The loop calls subroutines , each of which manages a part of the hardware or software.

Advanced Power Management in Embedded Memory Subsystems

Hence it is called a simple control loop or control loop. Some embedded systems are predominantly controlled by interrupts. This means that tasks performed by the system are triggered by different kinds of events; an interrupt could be generated, for example, by a timer in a predefined frequency, or by a serial port controller receiving a byte. These kinds of systems are used if event handlers need low latency, and the event handlers are short and simple. Usually, these kinds of systems run a simple task in a main loop also, but this task is not very sensitive to unexpected delays. Sometimes the interrupt handler will add longer tasks to a queue structure.

Later, after the interrupt handler has finished, these tasks are executed by the main loop. This method brings the system close to a multitasking kernel with discrete processes. A nonpreemptive multitasking system is very similar to the simple control loop scheme, except that the loop is hidden in an API.

The advantages and disadvantages are similar to that of the control loop, except that adding new software is easier, by simply writing a new task, or adding to the queue. In this type of system, a low-level piece of code switches between tasks or threads based on a timer connected to an interrupt. This is the level at which the system is generally considered to have an "operating system" kernel.

Depending on how much functionality is required, it introduces more or less of the complexities of managing multiple tasks running conceptually in parallel. As any code can potentially damage the data of another task except in larger systems using an MMU programs must be carefully designed and tested, and access to shared data must be controlled by some synchronization strategy, such as message queues , semaphores or a non-blocking synchronization scheme.

Because of these complexities, it is common for organizations to use a real-time operating system RTOS , allowing the application programmers to concentrate on device functionality rather than operating system services, at least for large systems; smaller systems often cannot afford the overhead associated with a generic real-time system, due to limitations regarding memory size, performance, or battery life.

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  • The choice that an RTOS is required brings in its own issues, however, as the selection must be done prior to starting to the application development process. This timing forces developers to choose the embedded operating system for their device based upon current requirements and so restricts future options to a large extent. These trends are leading to the uptake of embedded middleware in addition to a real-time operating system.

    A microkernel is a logical step up from a real-time OS. The usual arrangement is that the operating system kernel allocates memory and switches the CPU to different threads of execution. User mode processes implement major functions such as file systems, network interfaces, etc. In general, microkernels succeed when the task switching and intertask communication is fast and fail when they are slow.

    Advanced Memory Optimization Techniques for Low Power Embedded Processors

    Exokernels communicate efficiently by normal subroutine calls. The hardware and all the software in the system are available to and extensible by application programmers. In this case, a relatively large kernel with sophisticated capabilities is adapted to suit an embedded environment. This gives programmers an environment similar to a desktop operating system like Linux or Microsoft Windows , and is therefore very productive for development; on the downside, it requires considerably more hardware resources, is often more expensive, and, because of the complexity of these kernels, can be less predictable and reliable.

    Despite the increased cost in hardware, this type of embedded system is increasing in popularity, especially on the more powerful embedded devices such as wireless routers and GPS navigation systems. Here are some of the reasons:. In addition to the core operating system, many embedded systems have additional upper-layer software components. If the embedded device has audio and video capabilities, then the appropriate drivers and codecs will be present in the system. In the case of the monolithic kernels, many of these software layers are included.

    In the RTOS category, the availability of the additional software components depends upon the commercial offering. From Wikipedia, the free encyclopedia. Main article: Embedded software. Electronics portal. Neutrino Technical Library. Retrieved Embedded systems design. EDN series for design engineers 2 ed. An embedded system is a microprocessor based system that is built to control a function or a range of functions. Massa Programming embedded systems: with C and GNU development tools.

    Embedded Systems Design. TechInsights United Business Media. Electronic Frontier Foundation. Retrieved on Alippi: Intelligence for Embedded Systems. SenSys ' Electronic Engineering Journal. IPSN ' Prove it! Retrieved 2 February Computer sizes. Classes of computers. Ultra-mobile PC 2-in-1 Phablet Tabletop. Scientific Programmable Graphing. Digital wristwatch Calculator watch Smartwatch Smartglasses Smart ring. Server Minicomputer Supermini.

    Super Mainframe Minisuper. Microcontroller Nanocomputer Single-board computer Smartdust Wireless sensor network.

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    Computer science. Computer architecture Embedded system Real-time computing Dependability. Network architecture Network protocol Network components Network scheduler Network performance evaluation Network service. For maximum power efficiency, however, this tool requires that we design our software carefully. We have all written code that polls a status register and waits until a flag is set. Perhaps we're checking a FIFO status flag in a serial port to see if data has been received.

    Or maybe we're monitoring a dual-ported memory location to see if another processor or device in the system has written a variable, giving us control of a shared resource. While seemingly benign, polling a register in a loop represents a missed opportunity to extend battery life on handhelds. The better solution is to use an external interrupt to signal when the status flag has changed.

    In a single-threaded software environment, you can then invoke the processor's idle mode to reduce power consumption until the actual event occurrs. When the interrupt occurs, the processor automatically wakes up and continues executing your code. Idle mode can even be used in cases where the event cannot be directly tied to an external interrupt.

    In these situations, using a system timer to periodically wake the processor is still preferable to polling. For instance, if you are waiting for an event and know you can process it quickly enough as long as you check its status every millisecond, enable a 1ms timer and place the processor into idle mode. Check the event's status every time the interrupt fires; if the status hasn't changed, you can return to idle mode immediately. This type of waiting mechanism is very common. The vast majority of today's PDAs and smart phones are powered by processors and operating systems that have idle-mode capabilities.

    In fact, most of these devices hop into and out of idle many times per second, awakened whenever a touchscreen tap, keypress, or timeout occurs. Another technique to consider is event reduction. Whereas intelligent waiting enables the processor to enter its idle mode as often as possible, event reduction attempts to keep the processor in idle as long as possible. It is implemented by analyzing your code and system requirements to determine if you can alter the way you process interrupts.

    For example, if you are working with a multitasking operating system that uses time-slicing to schedule threads, the operating system will typically set a timer interrupt to occur at the slice interval, which is often as small as 1ms. Assuming your code makes good use of intelligent waiting techniques, the operating system will frequently find opportunities to place the processor into idle mode, where it stays until it's awakened by an interrupt.

    Of course, in this scenario, the interrupt most likely to awaken the processor is the timer interrupt itself. Even if all other threads are blocked-pending other interrupts, pending internal events, pending long delays-the timer interrupt will wake the processor from idle mode 1, times every second to run the scheduler. Even if the scheduler determines that all threads are blocked and quickly returns the processor to idle mode, this frequent operation can waste considerable power.

    In these situations, the time-slice interrupt should be disabled when idle mode is entered, waking only when another interrupt occurs. Of course, it is usually inappropriate to disable the time-slice interrupt altogether. While most blocked threads may be waiting-directly or indirectly-on external interrupts, some may have yielded to the operating system for a specific time period.

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    A driver, for instance, might sleep for ms while waiting for a peripheral. In this case, completely disabling the system timer on idle might mean the thread doesn't resume execution on time. Ideally, your operating system should be able to set variable timeouts for its scheduler. The operating system knows whether each thread is waiting indefinitely for an external or internal event or is scheduled to run again at a specific time. The operating system can then calculate when the first thread is scheduled to run and set the timer to fire accordingly before placing the processor in idle mode.

    Variable timeouts do not impose a significant burden on the scheduler and can save both power and processing time. But variable scheduling timeouts are just one means of reducing events. Direct memory access DMA allows the processor to remain in idle mode for significant periods even while data is being sent to or received from peripherals.

    DMA should be used in peripheral drivers whenever possible. The savings can be quite impressive. At , bits-per-second, an 11KB burst of data sent to this port would cause the processor core to be interrupted-and possibly awakened from idle mode-almost 1, times in one second. If you don't actually need to process data in these small, 8-byte chunks, the waste is tremendous. Ideally, DMA would be used with larger buffer sizes, causing interrupts to occur at a much more manageable level-perhaps 10 or times per second-allowing the processor to idle in between. Dynamic clock and voltage adjustments represent the cutting edge of power reduction capabilities in microcontrollers.

    This advance in power management is based on the following observation: the energy consumed by a processor is directly proportional to the clock frequency driving it and to the square of the voltage applied to its core. Processors that allow dynamic reductions in clock speed provide a first step toward power savings: cut the clock speed in half and the power consumption drops proportionately.

    It's tricky, however, to implement effective strategies using this technique alone since the code being executed may take twice as long to complete. In that case, no power may be saved. Dynamic voltage reduction is another story.

    Advanced Memory Optimization Techniques for Low-Power Embedded Processors

    An increasing number of processors allow voltage to be dropped in concert with a drop in processor clock speed, resulting in a power savings even in cases when a clock-speed reduction alone offers no advantage. In fact, as long as the processor does not saturate, the frequency and voltage can be continually reduced. In this way, work is still being completed, but the energy consumed is lower overall. Even these approaches can be improved upon by considering that not all threads are equally productive consumers of processor bandwidth.

    Threads that are efficient users of processor bandwidth will take longer to complete as the processor's clock speed is dropped; these threads make use of every cycle allocated to them. When data is written to a flash memory card, the bottleneck in the system is not the speed of the processor but the physical bus interface and the time the card's firmware takes to erase and reprogram flash. Ideally, the intelligent waiting techniques discussed above would be used to minimize power consumption in this case, but wait times are often highly variable and much smaller than the operating system's time quantum.

    As a result, intelligent waiting would injure performance, so these drivers often resort to polling status registers; reducing clock speed in these cases would conserve power, but it would have negligible impact on the time required to write data to the card most of the processor cycles used are used for polling. The challenge, of course, is knowing when it's possible to decrease clock frequency and voltage without noticeably affecting performance. As a software developer, it's unwieldy to have to consider when it's appropriate to drop clock speed in your driver and application code, and this technique becomes even trickier in multitasking environments.

    So far, we've discussed only what to do when the device is running; now, let's consider what happens when it is turned off. Most of us take for granted that we can turn on our PDA and have it pick right up where it was when we last used it; if we were in the middle of entering a new contact, that's where we'll be when we turn it back on a week or month later. This is accomplished with an intelligent shutdown procedure that effectively tricks any executing application software into thinking the device was never turned off at all.

    When the user turns the device off by pressing the power button, an interrupt signals the operating system to begin a graceful shutdown that includes saving the context of the lowest-level registers in the system. The operating system does not actually shut programs down, but leaves their contents code, stack, heap, static data in memory.

    Advanced Memory Optimization Techniques for Low-Power Embedded Processors Advanced Memory Optimization Techniques for Low-Power Embedded Processors
    Advanced Memory Optimization Techniques for Low-Power Embedded Processors Advanced Memory Optimization Techniques for Low-Power Embedded Processors
    Advanced Memory Optimization Techniques for Low-Power Embedded Processors Advanced Memory Optimization Techniques for Low-Power Embedded Processors
    Advanced Memory Optimization Techniques for Low-Power Embedded Processors Advanced Memory Optimization Techniques for Low-Power Embedded Processors
    Advanced Memory Optimization Techniques for Low-Power Embedded Processors Advanced Memory Optimization Techniques for Low-Power Embedded Processors
    Advanced Memory Optimization Techniques for Low-Power Embedded Processors Advanced Memory Optimization Techniques for Low-Power Embedded Processors

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