Embedded system

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An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electrical system. Embedded system_sentence_0

It is embedded as part of a complete device often including electrical or electronic hardware and mechanical parts. Embedded system_sentence_1

Because an embedded system typically controls physical operations of the machine that it is embedded within, it often has real-time computing constraints. Embedded system_sentence_2

Embedded systems control many devices in common use today. Embedded system_sentence_3

Ninety-eight percent of all microprocessors manufactured are used in embedded systems. Embedded system_sentence_4

Modern embedded systems are often based on microcontrollers (i.e. microprocessors with integrated memory and peripheral interfaces), but ordinary microprocessors (using external chips for memory and peripheral interface circuits) are also common, especially in more complex systems. Embedded system_sentence_5

In either case, the processor(s) used may be types ranging from general purpose to those specialized in a certain class of computations, or even custom designed for the application at hand. Embedded system_sentence_6

A common standard class of dedicated processors is the digital signal processor (DSP). Embedded system_sentence_7

Since the embedded system is dedicated to specific tasks, design engineers can optimize it to reduce the size and cost of the product and increase the reliability and performance. Embedded system_sentence_8

Some embedded systems are mass-produced, benefiting from economies of scale. Embedded system_sentence_9

Embedded systems range from portable devices such as digital watches and MP3 players, to large stationary installations like traffic light controllers, programmable logic controllers, and large complex systems like hybrid vehicles, medical imaging systems, and avionics. Embedded system_sentence_10

Complexity varies from low, with a single microcontroller chip, to very high with multiple units, peripherals and networks mounted inside a large equipment rack. Embedded system_sentence_11

History Embedded system_section_0

Background Embedded system_section_1

See also: Microprocessor chronology Embedded system_sentence_12

The origins of the microprocessor and the microcontroller can be traced back to the MOS integrated circuit, which is an integrated circuit chip fabricated from MOSFETs (metal-oxide-semiconductor field-effect transistors) and was developed in the early 1960s. Embedded system_sentence_13

By 1964, MOS chips had reached higher transistor density and lower manufacturing costs than bipolar chips. Embedded system_sentence_14

MOS chips further increased in complexity at a rate predicted by Moore's law, leading to large-scale integration (LSI) with hundreds of transistors on a single MOS chip by the late 1960s. Embedded system_sentence_15

The application of MOS LSI chips to computing was the basis for the first microprocessors, as engineers began recognizing that a complete computer processor system could be contained on several MOS LSI chips. Embedded system_sentence_16

The first multi-chip microprocessors, the Four-Phase Systems AL1 in 1969 and the Garrett AiResearch MP944 in 1970, were developed with multiple MOS LSI chips. Embedded system_sentence_17

The first single-chip microprocessor was the Intel 4004, released in 1971. Embedded system_sentence_18

It was developed by Federico Faggin, using his silicon-gate MOS technology, along with Intel engineers Marcian Hoff and Stan Mazor, and Busicom engineer Masatoshi Shima. Embedded system_sentence_19

Development Embedded system_section_2

One of the first recognizably modern embedded systems was the Apollo Guidance Computer, developed ca. 1965 by Charles Stark Draper at the MIT Instrumentation Laboratory. Embedded system_sentence_20

At the project's inception, the Apollo guidance computer was considered the riskiest item in the Apollo project as it employed the then newly developed monolithic integrated circuits to reduce the computer's size and weight. Embedded system_sentence_21

An early mass-produced embedded system was the Autonetics D-17 guidance computer for the Minuteman missile, released in 1961. Embedded system_sentence_22

When the Minuteman II went into production in 1966, the D-17 was replaced with a new computer that represented the first high-volume use of integrated circuits. Embedded system_sentence_23

Since these early applications in the 1960s, embedded systems have come down in price and there has been a dramatic rise in processing power and functionality. Embedded system_sentence_24

An early microprocessor, the Intel 4004 (released in 1971), was designed for calculators and other small systems but still required external memory and support chips. Embedded system_sentence_25

By the early 1980s, memory, input and output system components had been integrated into the same chip as the processor forming a microcontroller. Embedded system_sentence_26

Microcontrollers find applications where a general-purpose computer would be too costly. Embedded system_sentence_27

As the cost of microprocessors and microcontrollers fell the prevalence of embedded systems increased. Embedded system_sentence_28

Today, a comparatively low-cost microcontroller may be programmed to fulfill the same role as a large number of separate components. Embedded system_sentence_29

With microcontrollers, it became feasible to replace, even in consumer products, expensive knob-based analog components such as potentiometers and variable capacitors with up/down buttons or knobs read out by a microprocessor. Embedded system_sentence_30

Although in this context an embedded system is usually more complex than a traditional solution, most of the complexity is contained within the microcontroller itself. Embedded system_sentence_31

Very few additional components may be needed and most of the design effort is in the software. Embedded system_sentence_32

Software prototype and test can be quicker compared with the design and construction of a new circuit not using an embedded processor. Embedded system_sentence_33

Applications Embedded system_section_3

Embedded systems are commonly found in consumer, industrial, automotive, home appliances, medical, telecommunication, commercial and military applications. Embedded system_sentence_34

Telecommunications systems employ numerous embedded systems from telephone switches for the network to cell phones at the end user. Embedded system_sentence_35

Computer networking uses dedicated routers and network bridges to route data. Embedded system_sentence_36

Consumer electronics include MP3 players, television sets, mobile phones, video game consoles, digital cameras, GPS receivers, and printers. Embedded system_sentence_37

Household appliances, such as microwave ovens, washing machines and dishwashers, include embedded systems to provide flexibility, efficiency and features. Embedded system_sentence_38

Advanced HVAC systems use networked thermostats to more accurately and efficiently control temperature that can change by time of day and season. Embedded system_sentence_39

Home automation uses wired- and wireless-networking that can be used to control lights, climate, security, audio/visual, surveillance, etc., all of which use embedded devices for sensing and controlling. Embedded system_sentence_40

Transportation systems from flight to automobiles increasingly use embedded systems. Embedded system_sentence_41

New airplanes contain advanced avionics such as inertial guidance systems and GPS receivers that also have considerable safety requirements. Embedded system_sentence_42

Various electric motors — brushless DC motors, induction motors and DC motors — use electronic motor controllers. Embedded system_sentence_43

Automobiles, electric vehicles, and hybrid vehicles increasingly use embedded systems to maximize efficiency and reduce pollution. Embedded system_sentence_44

Other automotive safety systems using embedded systems include anti-lock braking system (ABS), Electronic Stability Control (ESC/ESP), traction control (TCS) and automatic four-wheel drive. Embedded system_sentence_45

Medical equipment uses embedded systems for monitoring, and various medical imaging (PET, SPECT, CT, and MRI) for non-invasive internal inspections. Embedded system_sentence_46

Embedded systems within medical equipment are often powered by industrial computers. Embedded system_sentence_47

Embedded systems are used safety-critical systems. Embedded system_sentence_48

Unless connected to wired or wireless networks via on-chip 3G cellular or other methods for IoT monitoring and control purposes, these systems can be isolated from hacking and thus be more secure. Embedded system_sentence_49

For fire safety, the systems can be designed to have a greater ability to handle higher temperatures and continue to operate. Embedded system_sentence_50

In dealing with security, the embedded systems can be self-sufficient and be able to deal with cut electrical and communication systems. Embedded system_sentence_51

Miniature wireless devices called motes are networked wireless sensors. Embedded system_sentence_52

Wireless sensor networking makes use of miniaturization made possible by advanced IC design to couple full wireless subsystems to sophisticated sensors, enabling people and companies to measure a myriad of things in the physical world and act on this information through monitoring and control systems. Embedded system_sentence_53

These motes are completely self-contained and will typically run off a battery source for years before the batteries need to be changed or charged. Embedded system_sentence_54

Characteristics Embedded system_section_4

Embedded systems are designed to do some specific task, rather than be a general-purpose computer for multiple tasks. Embedded system_sentence_55

Some also have real-time performance constraints that must be met, for reasons such as safety and usability; others may have low or no performance requirements, allowing the system hardware to be simplified to reduce costs. Embedded system_sentence_56

Embedded systems are not always standalone devices. Embedded system_sentence_57

Many embedded systems consist of small parts within a larger device that serves a more general purpose. Embedded system_sentence_58

For example, the Gibson Robot Guitar features an embedded system for tuning the strings, but the overall purpose of the Robot Guitar is, of course, to play music. Embedded system_sentence_59

Similarly, an embedded system in an automobile provides a specific function as a subsystem of the car itself. Embedded system_sentence_60

The program instructions written for embedded systems are referred to as firmware, and are stored in read-only memory or flash memory chips. Embedded system_sentence_61

They run with limited computer hardware resources: little memory, small or non-existent keyboard or screen. Embedded system_sentence_62

User interface Embedded system_section_5

Embedded systems range from no user interface at all, in systems dedicated only to one task, to complex graphical user interfaces that resemble modern computer desktop operating systems. Embedded system_sentence_63

Simple embedded devices use buttons, LEDs, graphic or character LCDs (HD44780 LCD for example) with a simple menu system. Embedded system_sentence_64

More sophisticated devices that use a graphical screen with touch sensing or screen-edge buttons provide flexibility while minimizing space used: the meaning of the buttons can change with the screen, and selection involves the natural behavior of pointing at what is desired. Embedded system_sentence_65

Handheld systems often have a screen with a "joystick button" for a pointing device. Embedded system_sentence_66

Some systems provide user interface remotely with the help of a serial (e.g. RS-232, USB, I²C, etc.) or network (e.g. Ethernet) connection. Embedded system_sentence_67

This approach gives several advantages: extends the capabilities of embedded system, avoids the cost of a display, simplifies BSP and allows one to build a rich user interface on the PC. Embedded system_sentence_68

A good example of this is the combination of an embedded web server running on an embedded device (such as an IP camera) or a network router. Embedded system_sentence_69

The user interface is displayed in a web browser on a PC connected to the device, therefore needing no software to be installed. Embedded system_sentence_70

Processors in embedded systems Embedded system_section_6

Examples of properties of typical embedded computers, when compared with general-purpose counterparts, are low power consumption, small size, rugged operating ranges, and low per-unit cost. Embedded system_sentence_71

This comes at the price of limited processing resources, which make them significantly more difficult to program and to interact with. Embedded system_sentence_72

However, by building intelligence mechanisms on top of the hardware, taking advantage of possible existing sensors and the existence of a network of embedded units, one can both optimally manage available resources at the unit and network levels as well as provide augmented functions, well beyond those available. Embedded system_sentence_73

For example, intelligent techniques can be designed to manage power consumption of embedded systems. Embedded system_sentence_74

Embedded processors can be broken into two broad categories. Embedded system_sentence_75

Ordinary microprocessors (μP) use separate integrated circuits for memory and peripherals. Embedded system_sentence_76

Microcontrollers (μC) have on-chip peripherals, thus reducing power consumption, size and cost. Embedded system_sentence_77

In contrast to the personal computer market, many different basic CPU architectures are used since the software is custom-developed for an application and is not a commodity product installed by the end user. Embedded system_sentence_78

Both Von Neumann, as well as various degrees of Harvard architectures, are used. Embedded system_sentence_79

RISC as well as non-RISC processors are found. Embedded system_sentence_80

Word lengths vary from 4-bit to 64-bits and beyond, although the most typical remain 8/16-bit. Embedded system_sentence_81

Most architectures come in a large number of different variants and shapes, many of which are also manufactured by several different companies. Embedded system_sentence_82

Numerous microcontrollers have been developed for embedded systems use. Embedded system_sentence_83

General-purpose microprocessors are also used in embedded systems, but generally, require more support circuitry than microcontrollers. Embedded system_sentence_84

Ready-made computer boards Embedded system_section_7

PC/104 and PC/104+ are examples of standards for ready-made computer boards intended for small, low-volume embedded and ruggedized systems, mostly x86-based. Embedded system_sentence_85

These are often physically small compared to a standard PC, although still quite large compared to most simple (8/16-bit) embedded systems. Embedded system_sentence_86

They often use DOS, Linux, NetBSD, or an embedded real-time operating system such as MicroC/OS-II, QNX or VxWorks. Embedded system_sentence_87

Sometimes these boards use non-x86 processors. Embedded system_sentence_88

In certain applications, where small size or power efficiency are not primary concerns, the components used may be compatible with those used in general-purpose x86 personal computers. Embedded system_sentence_89

Boards such as the VIA EPIA range help to bridge the gap by being PC-compatible but highly integrated, physically smaller or have other attributes making them attractive to embedded engineers. Embedded system_sentence_90

The advantage of this approach is that low-cost commodity components may be used along with the same software development tools used for general software development. Embedded system_sentence_91

Systems built in this way are still regarded as embedded since they are integrated into larger devices and fulfill a single role. Embedded system_sentence_92

Examples of devices that may adopt this approach are ATMs and arcade machines, which contain code specific to the application. Embedded system_sentence_93

However, most ready-made embedded systems boards are not PC-centered and do not use the ISA or PCI busses. Embedded system_sentence_94

When a system-on-a-chip processor is involved, there may be little benefit to having a standardized bus connecting discrete components, and the environment for both hardware and software tools may be very different. Embedded system_sentence_95

One common design style uses a small system module, perhaps the size of a business card, holding high density BGA chips such as an ARM-based system-on-a-chip processor and peripherals, external flash memory for storage, and DRAM for runtime memory. Embedded system_sentence_96

The module vendor will usually provide boot software and make sure there is a selection of operating systems, usually including Linux and some real-time choices. Embedded system_sentence_97

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. Embedded system_sentence_98

Implementation of embedded systems has advanced so that they can easily be implemented with already-made boards that are based on worldwide accepted platforms. Embedded system_sentence_99

These platforms include, but are not limited to, Arduino and Raspberry Pi. Embedded system_sentence_100

ASIC and FPGA solutions Embedded system_section_8

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 system_sentence_101

SoCs can be implemented as an application-specific integrated circuit (ASIC) or using a field-programmable gate array (FPGA). Embedded system_sentence_102

Peripherals Embedded system_section_9

Embedded systems talk with the outside world via peripherals, such as: Embedded system_sentence_103

Embedded system_unordered_list_0

Tools Embedded system_section_10

As with other software, embedded system designers use compilers, assemblers, and debuggers to develop embedded system software. Embedded system_sentence_104

However, they may also use some more specific tools: Embedded system_sentence_105

Embedded system_unordered_list_1

  • In circuit debuggers or emulators (see next section).Embedded system_item_1_10
  • Utilities to add a checksum or CRC to a program, so the embedded system can check if the program is valid.Embedded system_item_1_11
  • For systems using digital signal processing, developers may use a math workbench to simulate the mathematics.Embedded system_item_1_12
  • System-level modeling and simulation tools help designers to construct simulation models of a system with hardware components such as processors, memories, DMA, interfaces, buses and software behavior flow as a state diagram or flow diagram using configurable library blocks. Simulation is conducted to select the right components by performing power vs. performance trade-off, reliability analysis and bottleneck analysis. Typical reports that help a designer to make architecture decisions includes application latency, device throughput, device utilization, power consumption of the full system as well as device-level power consumption.Embedded system_item_1_13
  • A model-based development tool creates and simulates graphical data flow and UML state chart diagrams of components like digital filters, motor controllers, communication protocol decoding and multi-rate tasks.Embedded system_item_1_14
  • Custom compilers and linkers may be used to optimize specialized hardware.Embedded system_item_1_15
  • An embedded system may have its own special language or design tool, or add enhancements to an existing language such as Forth or Basic.Embedded system_item_1_16
  • Another alternative is to add a real-time operating system or embedded operating systemEmbedded system_item_1_17
  • Modeling and code generating tools often based on state machinesEmbedded system_item_1_18

Software tools can come from several sources: Embedded system_sentence_106

Embedded system_unordered_list_2

  • Software companies that specialize in the embedded marketEmbedded system_item_2_19
  • Ported from the GNU software development toolsEmbedded system_item_2_20
  • Sometimes, development tools for a personal computer can be used if the embedded processor is a close relative to a common PC processorEmbedded system_item_2_21

As the complexity of embedded systems grows, higher-level tools and operating systems are migrating into machinery where it makes sense. Embedded system_sentence_107

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 system_sentence_108

In these systems, an open programming environment such as Linux, NetBSD, OSGi or Embedded Java is required so that the third-party software provider can sell to a large market. Embedded system_sentence_109

Embedded systems are commonly found in consumer, cooking, industrial, automotive, and medical applications. Embedded system_sentence_110

Some examples of embedded systems are MP3 players, mobile phones, video game consoles, digital cameras, DVD players, and GPS. Embedded system_sentence_111

Household appliances, such as microwave ovens, washing machines and dishwashers, include embedded systems to provide flexibility and efficiency. Embedded system_sentence_112

Debugging Embedded system_section_11

Embedded debugging may be performed at different levels, depending on the facilities available. Embedded system_sentence_113

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.g., inspecting the memory when a particular program counter value is reached), and what can be inspected in the debugging process (such as, only memory, or memory and registers, etc.). Embedded system_sentence_114

From simplest to most sophisticated they can be roughly grouped into the following areas: Embedded system_sentence_115

Embedded system_unordered_list_3

  • Interactive resident debugging, using the simple shell provided by the embedded operating system (e.g. Forth and Basic)Embedded system_item_3_22
  • External debugging using logging or serial port output to trace operation using either a monitor in flash or using a debug server like the Remedy Debugger that even works for heterogeneous multicore systems.Embedded system_item_3_23
  • An in-circuit debugger (ICD), a hardware device that connects to the microprocessor via a JTAG or Nexus interface. This allows the operation of the microprocessor to be controlled externally, but is typically restricted to specific debugging capabilities in the processor.Embedded system_item_3_24
  • An in-circuit emulator (ICE) replaces the microprocessor with a simulated equivalent, providing full control over all aspects of the microprocessor.Embedded system_item_3_25
  • A complete emulator provides a simulation of all aspects of the hardware, allowing all of it to be controlled and modified, and allowing debugging on a normal PC. The downsides are expense and slow operation, in some cases up to 100 times slower than the final system.Embedded system_item_3_26
  • For SoC designs, the typical approach is to verify and debug the design on an FPGA prototype board. Tools such as Certus are used to insert probes in the FPGA RTL that make signals available for observation. This is used to debug hardware, firmware and software interactions across multiple FPGA with capabilities similar to a logic analyzer.Embedded system_item_3_27
  • Software-only debuggers have the benefit that they do not need any hardware modification but have to carefully control what they record in order to conserve time and storage space.Embedded system_item_3_28

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. Embedded system_sentence_116

The view of the code may be as HLL source-code, assembly code or mixture of both. Embedded system_sentence_117

Because an embedded system is often composed of a wide variety of elements, the debugging strategy may vary. Embedded system_sentence_118

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). Embedded system_sentence_119

An increasing number of embedded systems today use more than one single processor core. Embedded system_sentence_120

A common problem with multi-core development is the proper synchronization of software execution. Embedded system_sentence_121

In this case, the embedded system design may wish to check the data traffic on the busses between the processor cores, which requires very low-level debugging, at signal/bus level, with a logic analyzer, for instance. Embedded system_sentence_122

Tracing Embedded system_section_12

Real-time operating systems (RTOS) often supports tracing of operating system events. Embedded system_sentence_123

A graphical view is presented by a host PC tool, based on a recording of the system behavior. Embedded system_sentence_124

The trace recording can be performed in software, by the RTOS, or by special tracing hardware. Embedded system_sentence_125

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 system_sentence_126

Reliability Embedded system_section_13

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. Embedded system_sentence_127

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. Embedded system_sentence_128

Specific reliability issues may include: Embedded system_sentence_129

Embedded system_unordered_list_4

  • The system cannot safely be shut down for repair, or it is too inaccessible to repair. Examples include space systems, undersea cables, navigational beacons, bore-hole systems, and automobiles.Embedded system_item_4_29
  • The system must be kept running for safety reasons. "Limp modes" are less tolerable. Often backups are selected by an operator. Examples include aircraft navigation, reactor control systems, safety-critical chemical factory controls, train signals.Embedded system_item_4_30
  • The system will lose large amounts of money when shut down: Telephone switches, factory controls, bridge and elevator controls, funds transfer and market making, automated sales and service.Embedded system_item_4_31

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: Embedded system_sentence_130

Embedded system_unordered_list_5

  • watchdog timer that resets the computer unless the software periodically notifies the watchdog subsystems with redundant spares that can be switched over to software "limp modes" that provide partial functionEmbedded system_item_5_32
  • Designing with a Trusted Computing Base (TCB) architecture ensures a highly secure & reliable system environmentEmbedded system_item_5_33
  • A hypervisor designed for embedded systems is able to provide secure encapsulation for any subsystem component so that a compromised software component cannot interfere with other subsystems, or privileged-level system software. This encapsulation keeps faults from propagating from one subsystem to another, thereby improving reliability. This may also allow a subsystem to be automatically shut down and restarted on fault detection.Embedded system_item_5_34
  • Immunity Aware ProgrammingEmbedded system_item_5_35

High vs. low volume Embedded system_section_14

For high volume systems such as portable music players or mobile phones, minimizing cost is usually the primary design consideration. Embedded system_sentence_131

Engineers typically select hardware that is just “good enough” to implement the necessary functions. Embedded system_sentence_132

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. Embedded system_sentence_133

Embedded software architectures Embedded system_section_15

Main article: Embedded software Embedded system_sentence_134

In 1978 National Electrical Manufacturers Association released a standard for programmable microcontrollers, including almost any computer-based controllers, such as single board computers, numerical, and event-based controllers. Embedded system_sentence_135

There are several different types of software architecture in common use today. Embedded system_sentence_136

Simple control loop Embedded system_section_16

In this design, the software simply has a loop. Embedded system_sentence_137

The loop calls subroutines, each of which manages a part of the hardware or software. Embedded system_sentence_138

Hence it is called a simple control loop or control loop. Embedded system_sentence_139

Interrupt-controlled system Embedded system_section_17

Some embedded systems are predominantly controlled by interrupts. Embedded system_sentence_140

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. Embedded system_sentence_141

These kinds of systems are used if event handlers need low latency, and the event handlers are short and simple. Embedded system_sentence_142

Usually, these kinds of systems run a simple task in a main loop also, but this task is not very sensitive to unexpected delays. Embedded system_sentence_143

Sometimes the interrupt handler will add longer tasks to a queue structure. Embedded system_sentence_144

Later, after the interrupt handler has finished, these tasks are executed by the main loop. Embedded system_sentence_145

This method brings the system close to a multitasking kernel with discrete processes. Embedded system_sentence_146

Cooperative multitasking Embedded system_section_18

A non-preemptive multitasking system is very similar to the simple control loop scheme, except that the loop is hidden in an API. Embedded system_sentence_147

The programmer defines a series of tasks, and each task gets its own environment to “run” in. Embedded system_sentence_148

When a task is idle, it calls an idle routine, usually called “pause”, “wait”, “yield”, “nop” (stands for no operation), etc. Embedded system_sentence_149

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. Embedded system_sentence_150

Preemptive multitasking or multi-threading Embedded system_section_19

In this type of system, a low-level piece of code switches between tasks or threads based on a timer (connected to an interrupt). Embedded system_sentence_151

This is the level at which the system is generally considered to have an "operating system" kernel. Embedded system_sentence_152

Depending on how much functionality is required, it introduces more or less of the complexities of managing multiple tasks running conceptually in parallel. Embedded system_sentence_153

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. Embedded system_sentence_154

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. Embedded system_sentence_155

The choice that an RTOS is required brings in its own issues, however, as the selection must be made prior to starting to the application development process. Embedded system_sentence_156

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. Embedded system_sentence_157

The restriction of future options becomes more of an issue as product life decreases. Embedded system_sentence_158

Additionally, the level of complexity is continuously growing as devices are required to manage variables such as serial, USB, TCP/IP, Bluetooth, Wireless LAN, trunk radio, multiple channels, data and voice, enhanced graphics, multiple states, multiple threads, numerous wait states and so on. Embedded system_sentence_159

These trends are leading to the uptake of embedded middleware in addition to a real-time operating system. Embedded system_sentence_160

Microkernels and exokernels Embedded system_section_20

A microkernel is a logical step up from a real-time OS. Embedded system_sentence_161

The usual arrangement is that the operating system kernel allocates memory and switches the CPU to different threads of execution. Embedded system_sentence_162

User-mode processes implement major functions such as file systems, network interfaces, etc. Embedded system_sentence_163

In general, microkernels succeed when task switching and intertask communication is fast and fail when they are slow. Embedded system_sentence_164

Exokernels communicate efficiently by normal subroutine calls. Embedded system_sentence_165

The hardware and all the software in the system are available to and extensible by application programmers. Embedded system_sentence_166

Monolithic kernels Embedded system_section_21

In this case, a relatively large kernel with sophisticated capabilities is adapted to suit an embedded environment. Embedded system_sentence_167

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. Embedded system_sentence_168

Common examples of embedded monolithic kernels are embedded Linux, VXWorks and Windows CE. Embedded system_sentence_169

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. Embedded system_sentence_170

Here are some of the reasons: Embedded system_sentence_171

Embedded system_unordered_list_6

  • Ports to common embedded chip sets are available.Embedded system_item_6_36
  • They permit re-use of publicly available code for device drivers, web servers, firewalls, and other code.Embedded system_item_6_37
  • Development systems can start out with broad feature-sets, and then the distribution can be configured to exclude unneeded functionality, and save the expense of the memory that it would consume.Embedded system_item_6_38
  • Many engineers believe that running application code in user mode is more reliable and easier to debug, thus making the development process easier and the code more portable.Embedded system_item_6_39
  • Features requiring faster response than can be guaranteed can often be placed in hardware.Embedded system_item_6_40

Additional software components Embedded system_section_22

In addition to the core operating system, many embedded systems have additional upper-layer software components. Embedded system_sentence_172

These components consist of networking protocol stacks like CAN, TCP/IP, FTP, HTTP, and HTTPS, and also included storage capabilities like and flash memory management systems. Embedded system_sentence_173

If the embedded device has audio and video capabilities, then the appropriate drivers and codecs will be present in the system. Embedded system_sentence_174

In the case of the monolithic kernels, many of these software layers are included. Embedded system_sentence_175

In the RTOS category, the availability of the additional software components depends upon the commercial offering. Embedded system_sentence_176

Domain-specific architectures Embedded system_section_23

In the automotive sector, AUTOSAR is a standard architecture for embedded software. Embedded system_sentence_177

See also Embedded system_section_24

Credits to the contents of this page go to the authors of the corresponding Wikipedia page: en.wikipedia.org/wiki/Embedded system.