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information regarding embedded systems

By Edited Jun 21, 2015 0 0

EMBEDDED SYSTEMS

"Things do not change but we change", as the saying goes, our attitude towards the advent of technology has been changing. The catastrophical growth of software has reached at its cumulative point of handshaking with the hardware and software is called "Embedded systems". The Embedded system applications have started dominating the entire consumer aspects, may be it the washing machine, scanners, Xerox machines, microwave ovens etc.

INTRODUCTION

An embedded system is a special-purpose computer system designed to perform one or a few dedicated functions, often with real-time computing constraints. It is usually embedded as part of a complete device including hardware and mechanical parts. In contrast, a general-purpose computer, such as a personal computer, can do many different tasks depending on programming. Embedded systems have become very important today as they control many of the common devices we use.

Since the embedded system is dedicated to specific tasks, design engineers can optimize it, reducing the size and cost of the product, or increasing the reliability and performance. Some embedded systems are mass-produced, benefiting from economies of scale.

Physically, embedded systems range from portable devices such as digital watches and MP3 players, to large stationary installations like traffic lights, factory controllers, or the systems controlling nuclear power plants. Complexity varies from low, with a single microcontroller chip, to very high with multiple units, peripherals and networks mounted inside a large chassis or enclosure.

CHARACTERISTICS OF EMBEDDED SYSTEMS

1. Embedded systems are designed to do some specific task, rather than be a general-purpose computer for multiple tasks. Some also have real-time performance constraints that must be met, for reason such as safety and usability; others may have low or no performance requirements, allowing the system hardware to be simplified to reduce costs.

2. Embedded systems are not always separate devices. Most often they are physically built-in to the devices they control.

3. The software written for embedded systems is often called firmware, and is stored in read-only memory or Flash memory chips rather than a disk drive. It often runs with limited computer hardware resources: small or no keyboard, screen, and little memory.

Embedded system terminologies

The Embedded system as said above is the combination of the hardware and software designed for a specific task. The hardware aspect of the embedded system needs a microprocessor made up of semiconductor parts called chips and the micro controllers which is the basic controlling aspect. It may contain the microprocessor with memory. Timers/counters interrupt handlers etc. The basic block diagram of an embedded system is given below.

Embedded system needs memory for two purposes

To store its program (pre-programmable) to implement this, it uses "ROM" (Read Only Memory)

Another to store its data-input given by the user. For this RAM (Random Access Memory) is used.

Categories of Embedded systems

Embedded systems can be broadly classified into the following categories. This categorization is based on whether the system has to work as an independent unit or it has to be networked, whether it has to perform real-time operations or not.

Stand alone Embedded systems

As the name suggests, they operate in a stand-alone mode, taking input and producing output. The process control systems are an example of this category in which the inputs come from transducers that convert a physical entity, such as temperature into an electrical signal. The electrical signals becomes the output that can control devices such as valves. Other examples are toys, CD players and measuring instruments.

Real-time Embedded systems

Some Embedded systems are required to carry out specific tasks in a specified amount of time, such systems are called real time embedded systems. These are extensively used in process control, when time critical tasks constraints have to be strictly met are called hard real-time Embedded systems, systems in which real-time constraints are present, but not critical are called Soft real-time embedded systems.

Networked appliances

Some Embedded systems are connected to a network, typically one based on a TCP/IP (Transmission Control Protocol/Internet Protocol). The system constituting these web server-running protocols can send the data over a network to a centralized system for online monitoring. A typical example is the monitoring of equipment in a manufacturing unit. The system sends the data over the TCP/IP networks to a central management system, which can be a desktop computer running a web browser.

Mobile devices

Mobile devices are capable of supporting high data rates services and the accessing of Internet services can be enabled with the devices that monitor the mobile operating systems.

Peripherals

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

· Serial Communication Interfaces (SCI): RS-232, RS-422, RS-485 etc

· Synchronous Serial Communication Interface: I2C, JTAG, SPI, SSC and ESSI

· Universal Serial Bus (USB)

· Networks: Ethernet, Controller Area Network, Lon Works, etc

· Timers: PLL(s), Capture/Compare and Time Processing Units

· Discrete IO: aka General Purpose Input/Output (GPIO)

· Analog to Digital/Digital to Analog (ADC/DAC)

Tools

As for other software, embedded system designers use compilers, assemblers, and debuggers to develop embedded system software. However, they may also use some more specific tools:

· In circuit debuggers or emulators (see next section).

· Utilities to add a checksum or CRC to a program, so the embedded system can check if the program is valid.

· For systems using digital signal processing, developers may use a math workbench such as MATLAB, Simulink, MathCad, or Mathematica to simulate the mathematics. They might also use libraries for both the host and target which eliminates developing DSP routines as done in DSPnano RTOS and Unison Operating System.

· Custom compilers and linkers may be used to improve optimization for the particular hardware.

· An embedded system may have its own special language or design tool, or add enhancements to an existing language such as Forth or Basic.

· Another alternative is to add a Real-time operating system or Embedded operating system, which may have DSP capabilities like DSPnano RTOS.

Software tools can come from several sources:

· Software companies that specialize in the embedded market

· Ported from the GNU software development tools

· Sometimes, development tools for a personal computer can be used if the embedded processor is a close relative to a common PC processor

As the complexity of embedded systems grows, higher level tools and operating systems are migrating into machinery where it makes sense. For example 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. 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 software architectures

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

Simple control loop

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

Interrupt controlled system

Some embedded systems are predominantly interrupt controlled. 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.

Cooperative multitasking

A nonpreemptive multitasking system is very similar to the simple control loop scheme, except that the loop is hidden in an API. The programmer defines a series of tasks, and each task gets its own environment to "run" in. Then, when a task is idle, it calls an idle routine (usually called "pause", "wait", "yield", "nop" (Stands for no operation), etc.).

The advantages and disadvantages are very similar to the control loop, except that adding new software is easier, by simply writing a new task, or adding to the queue-interpreter.

Preemptive multitasking or multi-threading

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 buy a real-time operating system, 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, and/or battery life.

Microkernels and exokernels

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.

Exokernels communicate efficiently by normal subroutine calls. The hardware, and all the software in the system are available to, and extensible by application programmers.

Monolithic kernels

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.

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

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:

Ports to common embedded chip sets are available.

They permit re-use of publicly available code for Device Drivers, Web servers,Firewalls and other code.

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.

Many engineers believe that running application code in user mode is more reliable, easier to debug and that therefore the development process is easier and the code more portable.

Many embedded systems lack the tight real time requirements of a control system. A system such as Embedded Linux has fast enough response for many applications.

Features requiring faster response than can be guaranteed can often be placed in hardware.

Many RTOS systems have a per-unit cost. When used on a product that is or will become a commodity, that cost is significant.

Debugging

Embedded Debugging may be performed at different levels, depending on the facilities available. From simplest to most sophisticate they can be roughly grouped into the following areas:

Interactive resident debugging, using the simple shell provided by the embedded operating system (e.g. Forth and Basic)

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 which even works for heterogeneous multicore systems.

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.

An in-circuit emulator replaces the microprocessor with a simulated equivalent, providing full control over all aspects of the microprocessor.

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.

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 assembly or source code.

Reliability

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.

Specific reliability issues may include:

1. 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.

2. 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, engines on single-engine aircraft.

3. 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.

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:

· 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 function

· Immunity Aware Programming

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