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INTRODUCTION TO EMBEDDED SYSTEMS
What is an Embedded System?
An embedded system can be defined as a computing device that does a specific focused job. Appliances such as the air-conditioner, VCD player, DVD player, printer, fax machine, mobile phone etc. are examples of embedded systems. Each of these appliances will have a processor and special hardware to meet the specific requirement of the application along with the embedded software that is executed by the processor for meeting that specific requirement. The embedded software is also called “firm ware”. The desktop/laptop computer is a general purpose computer. You can use it for a variety of applications such as playing games, word processing, accounting, software development and so on. In contrast, the software in the embedded systems is always fixed.
1.1 History:
In the earliest years of computers in the 1940s, computers were sometimes dedicated to a single task, but were too large to be considered "embedded". Over time however, the concept of programmable controllers developed from a mix of computer technology, solid state devices, and traditional electromechanical sequences.
The first recognizably modern embedded system was the Apollo Guidance Computer, developed by Charles Stark Draper at the MIT Instrumentation Laboratory. At the project's inception, the Apollo guidance computer was considered the riskiest item in the Apollo project. The use of the then new monolithic integrated circuits, to reduce the size and weight, increased this risk.
The first mass-produced embedded system was the Autonetics D-17 guidance computer for the Minuteman (missile), released in 1961. It was built from transistor logic and had a hard disk for main memory. When the Minuteman II went into production in 1966, the D-17 was replaced with a new computer that was the first high-volume use of integrated circuits. This program alone reduced prices on quad nand gate ICs from $1000/each to $3/each, permitting their use in commercial products.
Since these early applications in the 1960s, embedded systems have come down in price. There has also been an enormous rise in processing power and functionality. For example the first microprocessor was the Intel 4004, which found its way into calculators and other small systems, but required external memory and support chips.
In 1978 National Engineering Manufacturers Association released the standard for a programmable microcontroller. The definition was an almost any computer-based controller. They included single board computers, numerical controllers, and sequential controllers in order to perform event-based instructions.
By the mid-1980s, many of the previously external system components had been integrated into the same chip as the processor, resulting in integrated circuits called microcontrollers, and widespread use of embedded systems became feasible.
As the cost of a microcontroller fell below $1, it became feasible to replace expensive knob-based analog components such as potentiometers and variable capacitors with digital electronics controlled by a small microcontroller with up/down buttons or knobs. By the end of the 80s, embedded systems were the norm rather than the exception for almost all electronics devices, a trend which has continued since.
1.2 Embedded systems are characterized by some special features listed below:
1. Embedded systems do a very specific task; they cannot be programmed to do different things. . Embedded systems have very limited resources, particularly the memory. Generally, they do not have secondary storage devices such as the C DROM or the floppy disk. Embedded systems have to work against some deadlines. A specific job has to be completed within a specific time. In some embedded systems, called real-time systems, the deadlines are stringent. Missing a deadline may cause a catastrophe-loss of life or damage to property. Embedded systems are constrained for power. As many embedded systems operate through a battery, the power consumption has to be very low.
2. Embedded systems need to be highly reliable. Once in a while, pressing ALT-CTRL-OEL is OK on your desktop, but you cannot afford to reset your embedded system.
3. Some embedded systems have to operate in extreme environmental conditions such as very high temperatures and humidity.
4. Embedded systems that address the consumer market (for exam-ple, electronic toys) are very cost-sensitive: Even a reduction of $0.1 is lot of cost saving, because thousands or millions systems may be sold.
5. Unlike desktop computers in which the hardware platform is dominated by Intel and the operating system is dominated by Microsoft, there is a wide variety of processors and operating systems for the embedded systems. So, choosing the right plat-form is the most complex task.
1.3 Categories of Embedded Systems
Based on functionality and performance requirements, embedded systems can be categorized as:
1. Stand-alone embedded systems
2. Real-time systems
3. Networked information appliances
4. Mobile devices
1.3.1 Stand alone Embedded Systems:
As the name implies, stand-alone systems work in stand-alone mode. They take inputs, process them and produce the desired output. The input can be electrical signals from transducers or commands from a human being such as the pressing of a button. The output can be electrical signals to drive another system, an LED display or LCD display for displaying of information to the users. Embedded systems used in process co~1’rol, automobiles, consumer electronic items etc. fall into this category. In a process control system, the inputs are from sensors that convert a physical entity such as temperature or pressure into its equivalent electrical signal. These electrical signals are processed by the system and the appropriate electrical signals are produced using which an action is taken such as opening a valve. A few embedded systems used at home are shown in fig