Components of an Embedded System
By this time we know where are our Embedded Systems and what makes them stand out from other systems like Calculators, Desktop Computers, and our Old Television Sets. We have also developed some 6th sense to guess the components of an RTES.
This is the heart of any RTES. The microprocessors used here are different from the general purpose microprocessors like Pentium Sun SPARC etc. They are designed to meet some specific requirements. For example Intel 8048 is a special purpose microprocessor which you will find in the Keyboards of your Desktop computer. It is used to scan the keystrokes and send them in a synchronous manner to your PC. Similarly mobile phones Digital Cameras use special purpose processors for voice and image processing. A washer and dryer may use some other type of processor for Real Time Control and Instrumentation.
The microprocessor and memory must co-exit on the same Power Circuit Board(PCB) or same chip. Compactness, speed and low power consumption are the characteristics required for the memory to be used in an RTES. Therefore, very low power semiconductor memories are used in almost all such devices. For housing the operating system Read Only Memory(ROM) is used. The program or data loaded might exist for considerable duration. It is like changing the setup of your Desktop Computer. Similar user defined setups exist in RTES. For example you may like to change the ring tone of your mobile and keep it for some time. You may like to change the screen color etc. In these cases the memory should be capable of retaining the information even after the power is removed. In other words the memory should be non-volatile and should be easily programmable too. It is achieved by using Flash1 memories.
3. Input Output Devices and Interfaces
Input/Output interfaces are necessary to make the RTES interact with the external world. They could be Visual Display Units such as TFT screens in a mobile phone, touch pad key board, antenna, microphones, speakers etc. These RTES should also have open interfaces to other devices such as Desktop Computers, Local Area Networks (LAN) and other RTES. For example you may like to download your address book into your personal digital assistant (PDA). Or you may like to download some mp3 songs from your favorite internet site into your mp3 player. These input/output devices along with standard software protocols in the RTOS provide the necessary interface to these standards.
The RTES is the just the physical body as long as it is not programmed. It is like the human body without life. Whenever you switch on your mobile telephone you might have marked some activities on the screen. Whenever you move from one city to the other you might have noticed the changes on your screen. Or when you are gone for a picnic away from your city you might have marked the no-signal sign. These activities are taken care of by the Real Time Operating System sitting on the non-volatile memory of the RTES. Besides the above an RTES may have various other components and Application Specific Integrated Circuits (ASIC) for specialized functions such as motor control, modulation, demodulation, CODEC. The design of a Real Time Embedded System has a number of constraints. The following section discusses these issues.
The constraints in the embedded systems design are imposed by external as well as internal specifications. Design metrics are introduced to measure the cost function taking into account the technical as well as economic considerations.
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