Example, Definitions, Common Architecture Instructional Objectives After going through this lesson the student would be able to • Know what an embedded system is • distinguish a Real Time Embedded System from other systems • tell the difference between real and non-real time • Learn more about a mobile phone • Know the architecture • Tell the major components of an Embedded system Pre-Requisite Digital Electronics, Microprocessors Introduction In the day-to-day life we come across a wide variety of consumer electronic products. We are habituated to use them easily and flawlessly to our advantage. Common examples are TV Remote Controllers, Mobile Phones, FAX machines, Xerox machines etc. However, we seldom ponder over the technology behind each of them. Each of these devices does have one or more programmable devices waiting to interact with the environment as effectively as possible. These are a class of “embedded systems” and they provide service in real time. i.e. we need not have to wait too long for the action. Let us see how an embedded system is characterized and how complex it could be? Take example of a mobile telephone: (Fig. 1.1)
Fig. 1.1 Mobile Phones
When we want to purchase any of them what do we look for? Let us see what are the choices available? Phone Price Phone 1 Rs 5000/- Weight / Size 88.1 x 47.6 x 23.6 mm 116 g Phone 2 Rs 6000/- 89 x 49 x 24.8 mm 123 g Phone 3 Rs 5000/- 133.7 x 69.7 x 20.2mm 137g Screen TFT1 65k Color 96x32 screen Games Stauntman2 & Monopoly3 included more downloadable TFT J2ME 65k Games: Color Stauntman 176x220 and screen Monopoly More downloadable 176 x Symbian and 208 Java pixel download backlit games or screen packaged on with MMC cards 4096 colors Camera Radio Ring tones Memory Yes 4 x Zoom No Polyphonic Integrated Digital Camera 1 M Pixel No Polyphonic and MP3 No FM Stereo 3.4 MB user memory built in. Besides the above tabulated facts about the mobile handset, being a student of technology you may also like to know the following Network type GSM2 or CDMA3 (Bandwidth), Battery: Type and ampere hour Talk-time per one charge, Standby time 1 Short for thin film transistor, a type of LCD flat-panel display screen, in which each pixel is controlled by from one to four transistors. The TFT technology provides better resolution of all the flat-panel techniques, but it is also the most expensive. TFT screens are sometimes called active-matrix LCDs. 2 short form of Global System for Mobile Communications, one of the leading digital cellular systems. GSM uses narrowband Time Division Multiple Access (TDMA), which allows eight simultaneous calls on the same radio frequency. GSM was first introduced in 1991. As of the end of 1997, GSM service was available in more than 100 countries and has become the de facto standard in Europe and Asia. 3 Short form of Code-Division Multiple Access, a digital cellular technology that uses spread-spectrum techniques. Unlike competing systems, such as GSM, that use TDMA, CDMA does not assign a specific frequency to each user. Instead, every channel uses the full available spectrum. Individual conversations are encoded with a pseudo-random digital sequence. CDMA is a military technology first used during World War II by the English allies to foil German attempts at jamming transmissions. The allies decided to transmit over several frequencies, instead of one, making it difficult for the Germans to pick up the complete signal.
From the above specifications it is clear that a mobile phone is a very complex device which houses a number of miniature gadgets functioning coherently on a single device. Moreover each of these embedded gadgets such as digital camera or an FM radio along with the telephone has a number of operating modes such as: • you may like to adjust the zoom of the digital camera, • you may like to reduce the screen brightness, • you may like to change the ring tone, • you may like to relay a specific song from your favorite FM station to your friend using your mobile • You may like to use it as a calculator, address book, emailing device etc. These variations in the functionality can only be achieved by a very flexible device. This flexible device sitting at the heart of the circuits is none other than a Customized Microprocessor better known as an Embedded Processor and the mobile phone housing a number of functionalities is known as an Embedded System. Since it satisfies the requirement of a number of users at the same time (you and your friend, you and the radio station, you and the telephone network etc) it is working within a timeconstraint, i.e. it has to satisfy everyone with the minimum acceptable delay. We call this as to work in “Real Time”. This is unlike your holidaying attitude when you take the clock on your stride. We can also say that it does not make us wait long for taking our words and relaying them as well as receiving them, unlike an email server, which might take days to receive/deliver your message when the network is congested or slow. Thus we can name the mobile telephone as a “Real Time Embedded System” (RTES) Definitions Now we are ready to take some definitions Real Time “‘Real’-time usually means time as prescribed by external sources” For example the time struck by clock (however fast or late it might be). The timings generated by your requirements. You may like to call someone at mid-night and send him a picture. This external timing requirements imposed by the user is the real-time for the embedded system.