Lesson 1 Introduction to Real Time Embedded Systems Part I
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
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)
When we want to purchase any of them what do we look for?
Let us see what are the choices available?
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 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)
Version 2 EE IIT, Kharagpur