Von-Neumann Architecture

Most of the modern computer system has been designed on the basis of an architecture called Von-Neumann Architecture.



The Memory stores the instructions as well as data. No one can distinguish an instruction and data. The CPU has to be directed to the address of the instruction codes.

The memory is connected to the CPU through the following lines
1. Address
2. Data
3. Control

The von Neumann architecture is a model for a computing machine that uses a single storage structure to hold both the set of instructions on how to perform the computation and the data required or generated by the computation. Such machines are also known as stored-program computers. The separation of storage from the processing unit is implicit in this model.

By treating the instructions in the same way as the data, a stored-program machine can easily change the instructions. In other words the machine is reprogrammable. One important motivation for such a facility was the need for a program to increment or otherwise modify the address portion of instructions. This became less important when index registers and indirect addressing became customary features of machine architecture.


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Questions And Answers

Q1. Enumerate the similarities and differences between the Microcontroller and Digital Signal Processor

Ans:
Microcontrollers usually have on chip RAM and ROM (or EPROM) in addition to on chip i/o hardware to minimize chip count in single chip solutions. As a result of using on chip hardware for I/O and RAM and ROM they usually have pretty low performance CPU. Microcontrollers also often have timers that generate interrupts and can thus be used with the CPU and on chip A/D D/A or parallel ports to get regularly timed I/O. The prime use of a microcontroller is to control the operations of a machine using a fixed program that is stored in ROM and does not change over the lifetime of the system. The microcontroller is concerned with getting data from and to its own pins; the architecture and instruction set are optimized to handle data in bit and byte size.

Digital Signal Processors have been designed based on the modified Harvard Architecture to handle real time signals. The features of these processors are suitable for implementing signal processing algorithms. One of the common operations required in such applications is array multiplication. For example convolution and correlation require array multiplication. This is accomplished by multiplication followed by accumulation and addition. This is generally carried out by Multiplier and Accumulator (MAC) units. Some times it is known as MACD, where D stands for Data move. Generally all the instructions are executed in single cycle. These DSP units generally use Multiple Access and Multi Ported Memory units. Multiple access memory allows more than one access in one clock period. The Multi-ported Memory allows multiple addresses as well Data ports. This also increases the number of access per unit clock cycle.

Q2. Name few chips in each of the family of processors such as: Microcontroller, Digital Signal Processor, General Purpose Processor

Ans:
Microcontroller: Intel 8051, Intel 80196, Motorola 68705
Digital Signal Processors: TI 3206711, TI 3205000
General Purpose Processor: Intel Pentium IV, Power PC


Q3. Enlist the following in the increasing order of their access speed
Flash Memory, Dynamic Memory, Cache Memory, CDROM, Hard Disk, Magnetic Tape, Processor Memory


Ans:
Magnetic Tape, CDROM, Hard Disk, Dynamic Memory, Flash Memory, Cache Memory, Processor Memory

Q4. Draw the circuit of an anti-aliasing Filter using Operational amplifiers


Ans:



Q5. Is it possible to implement an anti-aliasing filter in the digital form?


Ans:
No it is not possible to implement an anti-aliasing filter in digital form. Because aliasing is an error introduced at the sampling phase of analog to digital converter. If the sampling frequency is less than twice of the highest frequency present the higher signal frequencies fold back to lower frequency band and hence can be distinguished in the digital/discrete domain.

Q6. State with justification if the following statements are right (or wrong)
Cache memory can be a static RAM
Dynamic RAMs occupy more space per word storage
The full-form of SDRAM is static-dynamic RAM
BIOS in your PC is not a Random Access Memory (RAM)


Ans:
Cache memory can be a static RAM right
The cache memory need to have very fast access time which is possible with static RAM.

Dynamic RAMs occupy more space per word storage wrong
DRAMs are basically simple MOS based capacitors. Therefore occupy much lower space as compared to static RAMs.

The full-form of SDRAM is static-dynamic RAM wrong
SDRAM is Synchronous Dynamic RAM.

BIOS in your PC is not a Random Access Memory (RAM) Wrong
The BIOS is a CMOS based memory which can be accessed uniformly.


Q7. Explain the function of the following units in a general purpose processor
Instruction Register
Program Counter
Instruction Queue
Control Unit


Ans:
Instruction Register: A register inside the CPU which holds the instruction code temporarily before sending it to the decoding unit.

Program Counter: It is a register inside the CPU which holds the address of the next instruction code in a program. It gets updated automatically by the address generation unit.

Instruction Queue: A set of memory locations inside the CPU to hold the instructions in a pipe-line before rending them to the next instruction decoding unit.

Control Unit: This is responsible in generating timing and control signals for various operations inside the CPU. It is very closely associated with the instruction decoding unit.

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Memory And Input/Output Devices

Memory

Memory serves processor short and long-term information storage requirements while registers serve the processor’s short-term storage requirements. Both the program and the data are stored in the memory. This is known as Princeton Architecture where the data and program occupy the same memory. In Harvard Architecture the program and the data occupy separate memory blocks. The former leads to simpler architecture. The later needs two separate connections and hence the data and program can be made parallel leading to parallel processing. The general purpose processors have the Princeton Architecture.

The memory may be Read-Only-Memory or Random Access Memory (RAM). It may exist on the same chip with the processor itself or may exist outside the chip. The on-chip memory is faster than the off-chip memory. To reduce the access (read-write) time a local copy of a portion of memory can be kept in a small but fast memory called the cache memory. The memory also can be categorized as Dynamic or Static. Dynamic memory dissipate less power and hence can be compact and cheaper. But the access time of these memories are slower than their Static counter parts. In Dynamic RAMs (or DRAM) the data is retained by periodic refreshing operation. While in the Static Memory (SRAM) the data is retained continuously. SRAMs are much faster than DRAMs but consume more power. The intermediate cache memory is an SRAM.

In a typical processor when the CPU needs data, it first looks in its own data registers. If the data isn't there, the CPU looks to see if it's in the nearby Level 1 cache. If that fails, it's off to the Level 2 cache. If it's nowhere in cache, the CPU looks in main memory. Not there? The CPU gets it from disk. All the while, the clock is ticking, and the CPU is sitting there waiting.


Input/Output Devices and Interface Chips

Typical RTES interact with the environment and users through some inbuilt hardware. Occasionally external circuits are required for communicating with user, other computers or a network.

In the mobile handset discussed earlier the input output devices are, keyboard, the display screen, the antenna, the microphone, speaker, LED indicators etc. The signal to these units may be analog or digital in nature. To generate an analog signal from the microprocessor we need an Digital to Analog Converter(DAC) and to accept analog signal we need and Analog to Digital Converter (ADC). These DAC and ADC again have certain control modes. They may also operate at different speed than the microprocessor. To synchronize and control these interface chips we may need another interface chip. Similarly we may have interface chips for keyboard, screen and antenna. These chips serve as relaying units to transfer data between the processor and input/output devices. The input/output devices are generally slower than the processor. Therefore, the processor may have to wait till they respond to any request for data transfer. Number of idle clock cycles may be wasted for doing so. However, the input-output interface chips carry out this task without making the processor to wait or idle.



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A Basic Microprocessors vs A Basic DSP

General Purpose Processor



Characterization of General Purpose Processor

1. CPUs for PCs and workstations E.g., Intel Pentium IV
2. Von Neumann architecture
3. Typically 1 access per cycle
4. Most operations take more than 1 cycle
5. General-purpose instructions Typically only one operation per instruction
6. Often, no separate address generation units
7. General-purpose addressing modes
8. Software loops only
9. Interrupts rarely disabled
10. Register shadowing common
11. Dynamic caches are common
12. Wide range of on-chip and off-chip peripherals and I/O interfaces
13. Asynchronous serial port...

A Basic DSP



DSP Characterization

1. Microprocessors specialized for signal processing applications.
2. Harvard architecture
3. Two to Four memory accesses per cycle
4. Dedicated hardware performs all key arithmetic operations in 1 cycle
5. Very limited SIMD(Single Instruction Multiple Data) features and Specialized, complex instructions
6. Multiple operations per instruction
7. Dedicated address generation units
8. Specialized addressing [• Auto-increment • Modulo (circular) Bit-reversed ]
9. Hardware looping.
10. Interrupts disabled during certain operations
11. Limited or no register Shadowing
12. Rarely have dynamic features
13. Relatively narrow range of DSP oriented on-chip peripherals and I/O interfaces
14. synchronous serial port



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Microprocessors vs Microcontrollers

Microprocessors

A microprocessor is a general-purpose digital computer’s central processing unit. To make a complete microcomputer, you add memory (ROM and RAM) memory decoders, an oscillator, and a number of I/O devices. The prime use of a microprocessor is to read data, perform extensive calculations on that data, and store the results in a mass storage device or display the results. These processors have complex architectures with multiple stages of pipelining and parallel processing. The memory is divided into stages such as multi-level cache and RAM. The development time of General Purpose Microprocessors is high because of a very complex VLSI design.




Microcontrollers

The design of the microcontroller is driven by the desire to make it as expandable and flexible as possible. Microcontrollers usually have on chip RAM and ROM (or EPROM) in addition to on chip i/o hardware to minimize chip count in single chip solutions. As a result of using on chip hardware for I/O and RAM and ROM they usually have pretty low performance CPU. Microcontrollers also often have timers that generate interrupts and can thus be used with the CPU and on chip A/D D/A or parallel ports to get regularly timed I/O. The prime use of a microcontroller is to control the operations of a machine using a fixed program that is stored in ROM and does not change over the lifetime of the system. The microcontroller is concerned with getting data from and to its own pins; the architecture and instruction set are optimized to handle data in bit and byte size.




The contrast between a microcontroller and a microprocessor is best exemplified by the fact that most microprocessors have many operation codes (opcodes) for moving data from external memory to the CPU; microcontrollers may have one or two. Microprocessors may have one or two types of bit-handling instructions; microcontrollers will have many.


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Digital Signal Processor (DSP)

These processors have been designed based on the modified Harvard Architecture to handle real time signals. The features of these processors are suitable for implementing signal processing algorithms. One of the common operations required in such applications is array multiplication. For example convolution and correlation require array multiplication. This is accomplished by multiplication followed by accumulation and addition. This is generally carried out by Multiplier and Accumulator (MAC) units. Some times it is known as MACD, where D stands for Data move. Generally all the instructions are executed in single cycle.



The MACD type of instructions can be executed faster by parallel implementation. This is possible by separately accessing the program and data memory in parallel. This can be accomplished by the modified architecture shown in Fig. 4.3. These DSP units generally use Multiple Access and Multi Ported Memory units. Multiple access memory allows more than one access in one clock period. The Multi-ported Memory allows multiple addresses as well Data ports. This also increases the number of access per unit clock cycle.



The Very Long Instruction Word (VLIW) architecture is also suitable for Signal Processing applications. This has got a number of functional units and data paths as seen in Fig. 4.5. The long instruction words are fetched from the memory. The operands and the operation to be performed by the various units are specified in the instruction itself. The multiple functional units share a common multi-ported register file for fetching the operands and storing the results. Parallel random access to the register file is possible through the read/write cross bar. Execution in the functional units is carried out concurrently with the load/store operation of data between RAM and the register file.



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Microcontroller

Just as you put all the major components of a Desktop PC on to a Single Board Computer (SBC) if you put all the major components of a Single Board Computer on to a single chip it will be called as a Microcontroller. Because of the limitations in the VLSI design most of the input/output functions exist in a simplified manner. Typical architecture of such a microprocessor is shown in Fig. 4.2.




* The double-lined blocks are core to the processor. Other blocks are on-chip.

The various units of the processors (Fig. 4.2) are as follows:

o The C500 Core contains the CPU which consists of the Instruction Decoder, Arithmetic Logic Unit (ALU) and Program Control section.

o The housekeeper unit generates internal signals for controlling the functions of the individual internal units within the microcontroller.

o Port 0 and Port 2 are required for accessing external code and data memory and for emulation purposes.

o The external control block handles the external control signals and the clock generation.

o The access control unit is responsible for the selection of the on-chip memory resources.

o The IRAM provides the internal RAM which includes the general purpose registers.

o The XRAM is another additional internal RAM sometimes provided.

o The interrupt requests from the peripheral units are handled by an Interrupt Controller Unit.

o Serial interfaces, timers, capture/compare units, A/D converters, watchdog units (WDU), or a multiply/divide unit (MDU) are typical examples for on-chip peripheral units. The external signals of these peripheral units are available at multifunctional parallel I/O ports or at dedicated pins.


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General Components of Embedded Systems

Introduction

You are now almost familiar with the various components of an embedded system. In this chapter we shall discuss some of the general components such as

• Processors
• Memory
• Input/Out Devices


Processors

The central processing unit is the most important component in an embedded system. It exists in an integrated manner along with memory and other peripherals. Depending on the type of applications the processors are broadly classified into 3 major categories

1. General Purpose Microprocessors
2. Microcontrollers
3. Digital Signal Processors

For more specific applications customized processors can also be designed. Unless the demand is high the design and manufacturing cost of such processors will be high. Therefore, in most of the applications the design is carried out using already available processors in the market. However, the Field Programmable Gate Arrays (FPGA) can be used to implement simple customized processors easily. An FPGA is a type of logic chip that can be programmed. They support thousands of gates which can be connected and disconnected like an EPROM (Erasable Programmable Read Only Memory). They are especially popular for prototyping integrated circuit designs. Once the design is set, hardwired chips are produced for faster performance.

General Purpose Processors

A general purpose processor is designed to solve problems in a large variety of applications as diverse as communications, automotive and industrial embedded systems. These processors are generally cheap because of the manufacturing of large number of units. The NRE (Non-recurring Engineering Cost: Lesson I) is spread over a large number of units. Being cheaper the manufacturer can invest more for improving the VLSI design with advanced optimized architectural features. Thus the performance, size and power consumption can be improved. Most cases, for such processors the design tools are provided by the manufacturer. Also the supporting hardware is cheap and easily available. However, only a part of the processor capability may be needed for a specific design and hence the over all embedded system will not be as optimized as it should have been as far as the space, power and reliability is concerned.






Pentium IV is such a general purpose processor with most advanced architectural features. Compared to its overall performance the cost is also low.
A general purpose processor consists of a data path, a control unit tightly linked with the memory. (Fig. 4.1)

The Data Path consists of a circuitry for transforming data and storing temporary data. It contains an arithmetic-logic-unit(ALU) capable of transforming data through operations such as addition, subtraction, logical AND, logical OR, inverting, shifting etc. The data-path also contains registers capable of storing temporary data generated out of ALU or related operations. The internal data-bus carries data within the data path while the external data bus carries data to and from the data memory. The size of the data path indicates the bit-size of the CPU. An 8-bit data path means an 8-bit CPU such as 8085 etc.

The Control Unit consists of circuitry for retrieving program instructions and for moving data to, from, and through the data-path according to those instructions. It has a program counter(PC) to hold the address of the next program instruction to fetch and an Instruction register(IR) to hold the fetched instruction. It also has a timing unit in the form of state registers and control logic. The controller sequences through the states and generates the control signals necessary to read instructions into the IR and control the flow of data in the data path. Generally the address size is specified by the control unit as it is responsible to communicate with the memory. For each instruction the controller typically sequences through several stages, such as fetching the instruction from memory, decoding it, fetching the operands, executing the instruction in the data path and storing the results. Each stage takes few clock cycles.


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Question and Answers

Q1. What are the Hierarchical components in a embedded system design.

Ans:




Q.2. What is LVDS?


Ans:

Known as Low Voltage Differential Signaling. The advantages of such a standard is low noise and low interference such that one can increase the data transmission rate. Instead of 0 and 5 V or ±5V a voltage level of 1.5 or 3.3 V is used for High and 0 or 1 V is used for Low. The Low to High voltage swing reduces interference. A differential mode rejects common mode noises.

Q.3. Is there any actuator in your mobile phone?


Ans:

There is a vibrator in a mobile phone which can be activated to indicate an incoming call or message. Generally there is a coreless motor which is operated by the microcontroller for generating the vibration.

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Structure of an Embedded System

On the other hand a desktop computer may contain all these units on a single Power Circuit Board (PCB) called as the Mother Board. Since these computers handle much larger dimension of data as compared to the embedded systems there has to be elaborate arrangements for storage and faster data transfer between the CPU and memory, CPU and input/output devices and memory and input/output devices. The storage is accomplished by cheaper secondary memories like Hard Disks and CDROM drives. The data transfer process is improved by incorporating multi-level cache and direct memory access methods. Generally no such arrangements are necessary for embedded systems. Because of the number of heterogeneous components in a desktop computer the power supply is required at multiple voltage-levels (typically ±12, ± 5, ± 3, 25 volts). On the other hand an Embedded Systems chip may just need one level DC power supply (typically +5V).
In a desktop computer various units operate at different speeds. Even the units inside a typical CPU such as Pentium-IV may operate at different speeds. The timing and control units are complex and provide multi-phase clock signal to the CPU and other peripherals at different voltage levels. The timing and control unit for an Embedded system may be much simpler.

The typical structure of an embedded system is shown in Fig. 3.2. This can be compared with that of a Desktop Computer as shown in Fig. 3.3. Normally in an embedded system the primary memory, central processing unit and many peripheral components including analog-to-digital converters are housed on a single chip. These single chips are called as Microcontrollers. This is shown by dotted lines in Fig. 3.2.

The typical structure of an Embedded System
(Fig. 3.2)



The structural layout of a desktop Computer
(Fig. 3.3)



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Components of Embedded Systems

Introduction

The various components of an Embedded System can be hierarchically grouped as System Level Components to Transistor Level Components. A system (subsystem) component is different than what is considered a "standard" electronic component. Standard components are the familiar active devices such as integrated circuits, microprocessors, memory, diodes, transistors, etc. along with passives such as resistors, capacitors, and inductors. These are the basic elements needed to mount on a circuit board for a customized, application-specific design.

A system component on the other hand, has active and passive components mounted on circuit boards that are configured for a specific task. (Fig. 3.1) System components can be either single- or multi-function modules that serve as highly integrated building blocks of a system. A system component can be as simple as a digital I/O board or as complex as a computer with video, memory, networking, and I/O all on a single board. System components support industry standards and are available from multiple sources worldwide.



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Questions and Answers of Real Time Embedded Systems

Q1. Give one example of a typical embedded system other than listed in this lecture. Draw the block diagram and discuss the function of the various blocks. What type of embedded processor they use?

Ans:
Example 1: A handheld Global Positioning System Receiver





A GPS receiver receives signals from a constellation of at least four out of a total of 24 satellites. Based on the timing and other information signals sent by these satellites the digital signal processor calculates the position using triangulation.

The major block diagram is divided into (1) Active Antenna System (2)RF/IF front end (3) The Digital Signal Processor(DSP)

The Active Antenna System houses the antenna a band pass filter and a low noise amplifier (LNA)

The RF/IF front end houses another band pass filter, the RF amplifier and the demodulator and A/D converter.

The DSP accepts the digital data and decodes the signal to retrieve the information sent by the GPS satellites.

Q2. Discuss about the Hard Disk Drive housed in your PC. Is it an RTES?

Ans:



Hard drives have two kinds of components: internal and external. External components are located on a printed circuit board called logic board while internal components are located in a sealed chamber called HDA or Hard Drive Assembly.

The big circuit is the controller. It is in charge of everything: exchanging data between the hard drive and the computer, controlling the motors on the hard drive, commanding the heads to read or write data, etc.

All these tasks are carried out as demanded by the processor sitting on the motherboard. It can be verified to be single-functioned, tightly constrained,
Therefore one can say that a Hard Disk Drive is an RTES.


Q3. Elaborate on the time-to-market design metric.


Ans:

The time required to develop a system to the point that it can be released and sold to customers. The main contributors are design time, manufacturing time, and testing time. This metric has become especially demanding in recent years. Introducing an embedded system to the marketplace early can make a big difference in the system’s profitability.


Q4. What is Moore’s Law? How was it conceived?

Moore's law is the empirical observation that the complexity of integrated circuits, with respect to minimum component cost, doubles every 24 months. It is attributed to Gordon E. Moor, a co-founder of Intel.



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Design Methodology

The design task can be further segregated into the following steps:

System level Design

Find out the possible subsystems of the system and the interconnections between them.

Sub-system or Node Level design

Each of these subsystems can be termed as the nodes. Elaborate on each of these subsystems and further make the block diagram and component level interconnections.

Processor Level Design

Each subsystem may consist of processor, memory, I/O devices. Specification and design at this level is required now.

Task Level Design

Complete interconnection of these subsystems depending on the tasks they would perform



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The Performance Design Metric

Performance of a system is a measure of how long the system takes to execute our desired tasks.

The two main measures of performance are:

Latency or response time:

This is the time between the start of the task’s execution and the end. For example, processing an image may take 0.25 second.


Throughput:

This is the number of tasks that can be processed per unit time. For example, a camera may be able to process 4 images per second
These are the some of the cost measures for developing an RTES. Optimization of the overall cost of design includes each of these factors taken with some multiplying factors depending on their importance. And the importance of each of these factors depends on the type of application. For instance in defense related applications while designing an anti-ballistic system the execution time is the deciding factor. On the other hand, for de-noising a photograph in an embedded camera in your mobile handset the execution time may be little relaxed if it can bring down the cost and complexity of the embedded Digital Signal Processor.

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Design 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.

Design Metrics

A Design Metric is a measurable feature of the system’s performance, cost, time for implementation and safety etc. Most of these are conflicting requirements i.e. optimizing one shall not optimize the other: e.g. a cheaper processor may have a lousy performance as far as speed and throughput is concerned.

Following metrics are generally taken into account while designing embedded systems:

NRE cost (nonrecurring engineering cost)


It is one-time cost of designing the system. Once the system is designed, any number of units can be manufactured without incurring any additional design cost; hence the term nonrecurring.

Suppose three technologies are available for use in a particular product. Assume that implementing the product using technology ‘A’ would result in an NRE cost of $2,000 and unit cost of $100, that technology B would have an NRE cost of $30,000 and unit cost of $30, and that technology C would have an NRE cost of $100,000 and unit cost of $2. Ignoring all other design metrics, like time-to-market, the best technology choice will depend on the number of units we plan to produce.


Unit cost

The monetary cost of manufacturing each copy of the system, excluding NRE cost.


Size

The physical space required by the system, often measured in bytes for software, and gates or transistors for hardware.


Performance

The execution time of the system.

Power Consumption

It is the amount of power consumed by the system, which may determine the lifetime of a battery, or the cooling requirements of the IC, since more power means more heat.


Flexibility

The ability to change the functionality of the system without incurring heavy NRE cost. Software is typically considered very flexible.


Time-to-prototype

The time needed to build a working version of the system, which may be bigger or more expensive than the final system implementation, but it can be used to verify the system’s usefulness and correctness and to refine the system’s functionality.

Time-to-market

The time required to develop a system to the point that it can be released and sold to customers. The main contributors are design time, manufacturing time, and testing time. This metric has become especially demanding in recent years. Introducing an embedded system to the marketplace early can make a big difference in the system’s profitability.

Maintainability

It is the ability to modify the system after its initial release, especially by designers who did not originally design the system.

Correctness

This is the measure of the confidence that we have implemented the system’s functionality correctly. We can check the functionality throughout the process of designing the system, and we can insert test circuitry to check that manufacturing was correct.

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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.

1. Microprocessor
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.

2. Memory
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.

4. Software
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.

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Typical Architecture of Mobile Phone

In general, a cell phone is composed of the following components:
• A Circuit board (Fig. 2.2)
• Antenna
• Microphone
• Speaker
• Liquid crystal display (LCD)
• Keyboard
• Battery



A typical mobile phone handset (Fig. 2.3) should include standard I/O devices (keyboard, LCD), plus a microphone, speaker and antenna for wireless communication. The Digital Signal Processor (DSP) performs the signal processing, and the micro-controller controls the user interface, battery management, call setup etc. The performance specification of the DSP is very crucial since the conversion has to take place in real time. This is why almost all cell phones contain such a special processor dedicated for making digital-to-analog (DA) and analog-to-digital(AD) conversions and real time processing such as modulation and demodulation etc. The Read Only Memory (ROM) and flash memory (Electrically Erasable and Programmable Memory) chips provide storage for the phone’s operating system(RTOS) and various data such as phone numbers, calendars information, games etc.

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Questions and Answers

Q1 Which of the following is a real time embedded system? Justify your answer
(a) Ceiling Fan
(b) Microwave Oven
(c) Television Set
(d) Desktop Key Board
(e) Digital Camera
Ans:
(b) and (e) are embedded systems
(
(a) Ceiling Fans: These are not programmable.
(b) & (e) obey all definitions of Embedded Systems such as
(i) Working in Real Time (ii) Programmable (iii) A number of systems coexist on a single platform to discharge one function(single functioned)
(c) Television Set: Only a small part of it is programmable. It can work without being programmable. It is not tightly constrained.
(d) Desktop Keyboard: Though it has a processor normally it is not programmable


Q.2 Write five advantages and five disadvantages of embodiment.
Ans:
Five advantages:
1. Smaller Size
2. Smaller Weight
3. Lower Power Consumption
4. Lower Electromagnetic Interference
5. Lower Price
Five disadvantages
1. Lower Mean Time Between Failure
2. Repair and Maintenance is not possible
3. Faster Obsolesce
4. Unmanageable Heat Loss
5. Difficult to Design

Q3. What do you mean by Reactive in Real Time. Cite an example.
Ans:
Many embedded systems must continually react to changes in the system’s environment and must compute certain results in real time without delay. For example, a car’s cruise controller continually monitors and reacts to speed and brake sensors. It must compute acceleration or deceleration amounts repeatedly within a limited time; a delayed computation could result in a failure to maintain control of the car. In contrast a desktop computer system typically focuses on computations, with relatively infrequent (from the computer’s perspective) reactions to input devices. In addition, a delay in those computations, while perhaps inconvenient to the computer user, typically does not result in a system failure.

Definition of Real Time Systems & Embedded System

Definition of Real Time Systems

An operation within a larger dynamic system is called a real-time operation if the combined reaction- and operation-time of a task operating on current events or input, is no longer than the maximum delay allowed, in view of circumstances outside the operation. The task must also occur before the system to be controlled becomes unstable. A real-time operation is not necessarily fast, as slow systems can allow slow real-time operations. This applies for all types of dynamically changing systems. The polar opposite of a real-time operation is a batch job with interactive timesharing falling somewhere in between the two extremes.
Alternately, a system is said to be hard real-time if the correctness of an operation depends not only upon the logical correctness of the operation but also upon the time at which it is performed. An operation performed after the deadline is, by definition, incorrect, and usually has no value. In a soft real-time system the value of an operation declines steadily after the deadline expires.

Embedded System

An embedded system is a special-purpose system in which the computer is completely encapsulated by the device it controls. Unlike a general-purpose computer, such as a personal computer, an embedded system performs pre-defined tasks, usually with very specific requirements. Since the system is dedicated to a specific task, design engineers can optimize it, reducing the size and cost of the product. Embedded systems are often mass-produced, so the cost savings may be multiplied by millions of items.

Handheld computers or PDAs are generally considered embedded devices because of the nature of their hardware design, even though they are more expandable in software terms. This line of definition continues to blur as devices expand.

Architecture of an Embedded System

ASIC: Application Specific Integrated Circuit: for specific functions like motor control, data modulation etc.

Microcontroller(μC): A family of microprocessors

Real Time Operating System (RTOS): contains all the software for the system control and user interface

Controller Process: The overall control algorithm for the external process. It also provides timing and control for the various units inside the embedded system.
Digital Signal Processor (DSP) a typical family of microprocessors

DSP assembly code: code for DSP stored in program memory

Dual Ported Memory: Data Memory accessible by two processors at the same time

CODEC: Compressor/Decompressor of the data

User Interface Process: The part of the RTOS that runs the software for User Interface activities

Controller Process: The part of the RTOS that runs the software for Timing and Control amongst the various units of the embedded system



The above architecture represents a hypothetical Embedded System (we will see more realistic ones in subsequent examples). More than one microprocessor (2 DSPs and 1 μC) are employed here to carry out different tasks. As we will learn later, the μC is generally meant for simpler and slower jobs such as carrying out a Proportional Integral (PI) control action or interpreting the user commands etc. The DSP is a more heavy duty processor capable of doing real time signal processing and control. Both the DSPs along with their operating systems and codes are independent of each other. They share the same memory without interfering with each other. This kind of memory is known as dual ported memory or two-way post-box memory. The Real Time Operating System (RTOS) controls the timing requirement of all the devices. It executes the over all control algorithm of the process while diverting more complex tasks to the DSPs. It also specifically controls the μC for the necessary user interactivity. The ASICs are specialized units capable of specialized functions such as motor control, voice encoding, modulation/demodulation (MODEM) action etc. They can be digital, analog or mixed signal VLSI circuits. CODECs are generally used for interfacing low power serial Analog-to-Digital Converters (ADCs). The analog signals from the controlled process can be monitored through an ADC interfaced through this CODEC.

Common Architecture of Real Time Embedded Systems -part1

The red and grey spheres in Fig.1.2 represent interface standards. When a system is assembled it starts with some chassis or a single subsystem. Subsequently subsystems are added onto it to make it a complete system.
Let us take the example of a Desktop Computer. Though not an Embedded System it can give us a nice example of assembling a system from its subsystems.
You can start assembling a desktop computer (Fig.1.3) starting with the chassis and then take the SMPS (switched mode power supply), motherboard, followed by hard disk drive, CDROM drive, Graphic Cards, Ethernet Cards etc. Each of these subsystems consists of several components e.g. Application Specific Integrated Circuits (ASICs), microprocessors, Analog as well as Digital VLSI circuits, Miniature Motor and its control electronics, Multilevel Power supply units crystal clock generators, Surface mounted capacitors and resistors etc. In the end you close the chassis and connect Keyboard, Mouse, Speakers, Visual Display Units, Ethernet Cable, Microphone, Camera etc fitting them into certain well-defined sockets.

Common Architecture of Real Time Embedded Systems

Unlike general purpose computers a generic architecture can not be defined for a Real Time Embedded Systems. There are as many architecture as the number of manufacturers. Generalizing them would severely dilute the soul purpose of embodiment and specialization.
However for the sake of our understanding we can discuss some common form of systems at the block diagram level. Any system can hierarchically divided into subsystems. Each sub-system may be further segregated into smaller systems. And each of these smaller systems may consist of some discrete parts. This is called Hardware configuration.
Some of these parts may be programmable and therefore must have some place to keep these programs. In RTES the on-chip or on-board non-volatile memory does keep these programs. These programs are the part of the Real Time Operating System (RTOS) and continually run as long as the gadget is receiving power. A part of the RTOS also executes itself in the stand-by mode while taking a very little power from the battery. This is also called the sleep mode of the system.
Both the hardware and software coexist in a coherent manner. Tasks which can be both carried out by software and hardware affect the design process of the system. For example a multiplication action may be done by hardware or it can be done by software by repeated additions. Hardware based multiplication improves the speed at the cost of increased complexity of the arithmetic logic unit (ALU) of the embedded processor. On the other hand software based multiplication is slower but the ALU is simpler to design. These are some of the conflicting requirements which need to be resolved on the requirements as imposed by the overall system. This is known as Hardware-Software Codesign or simply Codesign.
Let us treat both the hardware and the imbibed software in the same spirit and treat them as systems or subsystems. Later on we shall know where to put them together and how. Thus we can now draw a hierarchical block diagram representation of the whole system as follows:
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