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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