ELECTRICALELECTRICAL
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`ENGINEER’SENGINEER’S
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`REFERENCEREFERENCE
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`BOOKBOOK
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`Electrical
`En ineer’s
`Re erence
`Book
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`Sixteenth edition
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`M. A. Laughton CEng.. FIEE
`
`D. J. Warne cEng., FIEE
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`lrnrnttuction
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`tsra
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`15.1
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`Introduction
`
`Digital systems are used to process discrete cletncnts of
`inforrnation. They are built from digital electronic circuits
`that process discrete electrical signals using simple logic and
`arithmetic operations. A digital electronic system can also
`be used to hold or store discrete elements of information
`and this gives the system a memory capability. The ability
`to store information or data and to process the data by
`logical or aritlunctic operations is central to the design of
`nearly all digital inforrnatr'on—processing systems including
`digital computers. The function of a digital system is deter-
`mined by the sequence of operations that are performed on
`the infonnarion or data being processed. A digital system
`can be classified by the way in which its sequence of oper-
`ations is implemented.
`A digital system is considered to be hardwired ii’ the
`sequence of operations is governed by the physical intercon-
`nection of the digital processing elements. For example. in
`hardwired logic systems the physical
`interconnections of
`the elements govern the routes by which data flows between
`the processing elements and thus the sequence ofproccssing
`operations perfonned on the data. Conventionally. a hard-
`wired system is considered to be inflexible because the
`design is specific to a particular processing function: ifthe pro-
`cessing function is changed. then the processing elements
`and their interconnections have to be altered.
`
`The flexibility of hardwired systems has been much
`improved by the introduction of programmable {i.e. config-
`urable] logic devices such as Programmable Logic Arrays
`t'PLits} and Field Programmable Gate Arrays lFPGA.sl
`that can be programmed or configured to implement an
`application-specific
`digi tal
`signal processing function.
`Flexibility has been improved further with the introduction
`of re-progratrunahle i‘.i.e. re-configurable} devices that can
`be reprogrammed easily during fast-prototype system devel-
`opment. and can be reprogrammed after a product has been
`deployed to provide enhanced features or perfonnancc.
`Progress in this area has been rapid and the latest genera-
`tion ofre-progranunable FPO.-it device can be configured with
`a wide variety of communication interfaces. This opens
`the possibility of using advanced communication technology.
`such as the Internet. to re-program or to-configure a remote
`hardwired system.
`A digital system is considered to be genuinely program-
`mable if a prescriptive program of instructions 1[i.e. soft-
`ware} can be used to control the data—proce-ssing function
`of the system. This type of system usually incorporates a
`general-purpose processing element which is programmed
`to iinplcrrtcrrt a specific function in lit. predeterntined way.
`The coded instructions are normally stored in the memory
`part ofthe system and the program forms an integral part
`of the system. The ability to define the function of the
`digital system by programming introduces considerable fles-
`ibility into the system because the programming operation
`can talie place after the general-purpose digital elements
`have been designed. it also means that identical hardware
`designs can he used in a number t:-l'tlit'l'er1erIt applications. the
`system being tailored to the individual tasks by the applica-
`tions program. A wide range of simple llited—function
`prograrnrnable systems,
`such as
`sequencers and micro-
`programmed controllers. are used as controllers in etnbeddcd
`electronic systems. in this type of application the sequence
`of
`instructions
`is
`usually held in
`read-only memory
`(to. firmware} which increases the robustness of the system.
`The digital oontputcr is a very important class of stored
`program system. The computer or microprocessor is distin-
`guished by the fact that its processing function depends on
`
`both the prescriptive sequence of coded instructions and the
`value of the data being prooemed. In effect, the program
`prtsscrilacs a number of possible scquencu of operations
`and the conditions under which they may be carried out.
`The computer. under program control. assesses the data
`and determines which specific sequence of instruction is to
`be executed. It is the ability of the computer to take into
`account the value of the data being processed. when taking
`decisions about
`the type of processing to be perfonnetl.
`which makes the computer such a significant and powerful
`information-processing device.
`All three forms of digital electronic system find wide-
`spread application. Traditionally hardwired logic has been
`used extensively to provide the control and interface logic
`for more complex digital components such as micropro-
`cessors and other very large scale integration {‘v'LSl}dev1ioes.
`it is also used in the design of high-speed signal processing
`circuits for FPGA. implementation. lncreasingly. hardwired
`logic is used to provide the interface circuits between the
`main functional components within a complex FPGA.
`Where llesihility is required, it is common to use reconfigur-
`ahle systems particularly in more cnrriplctt applications.
`Programmable systems are used in an extremely large
`range of applications. The simpler fined-function program-
`mable systems are often used in repetitive tasks such as
`input scanning and data acquisition. They art: also used in
`mass-produced products and as components of larger sys-
`tems such as telephony equipment. However, the continu-
`ally increasing computational power of the microprocessor
`and its derivatives. such as digital signal processing (D5?)
`devices or powerful reduced instruction set {RISE} proces-
`sors, has led to many of these applications being designed
`rising fully programmable digital systems.
`in addition,
`commercial oil-the-shell‘ (CUTS) microprocessors are com-
`monly used in both stand-alone and ernbeddetl systems.
`Such systems are providing eoonnntic solutions to design
`problems in an increasingly wide range of application.
`The increase in size of VLSI logic circuits has led to a new
`generation of reconfigurable devices that are large enough
`to contain a complete digital processing system within a
`t:l'lip_ called System on Chip {Set} devices. An Soil: device
`can be configured to include an embedded digital RISC
`processor, rnamnry. communication interfaces, cloclt man-
`agement.
`application-specific digital
`signal processing
`{hardwired logic functions}, and appropriate intental inter-
`faces and data buses. This allows the designer to partition a
`design into those pans that will be implemented as software
`executing on the embedded processor and those parts that
`will be implemented in hardwan: as high-speed application-
`specific logic circuits. This design approach.
`ltnowri as
`co—design or co-ware. has the significant advantage that
`established and high-ptrrlonnance parts of the design can
`be committed to application-specific hardware. and more
`adventurous parts of the design or low-speed functions can
`be committed to easily changed software. This minimises
`risli.
`facilitates time-to-marltel which gives competitive
`advantage, and provides a good path to post—deploymenl
`upgrades of the system's capahilitim and performance.
`
`15.2 Structured design of programmable
`logic systems
`
`The design ofan application-spot:il'rc digital system typically
`involves the so-called ‘top-down‘ approach and starts from
`a specification which includes a statement of the problem
`and the identification of the principal functional parts of
`
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