In this chapter we present basic definitions that relate to

Transcription

1 01_1_16.fm Page 1 Thursday, January 13, :22 AM C H A P T E R 1 Structured Design Concepts In this chapter we present basic definitions that relate to the design process. It is necessary to introduce them now so that other concepts can be explained. The reader should study them carefully in order to comprehend material introduced later. It will also be useful to revisit this chapter as one proceeds through the text since the full meaning of the terms will only become clear through use and example. 1.1 THE ABSTRACTION HIERARCHY In this section we present the abstraction hierarchy employed by digital designers. Abstraction can be expressed in the following two domains: Structural domain. A domain in which a component is described in terms of an interconnection of more primitive components. Behavioral domain. A domain in which a component is described by defining its input/ output response. Figure 1.1 shows structural and behavioral descriptions for a logic circuit, which detects two or more consecutive 1 s or two or more consecutive 0 s on its input X. The structural description is an interconnection of gate and flip-flop primitives. The behavioral description is expressed textually in a hardware description language (HDL). An abstraction hierarchy can be defined as follows: Abstraction hierarchy. A set of interrelated representation levels that allow a system to be represented in varying amounts of detail. Figure 1.2 shows a picture of a typical abstraction hierarchy. For each level i in the hierarchy there exists a transformation to level i+1. The level of detail usually increases monotonically as one moves down in the hierarchy. 1

2 01_1_16.fm Page 2 Thursday, January 13, :22 AM 2 Chapter 1 Structured Design Concepts Structural Description entity TWO_CONSECUTIVE is port(clk,r,x: in BIT;Z: out BIT); end TWO_CONSECUTIVE; architecture DATAFLOW of TWO_CONSECUTIVE is signal Y1,Y0: BIT; begin STATE: block((clk = '1'and not CLK'STABLE) or R = '0') begin Y1 <= guarded '0' when R = '0' else X; Y0 <= guarded '0' when R = '0' else '1'; end block STATE; Z <= Y0 and ((not Y1 and not X) or (Y1 and X)); end DATAFLOW; Behavorial Description Figure 1.1 Structural and behavioral descriptions of a sample circuit. V Figure 1.2 Abstraction hierarchy.

3 01_1_16.fm Page 3 Thursday, January 13, :22 AM The Abstraction Hierarchy 3 Table 1.1 Design abstraction hierarchy. Level of Detail Behavioral Domain Representation Structural Domain Primitive System Performance specifications (English) Computer, disk, unit, radar Chip Algorithm Microprocessor, RAM, ROM, UART, parallel port Register Data flow Register, ALU, COUNTER, MUX, ROM Gate Boolean equations AND, OR, XOR, FF Circuit Differential equations Transistor, R, L, C Layout/silicon Equations of electron and hole motion Geometrical shapes This book uses a design hierarchy that has six levels: silicon, circuit, gate, register, chip, and system. Table 1.1 illustrates this hierarchy. The silicon level is the lowest level in the hierarchy, the system level the highest. One can represent a design at any of these levels. As one moves down in the hierarchy, one is closer to a physical implementation and the design representation is less abstract. Correspondingly, the amount of detail required to represent a design increases as one descends in the hierarchy. As will be emphasized throughout the text, it is important that a particular design activity be carried out at a level which has sufficient but not excessive detail. A level with insufficient detail yields inaccurate results; whereas, a level with excessive detail can make the design activity too expensive. Table 1.1 shows the nature of this hierarchy in terms of the structural primitives and behavioral representation for each level. Structural primitives are interconnected to form a structural model at a given level. Figure 1.3 shows examples of the structural primitives at each level. The behavioral representation is the textual or pictorial form of a device s I/O response at that level. At the lowest level, the silicon level, the basic primitives are geometric shapes that represent areas of diffusion, polysilicon, and metal on the silicon surface. The interconnection of these patterns models the fabrication process from the designer s point of view. Behavioral description at this level are the physical equations that describe electron and hole motion in electrical materials. At the next level up, the circuit level, the representation is that of an interconnection of traditional passive and active electrical circuit elements: resistors, capacitors, and bipolar and MOS transistors. The interconnection of components is used to model circuit behavior in terms of voltage and current. The behavioral content at this level can be expressed in terms of differential equations. The third level up, the gate level, has traditionally been the major design level for digital devices. The basic primitives are the AND, OR, and INVERT operators and various types of flip-flops. Interconnection of these primitives forms combinational and sequential logic circuits. Boolean equations define the behavior at this level. The level above the gate level is the register level. Here the basic primitives are such things as registers, counters, multiplexers, and ALUs. These primitives are sometimes referred to as functional blocks. They also correspond to VLSI design macros. Thus, this level is also referred to as the functional or macro level. Although the register-level primitives can be expressed in

4 01_1_16.fm Page 4 Thursday, January 13, :22 AM 4 Chapter 1 Structured Design Concepts V+ G S P D Vin G D Vout S SILICON LEVEL Inverter CIRCUIT LEVEL S Q Select REG R Q CLK A MUX REG CLK B S R Q Q INC GATE LEVEL REGISTER LEVEL RAM 8 Microprocessor Parallel Port USART 8 8 A/B Computer IMU RADAR Interrupt Controller C/D CHIP LEVEL SYSTEM LEVEL Figure 1.3 Examples of structural domain primitives. terms of an interconnection of gates, when working at this level, one does not take this viewpoint. Register-level primitives are expressed in terms of truth tables and state tables; thus, these two forms can be used to represent the behavioral content at this level. Behavioral descriptions at this level are termed data flow, i.e., they reflect the way data is actually distributed in a real implementation. In this book, we will show how these data flow descriptions can be implemented in a hardware description language. The level above the register level is the chip level. At this level, the structural primitives are such things as miroprocessors, memories, serial ports, parallel ports, and interrupt controllers. Although chip boundaries are typically the model boundaries, other situations are possible.

5 01_1_16.fm Page 5 Thursday, January 13, :22 AM Textual vs. Pictorial Representations 5 For example, collections of chips, which together form a single functional unit, can be modeled as a single entity. Or alternatively, sections of a chip design could be modeled as separate entities during this design phase. The key aspect is that a large block of logic is to be represented in which long and frequently convergent data paths from inputs to outputs must be modeled. In the behavioral domain, at each level in the hierarchy, the primitives are behavioral descriptions which are not structural models that are created from more basic primitives. Each primitive is a distinct model entity. Thus, if a serial I/O port (UART) is to be modeled, the model is not created by interconnecting simpler functional models of such things as registers and counters the UART itself is the basic model entity. Behavioral domain models are important to system manufacturers who buy a chip from another manufacturer but have no knowledge of its proprietary gate-level structure. Chip level models of complicated circuits are viewed as Intellectual Property (IP) and are frequently sold by one company to another. The behavioral content of a chiplevel model is defined in terms of the I/O response of the device the algorithm that the chip implements. In this book, a hardware description language is used to code these algorithmic descriptions. The top level in the structural hierarchy is the system level. The primitive elements of this level are computers, bus interface units, disk units, radar units, etc. The behavioral content of this level is frequently expressed in terms of performance specifications, which give, for example, the MIPS rating (million instructions per second) of a processor or the bandwidth in bits per second of a bus, or use a statistical model to determine the percent utilization of a part of the system. If a deterministic model is used at this level, it employs very high-level data types to represent information being passed between systems. For example, if a radar system were modeled, information of type "frequency" would be passed between units in the system. 1.2 TEXTUAL VS. PICTORIAL REPRESENTATIONS Design representations can be either pictorial or textual. Figure 1.1 shows a pictorial logic schematic for the circuit, which detects two consecutive 1 s or two consecutive 0 s (referred to as TWO_CON). Figure 1.4 shows a block diagram, a state diagram, a timing diagram, a state table, state assignment, and truth tables (Kmaps) for the same circuit. These are all examples of pictorial forms. Common textual methods of representation are natural languages (e.g., English), equations (e.g., Boolean or differential), and computer languages. In this text we use a specialized computer language called a hardware description language, which can be defined as follows: Hardware description language. A high-level programming language with specialized constructs for modeling hardware. The behavioral description in Figure 1.1 illustrates a hardware description language textual description for TWO_CON. An important consideration in developing a design process is whether to use pictures or text. Historically, pictures have been the preferred representation for digital design, i.e., block, timing, and logic diagrams (schematics) were the principal forms of representation. However, with the advent of hardware description languages, textual design descriptions have gained in popularity. From inspecting Figures 1.1, 1.3, 1.4, and Table 1.1, one can see that structural

6 01_1_16.fm Page 6 Thursday, January 13, :22 AM 6 Chapter 1 Structured Design Concepts Block Diagram State Diagram Timing Diagram X 0 1 S0 S1/0 S2/0 S1 S1/1 S2/0 S2 S1/0 S2/1 State Table code X y1y Y1 code y1y0 S0 00 S1 01 S2 11 State Assignment code X y1y Y2 Truth Tables (Kmaps) code X y1y Z Figure 1.4 Pictorial representations of logic circuits. descriptions are primarily pictorial, and that behavioral representations are primarily textual. Some exceptions to this classification scheme are state tables, state diagrams, and timing diagrams, which are pictorial but represent behavior. As to the general question of when pictures should be employed or when text should be used, one can make the following generalization: Text is better for representing complex behavior; pictures are better for illustrating interrelationships. Excessive use of either text or pictures results in a loss of perspective, i.e., one cannot see the forest for the trees. Thus, real design systems balance the use of text and pictures this is the approach we use in this text.

7 01_1_16.fm Page 7 Thursday, January 13, :22 AM Types of Behavioral Descriptions 7 Architecture ALGORITHMIC of TWO_CONSECUTIVE is type STATE is (S0,S1,S2); signal Q: STATE := S0; begin process(r,x,clk,q) begin if (R'EVENT and R = '0') then --reset event Q <= S0; elsif (CLK'EVENT and CLK = '1') then --clock event if X = '0' then Q <= S1; else Q <= S2; end if; end if; If Q'EVENT or X'EVENT then --output function if (Q=S1 and X='0') or (Q=S2 and X='1') then A <= '1'; else z <= '0'; end if; end if; end process; and ALGORITHMIC; Figure 1.5 Algorithmic description of the example circuit in Figure TYPES OF BEHAVIORAL DESCRIPTIONS Behavioral descriptions in hardware description languages are frequently divided into two types: algorithmic and data flow. Algorithmic. A behavioral description in which the procedure defining the I/O response is not meant to imply any particular physical implementation. Thus, an algorithmic description is merely a procedure or program written to model the behavior of a device, to check that it is performing the correct function, without worrying about how it is to be built. Data flow. A behavioral description in which the data dependencies in the description match those in a real implementation. Data flow descriptions show how data moves between registers. Figure 1.1 gave a data flow description for the circuit which detects two or more consecutive 1 s or two or more consecutive 0 s. Figure 1.5 shows an algorithmic description for the same circuit.

8 01_1_16.fm Page 8 Thursday, January 13, :22 AM 8 Chapter 1 Structured Design Concepts Figure 1.6 A typical design track. 1.4 DESIGN PROCESS We will describe a structured design process in this text beginning with the following definitions. Design. A series of transformations from one representation of a system to another until a representation exists that can be fabricated. Our approach to design involves synthesis, which the dictionary defines as the combining of abstract entities into a single or unified entity. Thus, to synthesize is to put something together. For our context we make a more specialized definition. Synthesis. The process of transforming one representation in the design abstraction hierarchy into another representation. Each step in the design process is referred to as a synthesis step. Since the design process usually starts in the behavioral domain at a high level and ends in the structural domain at a low level, each synthesis step is a transformation from level i to level j with i j. Therefore, what we are putting together is a representation of a design at level j. The representation at level i is used as a guide in the synthesis process, in that the implementation at level j must implement the same function as that at level i. In most cases, j=i+1, or j=i, i.e., either the levels are adjacent, or the transformation is from the behavioral domain to the structural domain at the same level. However, there are situations where levels are skipped in the synthesis process.

9 01_1_16.fm Page 9 Thursday, January 13, :22 AM Structural Design Decomposition 9 The design cycle consists of a series of transformations (synthesis steps). Common synthesis steps include: 1. Transformation from English to an algorithmic representation (natural language synthesis). 2. Translation from an algorithmic representation to a data flow representation (algorithmic synthesis) or to a gate level representation (referred to in the industry as behavioral synthesis). Note: In this second case one moves from the behavioral to the structural domain. 3. Translation from data flow representation to a structural logic gate representation (logic synthesis). Note: In this process one also moves from the behavioral to the structural domain and skips the register level. 4. Translation from logic gate representation to layout representation (layout synthesis). In this synthesis step the circuit level is skipped. This completes the synthesis process since the layout information can be fabricated. The complete design cycle is sometimes referred to as design synthesis. Figure 1.6 shows a typical design track through the design hierarchy. The track begins in the behavioral domain and descends through the system and chip levels to the register level. At the register level the transformation is made from a behavioral data flow representation at this level to a structural gate level description. Or, the transformation can be made directly from an algorithmic representation to a structural gate level description. This gate-level description is then transformed to the circuit level or perhaps directly to the layout or silicon level. The design cycle steps can be carried out automatically in all stages in descending order, except the first, which is currently an active area of research. It is the purpose of this book to show students and design engineers how to carry out the steps themselves, and therefore, understand which functions are actually performed by automatic synthesis programs. Also, in some engineering organizations, manual transformations are still the only means employed. 1.5 STRUCTURAL DESIGN DECOMPOSITION The structural form of the design hierarchy implies a design decomposition process. This is because at any level we choose, the system model is composed by interconnection of the primitives defined for that level. In the structural domain, primitives are defined in terms of interconnections of primitives at the next lower level. Thus, as shown in Figure 1.7 a design can be represented as a tree, with the different levels of the tree corresponding to levels in the abstraction hierarchy. Eventually, even in structural models, primitives at the leaves of the tree must be represented by behavioral models. As defined above, a behavioral model is a primitive model in which the operation of the model is specified by a procedure as opposed to redefining it in terms of other components. Since a behavioral model can exist at any level in the design hierarchy, different parts of the design can have behavior specified at different levels. In Figure 1.7(a) the design tree is full, and all behavior is, therefore, specified at the same level. In Figure 1.7(b) a design that has the form of a partial tree is shown, where behavior is specified at different levels. This situation is

10 01_1_16.fm Page 10 Thursday, January 13, :22 AM 10 Chapter 1 Structured Design Concepts Figure 1.7 Structural decomposition. encountered because one frequently wants to evaluate the relationships between system components before they have all been completely designed. For example, multilevel simulation can be used to evaluate the interaction of one component whose design has reached the gate level with other components whose design is currently still at the system level. Thus, it is not required that all system components be specified at the gate-level in order to evaluate the gate level design of a specific component. The checking is done by employing a simulation in which the behavioral content of the component models occurs at different levels in the hierarchy. In fact, it may be impossible to fully simulate a large system with all components modeled at the gate level. Such a simulation might take months to execute using current gate-level simulators. Instead, one would do multiple simulations, with different sets of components modeled at the gate level in each simulation. The other components would be modeled at the systems level. These simulations would take much less time to execute because system level simulations are more efficient. A few hundred simulations with each simulation taking several hours is preferable to one giant simulation that takes several months. Two concepts related to the design tree are those of top-down and bottom-up design. Here the word top refers to the root of the tree; whereas, bottom refers to the leaves. In top-down design, the designer begins with knowledge of only the function of the root. He or she then partitions the root into a set of lower-level primitives. Each of these lower-level primitives is then partitioned into an interconnection of primitives at still lower levels. This process continues until the leaf nodes of the design are reached. At the leaf nodes, the models are always behavioral. An important point to make about top-down design is that the partitioning is optimized at each level according to some objective criterion, e.g., cost, speed, and chip area. The partitioning is not constrained by what's available.

11 01_1_16.fm Page 11 Thursday, January 13, :22 AM The Digital Design Space 11 Figure 1.8 A typical design space. The term bottom-up design is somewhat of a misnomer in that the process of design still begins with the definition of the root, but in this case the partitioning is conditioned by what is available. Lower parts of the tree will have been designed previously, perhaps on another project, and the designer is constrained (perhaps ordered!) to use them. Top-down design usage appears to be the most ideal situation, but its disadvantage is that it produces components that are not standard, thus, increasing the cost of the design. Bottom-up designs are more economical, but they may not meet the objective performance criterion as well as top-down designs. Most real designs are a combination of top-down and bottom-up techniques. A final concept related to the hierarchy is that of a design window. By this we mean a range of levels over which the designer works in developing a design-tree structure. The VLSI chip designer s window extends over the range of silicon, circuit, gate, register, and chip levels. The computer system designer, on the other hand, is currently concerned with a window consisting of the gate, register, chip, and system levels. Thus, the designer selects the window which is appropriate to his or her design activity and works at those levels of abstraction, which provide the necessary information for that activity, without involving unnecessary detail. CAD systems used to support design should allow easy movement between levels in the design window. 1.6 THE DIGITAL DESIGN SPACE In the preceding discussion on top-down design, we said that partitioning was carried out in order to meet some objective criteria. These criteria are the major factors one has to consider in arriving at a design. These factors can be considered to be dimensions in a space. Some useful dimensions for the Digital Design Space are: speed, chip area, and cost. Figure 1.8 illustrates such a design space. Various designs have different tracks through the space as they evolve. The designer trades off one factor for another. For example, in his quest for speed he may increase the chip area and cost of a design. Figure 1.9 gives a concrete example of this. Circuits A and B implement the same logic function. Circuit A uses fewer gates (area) than Circuit B but is slower. Circuit B is faster than Circuit A but requires more gates (area). Thus we are trading off area for speed.

12 01_1_16.fm Page 12 Thursday, January 13, :22 AM 12 Chapter 1 Structured Design Concepts Figure 1.9 An example of a design space trade-off. PROBLEMS 1.1 Draw a structural model of a 16-bit ripple carry adder as an interconnection of full adders. Develop a behavioral description of the 16-bit adder in C, C ++, or JAVA. 1.2 Draw a gate-level diagram of a full adder in terms of AND, OR, and INVERT primitives. Consult books on electronics and VLSI design and develop a CMOS circuit for the adder in terms of transistors, resistors, and capacitors. Compare the two circuits as follows: a. Count the number of primitives and wires required for each representation. b. Simulate both circuits on the same computer using a logic simulator and SPICE. Drive both circuits with all possible 8-input patterns. Compare simulation times. 1.3 Track the representation of a digital system through the abstraction hierarchy. Use the small computer example in Morris Mano s book, Computer System Architecture, 2nd Edition (Englewood Cliffs: Prentice Hall, 1982). Track the description of this system through the hierarchy as follows: a. Develop a one page, single-spaced, English, system-level description of the machine. b. Using your favorite graphics package, draw a chip-level diagram of a system that uses the Mano machine, RAM memory, and an I/O logic section. c. Draw a complete register-level diagram of the Mano machine. d. Using gates and J-K flip-flop primitives, draw a complete gate-level diagram of the Program Counter. e. Select a gate from the Program Counter, and draw its CMOS circuit equivalent. f. Draw the layout for the gate you described in the previous step. Which of the representations you developed are structural and which are behavioral? 1.4 Figure 1.10 shows a structural model of a simple RC series circuit. Write a behavioral description which involves current, i(t). Ploti(t). 1.5 Shown is a behavioral description for a gate-level circuit. F = AB or CD or EF (1.1) Develop two different, structural gate-level models that implement this function. 1.6 A textual description of the fabled family relationship from N. Wirth s Algorithms + Data Structures = Programs, (Englewood Cliffs: Prentice Hall, 1976) follows: I married a widow (let s call her W) who had a grown-up daughter (call her D). My father (F), who visited us quite often, fell in love with my stepdaughter and married her. Hence, my father became my son-in-law and my stepdaughter became my

13 01_1_16.fm Page 13 Thursday, January 13, :22 AM The Digital Design Space 13 R = 1k +5 v i(t) C = 1 uf Figure 1.10 RC circuit. mother. Some months later, my wife gave birth to a son (S1), who became the brother-in-law of my father, as well as my uncle. The wife of my father, that is, my stepdaughter, also had a son (S2). Question: Am I my own grandfather? Draw a pictorial representation of the relationship by means of a directed graph in which the nodes are the persons names and the arcs are the relationships between persons, e.g., father of. Which representation is easier to understand? Can you answer the question? 1.7 Figure 1.11 is a logic diagram of a counter circuit from Morris Mano s Computer System Architecture, 2nd Edition (Englewood Cliffs: Prentice Hall, 1982). Outline a behavioral description in either C or C ++ which describes the behavior. Which description gives a more readily understandable description of the circuit function? 1.8 Figure 1.12 shows a system interface between two devices: a sending device (Device 1) and a receiving device (Device 2). The interface between the two devices consists of a DATA line and three control signals READY, VALID, and ACCEPT. A communications protocol between two asynchronous devices functions as follows: a. Device 2 asserts READY. b. Device 1 detects the positive going change on READY, and places data on the DATA line and asserts VALID. c. Device 2 detects the positive going change on VALID, copies the data and resets READY, and asserts ACCEPT. Draw a timing diagram for DATA, READY, VALID, and ACCEPT. Use arrows between signals to show how one signal transition triggers another. 1.9 Given the following algorithmic description: for I=1 to 3 loop A(I) = B(I) + C(I) D(I) = E(I) * A(I) end for; Draw a data-flow graph in which the nodes are operations (+,*) and the arcs are inputs or computed values From your own experience, describe a design synthesis process that you are familiar with. Identify distinct synthesis steps, abstraction levels, and whether a representation is behavioral or structural. Draw a design track similar to Figure Design decomposition implies what kind of VHDL model? 1.12 Compare top-down and bottom-up design in two areas (a) cost and (b) whether the design is optimum or not Explain the difference between top-down and bottom-up design. What is the main advantage and disadvantage of each approach?

14 01_1_16.fm Page 14 Thursday, January 13, :22 AM 14 Chapter 1 Structured Design Concepts Figure 1.11 Counter circuit. READY DATA ACCEPT Device 1 Device 2 DVALID Figure 1.12 Interface protocol.

15 01_1_16.fm Page 15 Thursday, January 13, :22 AM The Digital Design Space Characterize microprocessor system design and design with application-specific integrated circuits (ASICs) as being top-down or bottom-up. Explain your answer Explain how top-down and bottom-up design concepts can be used for software design Use the design of a 16-bit adder to illustrate trade-offs in the design space. a. Design a 16-bit carry ripple adder. b. Design a 16-bit carry look-ahead adder. c. Compare gate counts and delay for the two adder implementations. d. Develop a general expression for the gate count and delay for the two adder implementations that is a function of n, the input word size to the adder.

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