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Introduction to Digital Design

Introduction to Digital Design. Text Book. Digital Design by - John F. Wakerly www.ddpp.com - you will find some solutions at this site. www.xilinx.com - Xlinix Web site. Digital Design. Digital Design is also know as “Logical Design”. The purpose is to build Digital Systems.

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Introduction to Digital Design

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  1. Introduction to Digital Design

  2. Text Book • Digital Design by - John F. Wakerly • www.ddpp.com - you will find some solutions at this site. • www.xilinx.com - Xlinix Web site

  3. Digital Design • Digital Design is also know as “Logical Design”. • The purpose is to build Digital Systems.

  4. Analog versus Digital • Analog systems process time-varying signals that can take on any value across a continuous range of voltages (in electrical/electronics systems). • Digital systems process time-varying signals that can take on only one of two discrete values of voltages (in electrical/electronics systems). • Discrete values are called 1 and 0 (ON and OFF, HIGH and LOW, TRUE and FALSE, etc.)

  5. DigitalRevolution • Digital systems started back in 1940s. • Digital systems cover all areas of life: • still pictures • digital video • digital audio • telephone • traffic lights • Animation

  6. Benefits of Digital over Analog • Reproducibility • Not effected by noise means quality • Ease of design • Data protection • Programmable • Speed • Economy

  7. Digital Devices • Gates • Flip-Flops • PLDs • FPGAs

  8. Gates • The most basic digital devices are called gates. • Gates got their name from their function of allowing or blocking (gating) the flow of digital information. • A gate has one or more inputs and produces an output depending on the input(s). • A gate is called a combinational circuit. • Three most important gates are: AND, OR, NOT

  9. Digital Logic • Binary system -- 0 & 1, LOW & HIGH, negated and asserted. • Basic building blocks -- AND, OR, NOT

  10. AND, OR, NOT Gates

  11. Electronic Aspects of Digital Design • How we represent digital information in electronic devices? • By discrete voltages.

  12. What is the Basic Digital Elementin Electronics? a Switch

  13. Using Switch to represent digital information

  14. Digital Abstraction • It is difficult to make ideal switches means a switch is completely ON or completely OFF. • So, we impose some rules that allow analog behavior to be ignored in most cases, so circuits can be modeled as if they really did process 0s and 1s, known as digital abstraction. • Digital abstraction allows us to associate a noise margin with each logic values (0 and 1).

  15. Real Switches to represent digital information 5v 5v 1k Output Output 5v 4.5v 10k

  16. Logic levels • Undefined regionis inherent • digital, not analog • amplification, weak => strong • Switching threshold varies with voltage, temp, process, phase of the moon • need “noise margin” • The more you push the technology, the more “analog” it becomes. • Logic voltage levels decreasing with new processors. • 5 -> 3.3 -> 2.5 -> 1.8 V

  17. MOS Transistors Voltage-controlled resistance PMOS NMOS

  18. CMOS Inverter

  19. Switch model

  20. Alternate transistor symbols

  21. CMOS Gate Characteristics • No DC current flow into MOS gate terminal • However gate has capacitance ==> current required for switching (CV2f power) • No current in output structure, except during switching • Both transistors partially on • Power consumption related to frequency • Slow input-signal rise times ==> more power • Symmetric output structure ==> equally strong drive in LOW and HIGH states

  22. Flip-flops • A device that stores either a 0 or 1. • Stored value can be changed only at certain times determined by a clock input. • New value depend on the current state and it’s control inputs • A digital circuit that contains filp-flops is called a sequential circuit

  23. Flip-flops S-R latch symbols D flip-flop J-K flip-flops

  24. Integrated Circuits • A collection of one or more gates fabricated on a single silicon chip is called an integrated circuit (IC). • ICs were classified by size: • SSI - small scale integration - 1~20 gates • MSI - medium scale integration - 20~200 gates • LSI - large scale integration - 200~200,000 gates • VLSI - very large scale integration - over 1M transistors • Pentium-III - 40 million transistors

  25. DIP Packages

  26. Gates in ICs

  27. Programmable Logic Devices • PLDs allow the function to be programmed into them after they are manufactured. • Complex PLDs (CPLD) are a collection of PLDs on the same chip. • Another programmable logic chip is FPGA - field-programmable gate arrays.

  28. CPLDs and FPGAs CPLD FPGA

  29. Application Specific Ics (ASICs) • Chips designed for a particular application are called semicustom ICs or application-specific ICs (ASICs). • ASICs generally reduce the total component and manufacturing cost of a product by reducing chip count, physical size, and power consumption, and they often provide higher performance. • But costly if not produced in bulk.

  30. Printed-Circuit Boards • An IC is normally mounted on a printed-circuit board (PCB) that connects it to other ICs in a system. • Individual wire connection or traces can be as narrow as 4 mils with 4 mils spacing (one-thousandth of an inch) • Now a days, most of the components use surface mount technology. • They are normally layered.

  31. Software Aspects of Digital Design • Today software tools are an essential part of digital design. • Software tools improve productivity, correctness and quality of designs • Software tools are: • Schematic entry • HDL (Hardware Description Language) Editors • Simulators - to verify the behaviour of the design • Synthesis tools - circuit design • Timing analyzers and verifiers

  32. Digital Design Levels • the lowest level of design is device physics and IC manufacturing processes. • design at the transistor level • level of functional building blocks • level of logic design using HDLs • computer design and overall system design.

  33. Different Design Levels Consider a simple design example: Build a multiplexer with two data inputs A and B, a control input S, and an output Z. Switch model for the example multiplexer

  34. Designing at the transistor level • Transistor-levelcircuit diagrams • Gate symbols (for simple elements)

  35. Logic designusing Truth tables

  36. Logic designusing boolean algebra Equations: Z = S¢ × A+ S × B • Logic diagrams

  37. Prepackaged building blocks, e.g. multiplexer

  38. Various hardware description languages ABEL VHDL We’ll start with gates and work our way up

  39. Name of the program module the type of PLD pin numbers ABEL statement to achieve the multiplexer Standard library and a set of definitions Inputs and outputs functions behaviour

  40. Structural VHDL program for the multiplexer

  41. Summery • Design to minimize cost. • Rule of thumb is to minimize the number of ICs. • Though PLDs costs more but uses less PCB area. • Unless mass production avoid ASIC design. • Design to solve real life problems.

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