Chapter1 Introduction

Types of Logic Circuits

• Logic circuits are classified into two types, “combinational” and “sequential”
• Combinational logic circuit is one whose outputs depend only on its current inputs.
• The output of sequential logic circuits not only depends on current inputs, but also on the past sequence of inputs.

Analog versus Digital

• Analog devices and systems process time-varying signals that can take on any value across a continuous range.
• Digital systems use digital circuits that process digital signals which can take on one of two values

0 and 1 (digits of the binary number system)
LOW and HIGH
FALSE and TRUE

• Digital Systems

1. Digital computers represent the most common digital systems.
2. Once-analog Systems that have now “gone digital”: Audio recorder (CDs, mp3), Phone system,Automobile engine control ,Movie effects,Still and video cameras….
   

Logic Circuits and Gates

• Gates: the most basic digital devices, have function of allowing or retarding (“gating”) the flow of digital information.
• Three most important kinds of gates :
  
• Flip-flop: a device that stores either a 0 or 1. The state can be changed only at certain state times determined by a “clock” input .

Software Aspects of Digital Design

• HDL(Hardware description language)
  1. “Programming” languages that allow the characteristics of a hardware module to be described.
  2. Many elements are analogous to “Software programming” languages.
  3. Three preeminent HDL’s are Verilog, VHDL and ABEL.
• HDL Compilers,simulators,and synthesis tools
• Test benches
• Timing analyzers and verifiers
• Word processors

Integrated Circuits

• An Integrated circuit (IC) is a number of logic gated fabricated on a single silicon chip.
• Large ICs with tens of millions of transistors: half an inch or more on a side
• die----IC chip,has pads around its periphery;
• “IC” or “chip” : the combination of a silicon die and its package
• In this book, “IC”: a packaged die
• ICs can be classified according to how many gates they contain as follows:
  1. Small-Scale Integration (SSI): Contain 1 to 20 gates.
  2. Medium-Scale Integration (MSI): Contain 20 to 200 gates.
     Examples: Registers, decoders, counters.
  3. Large-Scale Integration (LSI): Contain 200 to 200,000 gates.
     Include small memories, some microprocessors, programmable logic devices.
  4. Very Large-Scale Integration (VLSI): Usually stated in terms of transistors count rather than gate count. with over 1,000,000 transistors.
     Includes most microprocessors and memories.

CMOS Logic Circuits

• The symbols and characteristics of MOS transistors
  
  
  

Programmable Device

• PLAs: programmable logic arrays ,the first programmable logic devices, contained a two-level structure of AND and OR gates array with user-programmable connections.
• PAL: programmable array logic ,enhance PLA structure and reduce the costs, PLDs
• CPLD: complex PLD, merely a collection of multiple PLDs and an interconnection structure, all on the same chip, can be scaled to larger sizes.
• FPGA: field-programmable gate arrays, contains a much larger number of smaller individual logic blocks, provides a large, distributed interconnection structure that dominates the entire chip.
• both chip architecture
• each approach support a style of design in which products can be moved from design concept to prototype and production in a very short time.

Important words

Combinational Circuit ——组合逻辑电路
Sequential Circuit ——时序逻辑电路
Hardware Description Language ——硬件描述语言
Programmable Logic Device ——可编程逻辑器件
Flip-flop ——触发器
ASIC ——特定用途集成电路
FPGA ——现场可编程门阵列
CPLD ——复杂可编程逻辑器件