数字逻辑chapter3-1.ppt

Chapter 3-i: Combinational Logic Circuits (3.1--3.5) CSE221: Logic Design, Spring 2003 Chapter 2-i: Combinational Logic Circuits (Sections 2.1-- 2.5) 3.1 DEFINITION OF COMBINATIONAL LOGIC Logic circuits without feedback from output to the input, constructed from a functionally complete gate set, are said to be combinational. 3.1.1 Problem Statements to Truth Tables Before any combinational logic can be designed it must be defined. Proper statement of a problem is the most important part of any digital design task. Figure 3.2 illustrates the sequence of design tasks in a general way 3.1.1 Problem Statements to Truth Tables E.g : Conveyor system ( Ex. 3.1) 3.1.1 Problem Statements to Truth Tables E.g : Conveyor system (Ex. 3.1) 3.1.1 Problem Statements to Truth Tables E.g. A NASA system (Ex. 3.2) 3.1.1 Problem Statements to Truth Tables E.g. A NASA system(Ex. 3.2) 3.1.1 Problem Statements to Truth Tables E.g. Conveyor system( ex. 3.4) 3.1.1 Problem Statements to Truth Tables E.g. Conveyor system( ex. 3.4) 3.1.1 Problem Statements to Truth Tables In summary, the process of converting a verbal problem statement into a truth table involves the following steps(总之，将一个书面问题描述转换成真值表的过程包含以下几步): Determine the input variables and output variables that are involved (确定所包含的输入、输出变量) Assign mnemonic or letter or number symbols to each variable(为每个变量分配助记符或字母或标记) determine the size of the truth table(确定真值表的大小); how many input combinations exist: 2x=y where x=number of input variables and y=number of combinations. Construct a truth table containing all of the input variable combinations(构造一个包含所有输入变量组合的真值表) By careful reading of the problem statement determine the combinations of inputs that cause a given output to be true(仔细研究问题描述，确定使给定输出为真的输入组合) 3.1.1 PROBLEM STATEMENTS TO TRUTH TABLES Exe: Design a truth table to indicate a majority of three inputs is true 3.1.2 Deriving Switching Equations导出开关方程 Logic can be described in several ways Truth table L