Combinational Logic Design Chapter 0 Combinational Logic Design
Block Diagram of Combinational Circuit
Combinational Circuit Consists of logic gates whose outputs at any time are determined by logic operations on the input values. Performs an operation that can be specified logically by a set of Boolean equations. Consists of input variables, output variables, logic gates, & interconnections
Combinational Circuit For n input variables, there are 2n possible binary input combinations. For each binary combination of the input variables, there is one possible binary value on each output. Can be specified by a truth table that that lists the output values for each combination of the input variables.
Sequential circuit The outputs of a SC depend not only on the current input values, but also on past inputs. The behavior of the circuit must be specified by a sequence in time of inputs and internal stored bit values
Sequential circuit The outputs of a SC depend not only on the current input values, but also on past inputs. The behavior of the circuit must be specified by a sequence in time of inputs and internal stored bit values
Design Topics Modern digital design Some important concepts Techniques & tools for complex circuits & systems design Some important concepts CAD tools, HDL, logic synthesis, design hierarchy, top-down design
Design Hierarchy Approach to design complex digital system E.g a VLSI circuit – contains several tens of millions of gates “Divide & Conquer” approach Break up circuit into smaller pieces / blocks Interconnect the blocks to form complete circuit
Approach “Hierarchical design” Resulting symbols & schematics “hierarchy” Steps in blocks division “hierarchy levels”
Example : 9 – input odd function
Design Hierarchy Concepts Reduces complexity of schematic diagram Reuse of blocks One block is used multiple times Cascaded “Functional blocks”
Diagrams Representing Hierarchy
Top – down Design Describe circuit function by text, including constraints on cost, performance & reliability Divide circuit into hierarchy levels
Bottom – up Design Used to design certain portions of big design Starts with design of functional blocks So that functional blocks can be reused multiple times
Computer Aided Design (CAD) Schematic capture tools Drawing of blocks & interconnections at all hierarchy levels Libraries of graphic symbols Ready – made primitives & functional blocks design
Libraries in Max Plus II Prim Basic Fbs Mf Mega_lpm Edif Own libs from self-created symbols
CAD Logic simulator To verify behavior & timing of each hierarchy blocks / complete circuit Apply a set of inputs to determine the outputs
Hardware Description Language Important to design complex circuits Similar to other programming languages Describe hardware structures & behavior
Power of HDL Used to represent Can use hierarchical design approach Schematic information Boolean equations Truth tables Can use hierarchical design approach High – level description can be partitioned into lower – level descriptions
HDLs are portable across CAD design tools Logic synthesis An HDL description written at an intermediate level called Register Transfer Language (RTL) It can be converted into interconnection of primitive components that forms the circuit HDLs are portable across CAD design tools
HDL Languages VHDL Verilog HDL AHDL Very High – Speed ICs Developed by DoD, US IEEE standard Verilog HDL AHDL
The End