1. Combinational Logic Design
Chapter 0
Combinational Logic Design

2. Block Diagram of Combinational Circuit

3. 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

4. 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.

5. 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

6. 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

7. 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

8. 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

9. Approach  “Hierarchical design”
Resulting symbols & schematics  “hierarchy”
Steps in blocks division  “hierarchy levels”

10. Example : 9 – input odd function

11. Design Hierarchy Concepts
Reduces complexity of schematic diagram
Reuse of blocks
One block is used multiple times
Cascaded
“Functional blocks”

12. Diagrams Representing Hierarchy

13. Top – down Design
Describe circuit function by text, including constraints on cost, performance & reliability
Divide circuit into hierarchy levels

14. 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

15. 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

16. Libraries in Max Plus II
Prim
Basic Fbs Mf
Mega_lpm
Edif
Own libs from self-created symbols

17. CAD
Logic simulator
To verify behavior & timing of each hierarchy blocks / complete circuit
Apply a set of inputs to determine the outputs

18. Hardware Description Language
Important to design complex circuits
Similar to other programming languages
Describe hardware structures & behavior

19. 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

20. 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

21. HDL Languages VHDL Verilog HDL AHDL Very High – Speed ICs
Developed by DoD, US
IEEE standard
Verilog HDL
AHDL

22. The End