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EE 365

EE 365. Combinational-Circuit Synthesis. Combinational-Circuit Analysis. Combinational circuits -- outputs depend only on current inputs (not on history). Kinds of combinational analysis: exhaustive (truth table) algebraic (expressions) simulation / test bench

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EE 365

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  1. EE 365 Combinational-Circuit Synthesis

  2. Combinational-Circuit Analysis • Combinational circuits -- outputs depend only on current inputs (not on history). • Kinds of combinational analysis: • exhaustive (truth table) • algebraic (expressions) • simulation / test bench • Write functional description in HDL • Define test conditions / test vectors, including corner cases • Compare circuit output with functional description (or known-good realization) • Repeat for “random” test vectors

  3. Combinational-Circuit Design • Sometimes you can write an equation or equations directly using “logic” (the kind in your brain). • Example (alarm circuit): • Corresponding circuit:

  4. Alarm-circuit transformation • Sum-of-products form • Useful for programmable logic devices (next lec.) • “Multiply out”:

  5. AND-OR NAND-NAND Sum-of-products form

  6. OR-AND NOR-NOR Product-of-sums form

  7. row N3 N2 N1 N0 F 0 0 0 0 0 0 1 0 0 0 1 1 2 0 0 1 0 1 3 0 0 1 1 1 4 0 1 0 0 0 5 0 1 0 1 1 6 0 1 1 0 0 7 0 1 1 1 1 8 1 0 0 0 0 9 1 0 0 1 0 10 1 0 1 0 0 11 0 0 1 1 1 12 1 1 0 0 0 13 1 1 0 1 1 14 1 1 1 0 0 15 1 1 1 1 0 Brute-force design • Truth table --> canonical sum (sum of minterms) • Example:prime-number detector • 4-bit input, N3N2N1N0 F = SN3N2N1N0(1,2,3,5,7,11,13)

  8. Minterm list --> canonical sum

  9. Algebraic simplification • Theorem T8, • Reduce number of gates and gate inputs

  10. Resulting circuit

  11. Visualizing T10 -- Karnaugh maps

  12. 3-variable Karnaugh map

  13. Example: F = S(1,2,5,7)

  14. Karnaugh-map usage • Plot 1s corresponding to minterms of function. • Circle largest possible rectangular sets of 1s. • # of 1s in set must be power of 2 • OK to cross edges • Read off product terms, one per circled set. • Variable is 1 ==> include variable • Variable is 0 ==> include complement of variable • Variable is both 0 and 1 ==> variable not included • Circled sets and corresponding product terms are called “prime implicants” • Minimum number of gates and gate inputs

  15. Prime-number detector (again)

  16. When we solved algebraically, we missed one simplification -- the circuit below has three less gate inputs.

  17. Another example

  18. Yet another example • Distinguished 1 cells • Essential prime implicants

  19. Quine-McCluskey algorithm • This process can be made into a program, using appropriate algorithms and data structures. • Guaranteed to find “minimal” solution • Required computation has exponential complexity (run time and storage)-- works well for functions with up to 8-12 variables, but quickly blows up for larger problems. • Heuristic programs (e.g., Espresso) used for larger problems, usually give minimal results.

  20. Lots of possibilities • Can follow a “dual” procedure to find minimal products of sums (OR-AND realization) • Can modify procedure to handle don’t-care input combinations. • Can draw Karnaugh maps with up to six variables.

  21. Real-World Logic Design • Lots more than 6 inputs -- can’t use Karnaugh maps • Design correctness more important than gate minimization • Use “higher-level language” to specify logic operations • Use programs to manipulate logic expressions and minimize logic. • PALASM, ABEL, CUPL -- developed for PLDs • VHDL, Verilog -- developed for ASICs

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