1 EE121 John Wakerly Lecture #4 Combinational-Circuit Synthesis ABEL

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 (example in lab #2) Write functional description in HDL Define test conditions / test vecors, 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 Sum-of-products form AND-OR NAND-NAND

6 Product-of-sums form OR-AND NOR-NOR P-of-S preferred in CMOS, TTL (NAND-NAND)

7 Brute-force design Truth table --> canonical sum (sum of minterms) Example: prime-number detector –4-bit input, N 3 N 2 N 1 N 0 row N 3 N 2 N 1 N 0 F F =   (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 =  (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

22 ABEL Advanced Boolean Equation Language –Developed for use with programmable logic devices, which have a programmable AND-OR structure. Combinational logic functions –Operators: –Sets: –Relations: –Intermediate variables AND, OR, NOT, XOR, XNOR & # ! $ !$ XBUS = [X3,X2,X1,X0]; XBUS = [1,1,0,1]; XBUS = 13; (XBUS == YBUS) (XBUS > [1,1,0,1])

23 ABEL Program Structure

24 ABEL Example

25 ABEL Example (continued)

26 ABEL language processor (compiler) Checks syntax Checks device-pin capabilities Expands language statements and equations Reduces equations into sum-of-products form form for programmable logic device (PLD) Checks test vectors against equations Checks equation requirements (# of inputs, product terms) against PLD resources Determines “fuse map” to program the PLD

27 ABEL equations (.eq3) file

28 ABEL equations file (continued)

29 ABEL WHEN Statements

30 Nested WHEN statements Note: different variables can be used in different THEN and ELSE clauses

31 Next time PLDs (target of ABEL programs) Documentation standards