ELEC 7770 Advanced VLSI Design Spring 2007 Binary Decision Diagrams

1. ELEC 7770Advanced VLSI DesignSpring 2007Binary Decision Diagrams Vishwani D. Agrawal James J. Danaher Professor ECE Department, Auburn University Auburn, AL 36849 vagrawal@eng.auburn.edu http://www.eng.auburn.edu/~vagrawal/COURSE/E7770_Spr07 ELEC 7770: Advanced VLSI Design (Agrawal)

2. Methods of Equivalence Checking • Satisfiability algorithms • ATPG methods • Binary decision diagrams (BDD) ELEC 7770: Advanced VLSI Design (Agrawal)

3. Shannon’s Expansion Theorem • C. E. Shannon, “A Symbolic Analysis of Relay and Switching Circuits,” Trans. AIEE, vol. 57, pp. 713-723, 1938. • Consider: • Boolean variables, X1, X2, . . . , Xn • Boolean function, F(X1, X2, . . . , Xn) • Then F = Xi F(Xi=1) + Xi’ F(Xi=0) • Where • Xi’ is complement of Xi • Cofactors, F(Xi=j) = F(X1, X2, . . , Xi=j, . . , Xn), j = 0 or 1 ELEC 7770: Advanced VLSI Design (Agrawal)

4. Theorem (1) F = Xi F(Xi=1) + Xi’ F(Xi=0) ∀ i=1,2,3, . . . n (2) F = (Xi + F(Xi=0)) (Xi’ + F(Xi=1)) ∀ i=1,2,3, . . . n F(Xi=0) F(Xi=1) 0 1 Xi F ELEC 7770: Advanced VLSI Design (Agrawal)

5. Expansion About Two Inputs • F = XiXj F(Xi=1, Xj=1) + XiXj’ F(Xi=1, Xj=0) + Xi’Xj F(Xi=0, Xj=1) + Xi’Xj’ F(Xi=0, Xj=0) • In general, a Boolean function can be expanded about any number of input variables. • Expansion about k variables will have 2k terms. ELEC 7770: Advanced VLSI Design (Agrawal)

6. Binary Decision Tree a c b a 1 0 f b b 0 0 1 1 c c c c 0 0 1 1 0 1 1 0 Graph representation of a Boolean function. 0 0 1 0 0 1 1 1 Leaf nodes ELEC 7770: Advanced VLSI Design (Agrawal)

7. Binary Decision Diagrams • Binary decision diagram (BDD) is a graph representation of a Boolean function, directly derivable from Shannon’s expansion. • References: • C. Y. Lee, “Representation of Switching Circuits by Binary Decision Diagrams,” Bell Syst. Tech J., vol. 38, pp. 985-999, July 1959. • S. Akers, “Binary Decision Diagrams,” IEEE Trans. Computers, vol. C-27, no. 6, pp. 509-516, June 1978. • Ordered BDD (OBDD) and Reduced Order BDD (ROBDD). • Reference: • R. E. Bryant, “Graph-Based Algorithms for Boolean Function Manipulation,” IEEE Trans. Computers, vol. C-35, no. 8, pp. 677-691, August 1986. ELEC 7770: Advanced VLSI Design (Agrawal)

8. Binary Decision Diagram • BDD of an n-variable Boolean function is a tree: • Root node is any input variable. • All nodes in a level are labeled by the same input variable. • Each node has two outgoing edges, labeled as 0 and 1 indicating the state of the node variable. • Leaf nodes carry fixed 0 and 1 labels. • Levels from root to leaf nodes represent an ordering of input variables. • If we trace a path from the root to any leaf, the label of the leaf gives the value of the Boolean function when inputs are assigned the values from the path. ELEC 7770: Advanced VLSI Design (Agrawal)

9. Ordered Binary Decision Diagram (OBDD) a c b a 1 0 f b b 0 1 a 1 c c 0 0 1 0 1 0 1 b b 0 1 0 1 1 0 1 0 0 1 c c c c 0 1 0 1 1 0 0 1 0 0 1 0 0 1 1 1 OBDD Tree ELEC 7770: Advanced VLSI Design (Agrawal)

10. OBDD With Different Input Ordering a c b f c a 1 1 0 0 b b b b 1 0 0 1 a a 1 c c 1 0 0 0 1 0 0 1 0 1 1 0 1 0 1 0 1 1 0 1 0 0 1 ELEC 7770: Advanced VLSI Design (Agrawal)

11. Evaluating Function from OBDD • Start at leaf nodes and work toward the root – leaf node functions are 0 and 1. • Function at a node with variable x is f = x’.f(low) + x.f(high) x 0 1 low high ELEC 7770: Advanced VLSI Design (Agrawal)

12. Cannot Compare Two Circuits a c b a c b f f c c 1 0 0 1 b b b 1 0 a a a 1 1 0 0 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 ELEC 7770: Advanced VLSI Design (Agrawal)

13. OBDD Graph Isomorphism • Two OBDDs are isomorphic if there is one-to-one mapping between the vertex sets with respect to adjacency, labels and leaf values. • Two isomorphic OBDDs represent the same function. • Two identical circuits may not have identical OBDDs even when same variable ordering is used. • Comparison is possible if: • Same variable ordering is used. • Any redundancies in graphs are removed. ELEC 7770: Advanced VLSI Design (Agrawal)

14. Reduced Order BDD (ROBDD) • Directed acyclic graph (DAG) (*). • Contains just two leaf nodes labeled 0 and 1. • Variables are indexed, 1, 2, . . . n, such that the index of a node is greater than that of its child (*). • A node has exactly two child nodes, low and high, that low ≠ high. • Graph contains no pair of nodes such that subgraphs rooted in them are isomorphic. * Properties common to OBDD. ELEC 7770: Advanced VLSI Design (Agrawal)

15. ROBDDs a c b a c b f f c c 0 0 1 1 b b Isomorphic graphs 1 1 a a 0 0 1 1 0 0 0 1 0 1 ELEC 7770: Advanced VLSI Design (Agrawal)

16. Reduction: OBDD to ROBDD a c b f a a 1 0 0 1 b b b b 1 0 0 1 0 1 c c 1 c c 0 0 0 0 1 0 1 1 1 0 1 1 0 1 0 0 1 ELEC 7770: Advanced VLSI Design (Agrawal)

17. Properties of ROBDD • Unique for given variable ordering – graph isomorphism verifies logic equivalence. • Size (number of nodes) changes with variable ordering – worst-case size is exponential (e.g., integer multiplier). • Other applications: logic synthesis, testing. • For algorithms to derive ROBDD, see • R. E. Bryant, “Graph-Based Algorithms for Boolean Function Manipulation,” IEEE Trans. Computers, vol. C-35, no. 8, pp. 677-691, August 1986. • G. De Micheli, Synthesis and Optimization of Digital Circuits, New York: McGraw-Hill, 1994. • S. Devadas, A. Ghosh, and K. Keutzer, Logic Synthesis, New York: McGraw-Hill, 1994. ELEC 7770: Advanced VLSI Design (Agrawal)