ELEC 7770 Advanced VLSI Design Spring 2007 Retiming

1. ELEC 7770Advanced VLSI DesignSpring 2007Retiming 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. Retiming • Retiming is a function-preserving transformation of a synchronous sequential circuit. • Flip-flops are moved according to specific rules. • Original references: • C. E. Leiserson, F. Rose and J. B. Saxe, “Optimizing Synchronous Circuits by Retiming,” Proc. 3rd Caltech Conf. on VLSI, 1983, pp. 87-116. • C. E. Leiserson and J. B. Saxe, “Retiming Synchronous Circuitry,” Algorithmica, vol. 6, pp. 5-35, 1991. ELEC 7770: Advanced VLSI Design (Agrawal)

3. A Trivial Example: Reduced Hardware FF FF FF ELEC 7770: Advanced VLSI Design (Agrawal)

4. Example 2: Faster Clock FF FF ELEC 7770: Advanced VLSI Design (Agrawal)

5. Example 3: Reduced Flip-Flops FF FF FF ELEC 7770: Advanced VLSI Design (Agrawal)

6. Applications of Retiming • Performance optimization • Area optimization • Power optimization • Testability enhancement • FPGA optimization ELEC 7770: Advanced VLSI Design (Agrawal)

7. Fundamental Operation of Retiming • A retiming move in a circuit is caused by moving all of the memory elements at the input of a combinational block to all of its outputs, or vice-versa. FF Combinational logic Combinational logic FF ≡ FF ELEC 7770: Advanced VLSI Design (Agrawal)

8. A Correlator Circuit Adder + + + PO host PI = = = = Flip-flops Comparator ELEC 7770: Advanced VLSI Design (Agrawal)

9. Graph Model 0 0 7 7 7 0 0 0 0 0 0 1 3 3 3 3 1 1 1 Vertex, vi, combinational, delay = d(vi), assumed unchanged by retiming d(host) = 0 Edge, e(vi,vj) or eij, weight wij = number of flip-flops between vi and vj ELEC 7770: Advanced VLSI Design (Agrawal)

10. Path Delay and Path Weight • A set of connected nodes specify a path. • Path delay = ∑ d(vi) = comb. delay of path • Path weight = ∑ wij = clock delay of path • Retiming of a node i denoted by an integer ri • It represents the number of registers moved across, initially ri = 0 • Register moved from output to input, ri → ri + 1 • Register moved from input to output, ri → ri – 1 • After retiming, edge weight wij’ = wij + rj – ri ELEC 7770: Advanced VLSI Design (Agrawal)

11. Legal Retiming • Retiming is legal if the retimed circuit has no negative weights. • A legally retimed circuit is equivalent to the original circuit – proof by Leiserson and Saxe (1991) • Retiming is the most general method for changing the register count and position without knowing the functions of vertices. ELEC 7770: Advanced VLSI Design (Agrawal)

12. Example a c b x d c 1 0 x host 0 0 ELEC 7770: Advanced VLSI Design (Agrawal)

13. Example: Illegal Retiming a c b x d 0 0 c 1 c 1 → 0 0 0 x x host host 0 0 → –1 0 0 → –1 0 0 0 0 →1 Retiming vector = {0, 0, 0} Retiming vector = {0, 0, –1} ELEC 7770: Advanced VLSI Design (Agrawal)

14. Example: Legal Retiming a c b x d 0 →1 0 1 → 0 c 1 c 0 0 →1 x x host host 0 0 0 0 0 0 0 0 Retiming vector = {0, 0, 0} Retiming vector = {0, 1, 0} ELEC 7770: Advanced VLSI Design (Agrawal)

15. Correlator Circuit Critical path delay = 24 f g e 0 0 7 7 7 re=0 0 rf=0 rg=0 0 0 0 0 0 h rh=0 1 3 3 3 3 1 1 1 rd=0 rb=0 rc=0 ra=0 a c b d Initial retiming vector = {0,0,0,0,0,0,0,0} ELEC 7770: Advanced VLSI Design (Agrawal)

16. Retimed Correlator Circuit Critical path delay = 13 f g e 0→1 0→1 7 7 7 re= -2 0 rf= -1 rg=0 0 0 0 0→1 0 h rh=0 1→0 1→0 1 3 3 3 3 1 rd= -2 rb= -1 rc= -2 ra= -1 a c b d Initial retiming vector = {-1,-1,-2,-2,-2,-1,0,0} ELEC 7770: Advanced VLSI Design (Agrawal)

17. Retiming Theorem • Given a network G(V, E, W) and a cycle time T, (r1, . . . ) is a feasible retiming if and only if: • ri – rj ≤ wij for all edges (vi,vj) ε E • ri – rj ≤ W(vi,vj) – 1 for all node-pairs vi, vj such that D(vi,vj) > T Where, W(vi,vj) is the minimum weight path between vi and vj D(vi,vj) is the maximum delay among all minimum weight paths between vi and vj ELEC 7770: Advanced VLSI Design (Agrawal)

18. References • Two papers by Leiserson et al. (see slide 2). • G. De Micheli, Synthesis and Optimization of Digital Circuits, New York: McGraw-Hill, 1994. • N. Maheshwari and S. S. Sapatnekar, Timing Analysis and Optimization of Sequential Circuits, Boston: Springer, 1999. ELEC 7770: Advanced VLSI Design (Agrawal)