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Interconnect Optimizations

Explore interconnect scaling, wire sizing, buffer insertion, and analysis of RC circuits for optimal delay reduction in semiconductor design. Understand the impact of geometries and cross-sections on interconnect performance. Learn about buffer insertion techniques and combinational logic delay. A detailed guide for designing efficient interconnect systems.

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Interconnect Optimizations

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  1. Interconnect Optimizations

  2. G S D w S h l hs ls Ss ws A scaling primer • Ideal process scaling: • Device geometries shrink by S (= 0.7x) • Device delay shrinks by s • Wire geometries shrink by s • R/m: r/(ws.hs) = r/s2 • Cc/m : (hs).e/(Ss) = Cc • C/m: similar • R/m doubles, C/m and Cc/m unchanged

  3. Interconnect role • Short (local) interconnect • Used to connect nearby cells • Minimize wire C, i.e., use short min-width wires • Medium to long-distance (global) interconnect • Size wires to tradeoff area vs. delay • Increasing width  Capacitance increases, Resistance decreases Need to find acceptable tradeoff - wire sizing problem • “Fat” wires • Thicker cross-sections in higher metal layers • Useful for reducing delays for global wires • Inductance issues, sharing of limited resource

  4. Cross-Section of A Chip

  5. Block scaling • Block area often stays same • # cells, # nets doubles • Wiring histogram shape invariant • Global interconnect lengths don’t shrink • Local interconnect lengths shrink by s

  6. Interconnect delay scaling • Delay of a wire of length l : tint= (rl)(cl) = rcl2 (first order) • Local interconnects : tint : (r/s2)(c)(ls)2 = rcl2 • Local interconnect delay unchanged (compare to faster devices) • Global interconnects : tint : (r/s2)(c)(l)2 = (rcl2)/s2 • Global interconnect delay doubles – unsustainable! • Interconnect delay increasingly more dominant

  7. Buffer Insertion For Delay Reduction

  8. Analysis of Simple RC Circuit i(t) R v(t) vT(t) C ± state variable Input waveform

  9. v0u(t) v0 v0(1-e-t/RC)u(t) Analysis of Simple RC Circuit Step-input response: match initial state: output response for step-input:

  10. Delays of Simple RC Circuit • v(t) = v0(1 - e-t/RC) -- waveform under step input v0u(t) • v(t)=0.5v0  t = 0.69RC • i.e., delay = 0.69RC (50% delay) v(t)=0.1v0 t = 0.1RC v(t)=0.9v0 t = 2.3RC • i.e., rise time = 2.2RC (if defined as time from 10% to 90% of Vdd) • Commonly used metric TD = RC (= Elmore delay)

  11. Elmore Delay Delay

  12. Elmore Delay • Driver is modeled as R • Driver intrinsic gate delay t(B) • Delay = all Ri all Cj downstream from Ri Ri*Cj • Elmore delay at n2 R(B)*(C1+C2)+R(w)*C2 • Elmore delay at n1 R(B)*(C1+C2) n1 n2 R(B) B R(w) C1 C2

  13. Elmore Delay • For uniform wire • No matter how to lump, the Elmore delay is the same x unit wire capacitance c unit wire resistance r C

  14. Delay for Buffer u v u C(b) C Driver resistance Input capacitance Intrinsic buffer delay

  15. Buffers Reduce Wire Delay x/2 x/2 R C rx/2 R rx/2 cx/4 cx/4 cx/4 cx/4 C ∆t t_unbuf = R( cx + C ) + rx( cx/2 + C ) t_buf = 2R( cx/2 + C) + rx( cx/4 + C) + tb t_buf – t_unbuf = RC + tb– rcx2/4 x

  16. Combinational Logic Delay Combinational logic delay <= clock period Register Primary Input Register Primary Output Combinational Logic clock

  17. l l1 l2 l3 ln Buffered global interconnects: Intuition Interconnect delay = r.c.l2 Now, interconnect delay =  r.c.li2 < r.c.l2(where l = S lj ) sinceS (lj 2) < (S lj )2 (Of course, account for buffer delay also)

  18. … L Rd – On resistance of inverter Cg – Gate input capacitance r,c – Resistance, cap. per micron l Optimal inter-buffer length • First order (lumped parasitic, Elmore delay) analysis • Assume N identical buffers with equal inter-buffer length l(L = Nl) • For minimum delay,

  19. Optimal interconnect delay • Substituting lopt back into the interconnect delay expression: Delay grows linearly with L (instead of quadratically)

  20. Optimized interconnect delay scaling • Rewriting the optimal interconnect delay expression, • With optimally sized buffers (using dT/dh = 0),

  21. 80 clk-buf 70 buf 60 tot-buf 50 40 % cells used to buffer nets 30 20 10 0 90nm 65nm 45nm 32nm Total buffer count • Ever-increasing fractions of total cell count will be buffers • 70% in 32nm

  22. Feature size (nm) Relative 250 180 130 90 65 45 32 delay 100 Gate delay (fanout 4) Local interconnect (M1,2) Global interconnect with repeaters Global interconnect without repeaters 10 1 Source: ITRS, 2003 0.1 ITRS projections

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