EE290C Spring Lecture 2: High-Speed Link Overview and Environment. Elad Alon Dept. of EECS

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1 EE290C Spring 2011 Lecture 2: High-Speed Link Overview and Environment Elad Alon Dept. of EECS Most Basic Link Keep in mind that your goal is to receive the same bits that were sent EE290C Lecture 2 2

2 Why Wouldn t You Get What You Sent? EE290C Lecture 2 3 Eye Diagrams V 1 V 0 t b This is a 1 This is a 0 V e Eye Opening - space between 1 and 0 t e With voltage noise With Both! With timing noise EE290C Lecture 2 4

3 BER clk BER = Bit Error Rate Average # of wrong received bits / total transmitted bits Simplified example: (voltage only) V 1 in, ampl Voff BER = 2 erfc 2σ noise BER = : (V in,ampl V off ) = 7σ n BER = : (V in,ampl V off ) = 9.25σ n EE290C Lecture 2 5 What About That Wire Package Line card trace On-chip parasitic (termination resistance and device loading capacitance) Package via Back plane trace Back plane connector Line card via Backplane via [Kollipara, DesignCon03] EE290C Lecture 2 6

4 Wire Models ICs: usually use lumped models for wires Capacitance almost always matters Sometimes resistance Less often inductance Works because dimensions << λ Let s look at some example λ and size numbers for links EE290C Lecture 2 7 Links and Lengths Chip to chip on a PCB Short and relatively well controlled Packaging usually limits speed Distance: 3-6 Data-rate: 1-12Gb/s Wavelength in free space = Wavelength on PCB (FR4) = EE290C Lecture 2 8

5 Links and Lengths Cables connecting chips on two different PCBs Cables are lossy, but relatively clean if coax Connector transitions usually the bad part Distance: ~0.5m up to ~10 s of m (Ethernet) Data-rate: 1-10Gb/s Wavelength in free space = Wavelength on PCB (FR4) = EE290C Lecture 2 9 Links and Lengths High-speed board-to-board connectors Daughtercard (mezzanine-type) Backplane connectors Distance: 8 up to ~40 Data-rate: 5-20Gb/s Wavelength in free space = Wavelength on PCB (FR4) = EE290C Lecture 2 10

Transmission Lines Quick Review Delay Characteristic Impedance Reflections Loss EE290C Lecture 2 11 Reflections Z2 Z1 ------------------- Z1+ Z2 Z1 2Z2 ------------------- Z1+ Z2 Z2 (1) Energy

6 Transmission Lines Quick Review Delay Characteristic Impedance Reflections Loss EE290C Lecture 2 11 Reflections Z2 Z Z1+ Z2 Z1 2Z Z1+ Z2 Z2 (1) Energy conserved (2) Voltages equal Sources of Reflections : Z - Discontinuities PCB Z mismatch Connector Z mismatch Vias (through) Z mismatch Device parasitics - effective Z mismatch DC via Conn via BP EE290C Lecture 2 12

Skin Effect At high f, current crowds along the

-½ Model as if skin is δ thick Starts when skin depth

EE290C Lecture 2 13 Skin Effect cont d Skin depth =6.

7 Skin Effect At high f, current crowds along the surface of the conductor Skin depth proportional to f -½ Model as if skin is δ thick Starts when skin depth equals conductor radius (f s ) Figure 2001 Bill Dally EE290C Lecture 2 13 Skin Effect cont d Skin depth =6.6 um =2.95 um =2.08 um W=210um t=28um EE290C Lecture 2 14

8 Dielectric Loss High frequency signals jiggle molecules in the insulator Insulator absorbs energy Effect is approximately linear with frequency Modeled as conductance term in transmission line equations Dielectric loss often specified in terms of loss tangent Transfer function = e α D Length Table 2001 Bill Dally EE290C Lecture 2 15 Dielectric Loss cont d Attenuation mil wide and 1 m long 50 Ohm strip line FR4 Roger E+06 1.E+07 1.E+08 1.E+09 1.E+10 Frequency, Hz Kollipara DesignCon03 FR4 cheapest most widely used Rogers is most expensive high-end systems May not matter that much due to surface roughness EE290C Lecture 2 16

9 Skin + Dielectric Losses Attenuation FR4 dielectric, 8 mil wide and 1m long 50 Ohm strip line E E E E E+10 Frequency, Hz Kollipara DesignCon03 Total loss Conductor loss Dielectric loss Skin Loss f Dielectric loss f : bigger issue at high f EE290C Lecture 2 17 Everything Together: S21 S21: ratio of received vs. transmitted signals Breakdown of a 26" FR4 channel with 270 mil stubs Transfer function 1.0 PCB traces 0.9 PCB traces & connectors 0.8 PCB traces, connectors & vias 0.7 Entire channel E E E E E E E E E+09 Frequency, Hz EE290C Lecture 2 18

Real Backplane EE290C Lecture 2 19 Practical PCB Differential Lines µ -Strip Strip-line W W S S + - ε r H H H Differential signaling has nice properties Many

10 Real Backplane EE290C Lecture 2 19 Practical PCB Differential Lines µ -Strip Strip-line W W S S + - ε r H H H Differential signaling has nice properties Many sources of noise can be made common-mode Differential impedance raised as f(mutuals) between wires Strong mutual L, C can improve immunity EE290C Lecture 2 20

11 Coupling Crosstalk Near-end xtalk: NET (reverse wave) Far-end xtalk: FET (forward wave) NET in particular can be very destructive Full swing T vs. attenuated R signal Good news: can control through design NET typically 3-6%, FET typically 1-3% EE290C Lecture 2 21 NET: What Not To Do Tx Rx Tx Voltage, V Tx Rx T Time, ps EE290C Lecture 2 22

NET: Better Design Tx Rx 1 0.9 0.8 0.7 Voltage, V 0.6 0.5 0.4 Tx Rx T 0.3 0.2 0.

12 NET: Better Design Tx Rx Voltage, V Tx Rx T EE290C Time, Lecture ps 2 23 Connectors Particularly Tough NET FET 55 ps (20-80%) 55 ps (20-80%) 80ps (10-90%) 80ps (10-90%) AB 4.4% 3.7% DF 3.3% 2.6% GH 3.3% 2.6% JK 4.3% 3.5% Tight footprint constraints Hard to match pairs and even individual lines May compensate skew on line card Also big source of impedance discontinuities EE290C Lecture 2 24

DATA R DATA A T Connector-BP transitions A R B C T C R D 10 gh-gh conn.

13 Skew Within Link Need very tight control to maintain constant % of bit time 1% skew on 30 line 50ps skew Half of a bit time at 10Gb/s Good news: connectors relatively short (~200ps) EE290C Lecture 2 25 Reflections Revisited T DATA R DATA A T Connector-BP transitions A R B C T C R D 10 gh-gh conn. (baseline) : Normalized Raw and eq pulse response: PR length after main A T, T,R R C T, T,R R D 4 2 A2 T, T,R R B T -8 EE290C Lecture 2 26

Reflections Due To Via Stub Attenuation [db] 0-10 -20-30 -40-50 9" FR4, via stub 9" FR4 26" FR4-60 26"

leads to resonance (notch) Especially on thick backplanes, vias are a big culprit EE290C Lecture 2 27

14 Reflections Due To Via Stub Attenuation [db] " FR4, via stub 9" FR4 26" FR " FR4, via stub frequency [GHz] Stub : extra piece of T-line hanging off main path Usually leads to resonance (notch) Especially on thick backplanes, vias are a big culprit EE290C Lecture 2 27 Minimizing Via Stubs Thinner PCB? Better vias? counterbored blind via All expensive: 1.1-2x EE290C Lecture 2 28

Summary Packaging, chip connection, etc.

Length: 20", T/S: 30/270 mil Roger BP, Length: 1.5", T/S: 30/270 mil 1.0 1.0 0.9 0.9 Transfer function 0.8 0.7 0.6 0.5 0.

52 3.02 3.52 4.02 4.53 5.03 5.53 0.0 0.00 0.78 1.56 2.33 3.11 3.89 4.67 5.45 6.22 7.

15 Summary Packaging, chip connection, etc. can all have an effect Entire conferences dedicated to signal integrity (SI) EE290C Lecture 2 29 Implications FR-4 BP, Length: 20", T/S: 30/270 mil Roger BP, Length: 1.5", T/S: 30/270 mil Transfer function meas sim Transfer function (s21) meas sim frequency, GHz Frequency, GHz Need to know range of channels you will face Drives design of the link circuitry Start diving in to that next lecture Don t be a pure circuit weenie Simple fixes to channel may go a long way EE290C Lecture 2 30

Eye Diagrams. EE290C Spring Most Basic Link BER. What About That Wire. Why Wouldn t You Get What You Sent?

Eye Diagrams. EE290C Spring Most Basic Link BER. What About That Wire. Why Wouldn t You Get What You Sent? EE29C Spring 2 Lecture 2: High-Speed Link Overview and Environment Eye Diagrams V V t b This is a This is a V e Eye Opening - space between and Elad Alon Dept. of EECS t e With voltage noise With timing