EE290C – Spring 2011Lecture 2: High-Speed Link Overview and EnvironmentElad AlonDept. of EECSEE290C Lecture 2 2• Keep in mind that your goal is to receive the samebits that were sent…Most Basic “Link”EE290C Lecture 2 3Why Wouldn’t You Get What You Sent?EE290C Lecture 2 4This is a “1”This is a “0”Eye Opening - space between 1 and 0teVeWith voltage noiseWith timing noiseWith Both!V0V1tbEye DiagramsEE290C Lecture 2 5BERclk• BER = Bit Error Rate• Average # of wrong received bits / total transmitted bits• Simplified example:(voltage only)• BER = 10-12: (Vin,ampl–Voff) = 7σn• BER = 10-20: (Vin,ampl–Voff) = 9.25σn,122in ampl offnoiseVVBER erfcσ⎛⎞−=⎜⎟⎜⎟⎝⎠EE290C Lecture 2 6What About That “Wire”Back plane connectorLine card tracePackageOn-chip parasitic(termination resistance and device loading capacitance)Line card viaBack plane traceBackplane viaPackage viaBack plane connectorLine card tracePackageOn-chip parasitic(termination resistance and device loading capacitance)Line card viaBack plane traceBackplane viaPackage via[Kollipara, DesignCon03]EE290C Lecture 2 7• 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“Wire” ModelsEE290C Lecture 2 8Links 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 9Links 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 10Links 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 11Transmission Lines Quick Review• Delay• Characteristic Impedance• Reflections• LossEE290C Lecture 2 12Reflections• Sources of Reflections : Z - Discontinuities• PCB Z mismatch• Connector Z mismatch• Vias (through) Z mismatch• Device parasitics - effective Z mismatchZ1Z2Z2 Z1–Z1 Z2+--------------------2Z2Z1 Z2+--------------------DC via Conn via BP(1) Energy conserved(2) Voltages equalEE290C Lecture 2 13Skin 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 (fs)Figure © 2001 Bill DallyEE290C Lecture 2 14Skin Effect cont’d100100MHz500MHzMHz500MHz1GHz1GHzW=210um、t=28umδ=6.6 um δ=2.08 umδ=2.95 umSkin depthEE290C Lecture 2 15Dielectric 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 =Table © 2001 Bill DallyDLengtheα−EE290C Lecture 2 16Dielectric Loss cont’d• FR4 cheapest – most widely used• Rogers is most expensive –high-end systems• May not matter that much due to surface roughness8 mil wide and 1 m long 50 Ohm strip line-40.0-30.0-20.0-10.00.01.E+06 1.E+07 1.E+08 1.E+09 1.E+10Frequency, HzAttenuationFR4Roger 4350Kollipara DesignCon03EE290C Lecture 2 17Skin + Dielectric Losses• Skin Loss ∝√f• Dielectric loss ∝ f : bigger issue at high f00.10.20.30.40.50.60.70.80.911.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10AttenuationFrequency, HzFR4 dielectric, 8 mil wide and 1m long 50 Ohm strip lineTotal lossConductor lossDielectric lossKollipara DesignCon03EE290C Lecture 2 18Everything Together: S21• S21: ratio of received vs. transmitted signalsBreakdown of a 26" FR4 channel with 270 mil stubs0.00.10.20.30.40.50.60.70.80.91.00.0E+00 5.0E+08 1.0E+09 1.5E+0 9 2.0E+09 2.5E+09 3.0E+09 3.5E+09 4.0E+09Frequ ency, HzTransfer functionPCB tracesPCB traces & connectorsPCB traces, con nectors & viasEnt ir e ch an nelEE290C Lecture 2 19Real BackplaneEE290C Lecture 2 20Practical PCB Differential Lines• 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 immunityｔｔｔWS−HH＋SWεrｔｔH+-µ -StripStrip-lineEE290C Lecture 2 21Coupling Æ Crosstalk…• “Near-end” xtalk: NEXT (reverse wave)• “Far-end” xtalk: FEXT (forward wave)• NEXT in particular can be very destructive• Full swing TX vs. attenuated RX signal• Good news: can control through design• NEXT typically 3-6%, FEXT typically 1-3%EE290C Lecture 2 22NEXT: What Not To DoXXXXXXXXTxRx Tx00.10.20.30.40.50.60.70.80.910 100 200 300 400 500 600 700 800 900Time , psVoltage, VTxRxXTXEE290C Lecture 2 23NEXT: Better Design00.10.20.30.40.50.60.70.80.910 100 200 300 400 500 600 700 800 900Time , psVoltage, VTxRxXTXXXXXXXXXTxRxEE290C Lecture 2 24Connectors Particularly ToughNEXT FEXT55 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 discontinuitiesEE290C Lecture 2 25Skew Within Link• Need very tight control to maintain constant % of bit time• 1% skew on 30” line Æ 50ps skew• Half of a bit time at 10Gb/s• Good news: connectors relatively “short” (~200ps)EE290C Lecture 2 26Reflections RevisitedTXDATARXDATAATARCRCTDB-8-6-4-20246810gh-gh conn. (baseline) : Normalized Raw and eq pulse response: PR length aftermain 60AT,RA2T,RBCT,RD-8-6-4-20246810gh-gh conn. (baseline) : Normalized Raw and eq pulse response: PR length aftermain 60-8-6-4-20246810gh-gh conn.