Berkeley ELENG 240A - Lecture 18: High-Speed Link Overview

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EECS240 – Spring 2009Lecture 18: High-Speed Link OverviewElad AlonDept. of EECSEECS240 Lecture 17 2Links are everywhere…EECS240 Lecture 17 3Basic Link Issues• Signaling: getting bits from the TX to the RX• Timing: determining which bit is whichEECS240 Lecture 17 4Speed of Light• Why is a link (i.e., off-chip I/O) different than on-chip wires?• Both send info back and forth• Usually model on-chip wires with capacitor• Sometimes with resistance too• On-chip model works because dimensions << λ• Not true for off-chip wires…EECS240 Lecture 17 5Transmission Lines• Wire model when can’t ignore c:• Properties:• Delay• Characteristic impedance• Energy stored in E, B fieldsEECS240 Lecture 17 6Termination and Reflection• Two constraints at any junction:• Voltage are equal• Power is conservedEECS240 Lecture 17 7Loss• Real T-lines have loss too:• Skin loss α√f• Dielectric loss α fEECS240 Lecture 17 8Not Just Material Issues…• Energy splits at via• Via stub looks like a capacitor – reflectionsEECS240 Lecture 17 9Example Channels0 2 4 6 8 10-60-50-40-30-20-100frequency [GHz]Attenuation [dB]9" FR4, via stub26" FR4,via stub26" FR49" FR4• 20-30dB loss at 3GHz• How bad is that?• Two related issues:• (1) Noise and min. signal amplitude• (2) IntersymbolinterferenceEECS240 Lecture 17 10Noise and BER• RX circuits always have noise • If noise is ever larger than the input signal (at sampling point), RX will decode the bit incorrectly• BER = Bit Error Rate• I.e., average # of incorrectly received bits / total transmitted bitsclkEECS240 Lecture 17 11Min. Signal Amplitudeclk• Min. signal set by noise σ and residual offset:• BER = 10-12: (Vin,ampl–Voff) = 7σn• BER = 10-20: (Vin,ampl–Voff) = 9.25σn,122in ampl offnoiseVVBER erfcσ⎛⎞−=⎜⎟⎜⎟⎝⎠EECS240 Lecture 17 12So What?• Why not just hit the RX with a larger signal?• (Not a stupid question – this is often what people do)• Simple (hand-wavy) answers:• Transmission line Z usually low (~50Ω)• 1V swing Æ 20mW• Larger swing doesn’t help with ISI…• More next lecture• Bottom line: • If can use lower swing, can get lower power• Good application of EE240 material!EECS240 Lecture 17 13Link Circuits: “Current-Mode” TX• Often use differential signaling/circuits to reject supply/CM noise:EECS240 Lecture 17 14Receiver Termination OptionsEECS240 Lecture 17 15Basic ReceiverEECS240 Lecture 17 16Front-end Amp GainEECS240 Lecture 17 17Front-end Amp


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Berkeley ELENG 240A - Lecture 18: High-Speed Link Overview

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