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Passive Distortion Compensation for Package Level Interconnect
Chung-Kuan ChengUC San Diego
Dongsheng Ma & Janet Wang
Univ. of Arizona
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Outline
1. Motivation2. Review of High-Speed Serial Links3. Passive Distortion Compensation
1. Theory2. Implementation3. Simulation Results
4. Power Management and System Integration
5. Research Direction
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1. Motivation: ITRS Bandwidth Projection
• Abundant on-chip bandwidth• Off-chip bandwidth is the bottleneck• Many chip are I/O limited
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2018
1614
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#I/O padsOff-chip fclkAggr BWAggr BW (Fit)
Technology (nm) No
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Courtesy of Hamid Hatamkhani et al., DAC ‘06
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2. Review of High-Speed Serial Links
Techniques On-Chip Off-ChipPre-emphasis and equalization √ √
Clocked Discharging (M. Horowitz, ISVLSI’03)
√
Frequency Modulation (S. Wong, JSSC ’03; Jose ISVLSI ’05)
√ √
CDMA on wireline (Jongsun Kim et al.)
√ √
Non-linear Transmission Line (E. Hajimiri JSSC’05, E.C. Kan CICC ’05)
√
Resistive Termination (Tsuchiya et al., EPEP; M. Flynn ICCAD ’05)
√ √
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3. Passive Distortion Compensation
Typical RLC Transmission Line Distortionless Transmission Line
•Frequency dependent phase velocity (speed) and attenuation
• Intentionally make leakage conductance satisfy R/G=L/C• Frequency response becomes flat from DC mode to Giga Hz
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3.1 Theory: Telegrapher’s Equations• Telegrapher’s equations
• Propagation Constant
• Wave Propagation
• and correspond to attenuation and phase velocity. Both are frequency dependent in general.
• Characteristic Impedance
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3.1 Theory: Distortionless Lines
• Distortionless transmission line
If
Both attenuation and phase velocity become frequency independent
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3.1 Theory: Differential CaseCommon Mode – Current flowing in the same direction
Differential Mode – Current flowing in the opposite direction
Shunt between each line to ground Shunt between the two lines
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3.2 Implementation
• Evenly add shunt resistors between the signal line and the ground
• Non-ideality
Ideal Assumption
In Practice Implication
Homogeneous and distributive line
Discrete shuntsWhat’s the optimal spacing?
Are the shunt resistors realizable?
Frequency independent RLGC
Frequency dependent RLGC
What’s the optimal frequency for the matching?
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3.2 Implementation: MCM trace
MCM On-chip
Length
Series Resistance
Frequency dependency ofline parameters Large Small
~10 cm ~ 10 mm1 Ω/mm 1 Ω/μm
MCM trace vs. On-chip interconnect
Operation region RLC RC
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3.2 Implementation: A MCM Stripline Case
• Control the signal line thickness to minimize skin effect (cost vs. distortion)
• Assume LCP dielectric
Geometry based on IBM high-end AS/400 system
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3.3 Simulation: Methodology
• Transient simulation in Hspice• Each transmission line segment is modeled by W-
element using frequency-dependent tabular model
• Discrete resistors• Used CZ2D tool from IBM for RLGC extraction
• Part of IBM EIP (Electrical Interconnect & Packaging) suite.
• Fast and accurate• Ensures causality of transient simulation
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3.3 Simulation: RLGC vs. Frequency
R
C
• Match at DC• Boost up low frequency traveling speed• Balance low frequency attenuation and high frequency attenuation
Z0 = 78 Ω, delay = 57.78 ps/cm
R1MHz=11.07 Ω/cm, L1MHz=5.52e-3 μH/cm, C1MHz =0.74 pF/cm
Rshunt =L1MHz/R1MHzC1MHz = 669.5 Ω/cm
LG
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3.3 Simulation: Shunt Resistor Spacing
• Number of shunt resistors = N
• Resistors are implemented with embedded carbon paste film
• Spacing depends on the target data rate
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3.3 Attenuation
• W8μm/t2μm/b20μm
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3.3 Phase Velocity
• W8μm/t2μm/b20μm
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3.3 Simulation: Pulse Response
less severe ISI effect
DC saturation voltage determined by the resistor ladder
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3.3 Jitter and Eye opening for 2um case
10 cm 20 cm
Jitter (ps) Eye opening (volt)
Jitter (ns) Eye opening (volt)
1 shunt/1 cm 1 5.565 0.42563 9.369 0.095785
Terminated with Z0 2
5.0228 0.37449 13.87 0.14595
Terminated with Rdc 3 5.8183 0.33906 12.117 0.090432
Open end 22.5 0.51 > 70 < 0.14
• W8μm/t2μm/b20μm
1. Each shunt resistor is 669.5 ohm
2. Z0=78 ohm
3. For 10cm line, Rdc = 66.9 ohm; for 20 cm line, Rdc=33.5 ohm
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3.3 Jitter and Eye opening for 4.5um case
10 cm 20 cm
Jitter (ps) Eye opening (volt)
Jitter (ns) Eye opening (volt)
1 shunt/1 cm 1 22.83 0.525 23.34 0.238
Terminated with Z0 2
7.3764 0.48916 37.327 0.21423
Terminated with Rdc 3 12.026 0.57114 37.443 0.20064
Open end Unrecognizable Unrecognizable
• W8μm/t4.5μm/b20μm
1. Each shunt resistor is 1232 ohm
2. Z0=71.1 ohm
3. For 10cm line, Rdc = 123.2 ohm; for 20 cm line, Rdc=61.6 ohm
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3.3 Simulation: Eye Diagrams
With 10 shunts (each = 669.5)Without shunt resistors
Jitter = 22.5 psEye opening = 0.51 V
Jitter = 5.57 psEye opening = 0.426 V
• W8μm/t2μm/b20μm/L10cm• 1000 bit PRBS at 10Gbps• W-element + tabular RLGC model in HSpice
Clear eye opening
Reducedamplitude
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3.3 Best Eye Diagram for 2um thick case• W8μm/t2μm/b20μm/L10cm, 10 distributed resistors
Jitter & eye opening v.s. shunt valueBest case when each shunt is 500 ohmJitter = 4.63 psEye opening = 0.35645 V
Jitter
Eyeopening
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Best eye diagram when only terminator is used, 2um thick case
Jitter
Eyeopening
• W8μm/t2μm/b20μm/L10cm, terminator only
Jitter & eye opening v.s. R_term Best case when terminator 90 ohmJitter = 4.97 psEye opening = 0.40647 V
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• W8μm/t2μm/b20μm/L20cm, 20 distributed resistors
Jitter & eye opening v.s. shunt valueBest case when each shunt is 600 ohmJitter = 9.816 psEye opening = 0.08379 V
Jitter
Eyeopening
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• W8μm/t2μm/b20μm/L20cm, terminator only
Jitter & eye opening v.s. R_term Best case when terminator 40 ohmJitter = 11.95 psEye opening = 0.10111 V
Jitter
Eyeopening
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3.3 Eye Diagram for 4.5um thick case when matched at DC
Sleepy Eye Jitter = 22.8 pseye opening = 0.525 V
• W8μm/t4.5μm/b20μm/L10cm
Open ended 10 shunts matched at DC
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3.3 Best Eye Diagram for the 4.5um thick case
• W8μm/t4.5μm/b20μm/L10cm, 10 distributed resistors
Jitter & eye opening v.s. shunt value
Best case when each shunt is 500 ohmJitter = 11.97 psEye opening = 0.44036 V
Jitter
Eyeopening
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• W8μm/t4.5μm/b20μm/L10cm, terminator only
Jitter & eye opening v.s. R_term
Best case when the terminator is 80 ohm
Jitter = 7.18 psEye opening = 0.51672 V
Jitter
Eyeopening
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• W8μm/t4.5μm/b20μm/L20cm, 20 distributed resistors
Jitter
Eyeopening
Jitter & eye opening v.s. shunt value Best case when each shunt is 800 ohmJitter = 21.762 psEye opening = 0.18288 V
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• W8μm/t4.5μm/b20μm/L20cm, terminator only
Jitter
Eyeopening
Jitter & eye opening v.s. R_termBest case when the terminator is 110
ohmJitter = 37.595 psEye opening = 0.24859 V
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3.3 Eye Diagram for the MCM trace
Jitter = 38.834 psEye opening = 0.21418
Jitter = 23.24 pseye opening = 0.238 V
• W8μm/t4.5μm/b20μm/L20cm
Terminated with Z0 20 shunts matched at DC
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4. Adaptive Power Management (APM)
• The distortionless signaling simplifies the interface circuitry. However, the twice heavier attenuation due to passive compensation calls for adaptive power management;
• With adaptive power management, we adaptively regulate the power supply of the transmitter according to attenuation;
• The regulated supply voltage guarantees the speed of transmission while keeping the minimal power overhead and well-controlled bit-error rate.
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4. APM Preliminary Results
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4. APM Controller
CPUUtilization( , , , )
Performance Monitor
Frequency/Voltage TableTemperature/Voltage Table
Signal Processing
PerformanceRequest
Adaptive Power Controller (APC)
Intelligent Energy Manager *(IEM)
Core OperationsPropagatoin
Replica
EnergySource
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4. System Integration
• The reduction of the jitter leaves larger design margin for interface circuit design;
• To enable an effective and accurate communication, the operation of transmitter and receiver must be well synchronized. This requires accurate clock positioning and phase locking;
• Synergic method will be taken to achieve mutual compensation and joint leverage on signal accuracy, attenuation and system power.
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5. Research Direction• Develop analysis models for the
technology• Eye diagram analysis via step responses• Power consumption
• Optimize technologies• Chip carrier and board technologies• Redistribution• Physical dimensions• Shunts, terminators
• Prototype fabrication & measurement• More applications: clock trees, buses• Incorporate transmitter/receiver design
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Remark
• Distortion Compensation:• Source termination: Impedance• Receiver termination: Voltage Clamp, Matched
Z, Optimized Z.• Distributed shunts• Combination of above techniques
• Packaging• Current Products: Improve signal quality
based on current fabrication technologies.• Future Products: Devise the optimal
combination.
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The End
Thank you!