A. Aloisio
ATLASLevel–1 Muon Barrel
RODbusPreliminary Design Review
Mar.12, 2002Alberto AloisioINFN - Sezione di Napoli, Italy
e-mail: [email protected]
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Overview
� Muon Level-1 Trigger and Data path� ROD crate architecture� RODbus features� Measure, models & simulations� Test results� Conclusions
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From PAD to ROD/SL
ROD
TriggerSL
MTCPI
RXData
PAD Logic
� Optical out from PAD to:– SL– ROD
� 12 bit @ 40MHz on the trigger path; synch, fixedand low latency link required to guarantee timing.
� 16 bit + strobe on the data path.
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ROD Crate layout
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RODbus requirements
4848+48
48+48 4848
48
ROD RX
RODbus (SL)
SL
MTCPI
SL
MTCPI RODbus (RX)
RX12 16
� SL to ROD– 48 bit@40MHz– synchronous with low
latency
� RX to ROD– (16+8 bits x 2 = 48 bits)
� ROD to RX/SL(not shown)– Low skew, low jitter
clock distribution– control signals
(RST*, LVL1A, ...)
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Strategy
� Common platform for both SLs and RXs to RODbackbones:– SL asks for a synchronous design.
– Low skew, low jitter signals need a differentialsignaling scheme.
– Doubling data lines saturates the pin availability (andboard space) if a SerDes approach is not used.
– Low voltage, low power (and high performances)suggest LVDS.
– System level “non-timing” signals (as RST*) can runTTL.
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Choosing the chip-set
� DS90CR483/4 chip-set, a 48:8 bit + clock LVDSpseudo SerDes
� Deskew and Preemphasis for improved performances
� Data transfer rate @ 7x master clock frequency
backplane
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Partitioning the problem
� To optimize board layout and backplaneperformances, we decided to assign the SLsto ROD and the RXs to ROD channels totwo “add on” backplanes.
� In this review, we present the prototype ofthe RXs to ROD backplane, assigned to J2
� The SLs to ROD backplane will be doneusing the same technology, mapped to J0
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RODbus: RXs to ROD
� 3 slot backplane to link2 RXs to one ROD
� 10 layer stack-up
� Diff. microstrip forLVDS pairs and singleended for TTL lines onseparate planes
� Plug-in to the VME64rear side. TTL linesterminated as VME
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ROD slot pinout
� ROD distributes clock and 8 system control signals
� Receives 48bits@40MHz from each adjacent RX
TTL lines
RXB
SerDes
RXA
SerDes
clock
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Physical layout
� Diff. LVDS pairsrouted as edgecoupled microstrips
� All pairs have thesame length (10 miltolerance).
� Noisy TTL lines arerouted on a separateplane.
RXB clock
LVDS diff.
RXA clock
LVDS diff.
SerDes clock
LVDS diff.
SerDes clock
LVDS diff.
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TDR interface
� Two test fixtures havebeen designed to interfacethe RODbus to TDR
� Diff. Impedance profilecan be measured
� Impact of connectors,vias, stubs and holes(present in the realenvironment) can beevaluated
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TDR profile
� TDR step probesbackplane line andconnectors
� PCB stack-up is tested
� line length andimpedance are measured
� lumped L/C (connectors,vias, solder pads) arevisible
50 Ohm
coax cable
test
fixture
backplane
connectors
L=575ps
Open load
reflection
50 Ohm
LVDS line
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Testing the SerDes: ROD slot
� ROD emulation:– Distributes Clock to RXs (from ext. or local source)
– Receives serial streams from RXA and RXB
Interface to
ParBERT
Ext. Clock
Deserializer
(RXB)
Clock
LVDS driver
Deserializer
(RXA)Local Clock
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Testing the SerDes: RX slot
� RX emulation:– Receives Clock from ROD
– Transmits serial streams to ROD
Interface to
ParBERT
Clock
LVDS receiver
Serializer
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TDR system test
RODbus
VME
J2/P2
RXB
Emulator
ROD
EmulatorTDR step
launch point
� The entire RX-to-ROD connection is analyzed
� Unpopulated RX and ROD emulators are used toevaluate the signal integrity using TDR
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Impedance profile
� The TDR reflectedstep shows thediscontinuities (vias,holes, connectors, ...)
� The impedanceprofile is deriveddeconvolving themultiple reflections
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Modelling the system
� A PSpice model hasbeen developed, basedon the impedanceprofile
� The model is based onideal (lossless) linesegments
� Model validation is performed by simulatingthe TDR reflected step. Results are in excellentagreement with experimental data
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IBIS models
Typical output stage
Typical input stage
� IBIS is an ANSI standard tomodel I/O buffers mainly forsignal integrity issues
� Different from SPICE, IBISgives only a behavioraldescription - disclosing noproprietary circuit information
� The I/O buffer is characterizedby means of a standard templateof VI curves and stray L,C and R
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DS90CR483 LVDS Output
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System simulation
TX LVDS
PSpice models derived
from IBIS
RODbus
PSpice model
RX LVDS
PSpice models derived
from IBIS
� PSpice system simulation includes backplanes,connectors, stubs, SerDes TX and RX
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Simulation caveat
� Interconnection Model is based on idealtransmission lines
� Crosstalk, ground bounce, probe loading arenot modeled
� TDR-derived models are less accurate thelonger the step travels
� SerDes IBIS model only specs typ. values,pre-emphasis and deskew not modeled
� Standing wave needs long simulation runs(>15h to simulate 2us on a Pentium II – 350)
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SerDes LVDS clock - TX side
LVDS SerDes
Clock @ TX
simulation
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SerDes LVDS clock – RX side
LVDS SerDes
Clock @ RX
simulation
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Test results (preliminary)
� Tests with 2^15 PRBS show no errorsin the SerDes operations up to 60 MHz(420 Mbit/s). Fine tuning should allowreaching 80 MHz (560 Mbit/s).
� Latency is 5 clock cycles (125ns @ 40MHz), dominated by the SerDes specs.RODbus Tpd (570 ps) is negligible.
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More tests to be done
� RODbus performances in a real VME(noisy) environment
� EMI
� BER vs. clock frequency and differentpatterns
� 48-bit 2^16 Pseudo Random WordSequence (PRWS) test requires acustom platform (generator, analyzer,adapters, …) to be developed
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Conclusions (1/2)
� RODbus is the proposed backbone for all theROD crate interconnection needs
� It specifies the physical layer (backplane, lineimpedance, levels) and the logical layers(SerDes specs)
� Data transfer rate is 7x the master clockfrequency (280 Mbit/s @ 40MHz) requiringcareful PCB layout for backplane and users’boards. Emulators have been designed proof ofconcept, tests and PCB layout reference
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Conclusions (2/2)
� RODbus has been in-deep characterized byusing TDR techniques. SPICE models have beenderived and validated. SerDes SPICE modelshave been crafted starting from IBIS files.
� The entire system has been simulated withPSpice and results show good agreement withexperimental data.
� Analog simulations has been successfully usedto evaluate the system performances and signalintegrity issues.