The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 1
EE 382MVLSI–II: Advanced Circuit Design
I/O & ESD Design
Byron Krauter, IBM
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 2
Outline
2
• Logic Requirements• Other Complications• Transmission Line Behavior• Basic CMOS I/O and Receiver Design• Actual CMOS I/O and Receiver Design
– Impedance Matching & Slew Rate Control– Mixed Voltages– ESD and other extreme conditions
• Increasing Bandwidth– Source Synchronous I/O or Co-transmitted Clock– Pipelined Bus or Bus Pumping– Dual Data Rate– Simultaneous Bi-Directional– Pattern Based Driver Compensation
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 3
Logic Requirements
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 4
Logic Requirements
4
• Send 1’s and 0’s chip to chip– Can be accomplished with simple inverters??
Chip A Chip B
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 5
Complications• Pin Count Limitations
– Bi-directional signaling– Simultaneous switching noise
• Transmission Line Behavior– Limited net topologies work– Terminations required– Skin effect– Dielectric loss
• Other Noises– Reflections – Discontinuity noise– Crosstalk and connector noise
• Mixed Voltages • ESD and Other Handling Complications
5
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 6
Transmission Line Behavior
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 7
But First A Few Words onCommon Ground Interconnect
Models
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 8
Example - Two Wires & One Source• Twin lead transmission line modeled as a single section
and driven by a Thevenin source
0.5*Cwire0.5*Cwire
Rwire
Rwire
Rsource
L22
L11
M12
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 9
Example - Two Wires & One Source• Being concerned with local potentials only (i.e. capacitor
potentials) inductances and resistances can be combined
0.5*Cwire0.5*Cwire
Rwire RwireRsource L22L11
M12
0.5*Cwire0.5*Cwire 2*Rwire
Rsource L11+ L22 - 2*M12
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 10
Example - Three Wires & Two Sources• When multiple wires form a cutset, treat one wire as a
reference lead and fold it into the other wires*.
* Brian Young, “Digital Signal Integrity: Modeling and Simulation with Interconnects and Packages”
Rg
Lgg
0.5*C1g
R1Rs1 L11
M1g
0.5*C2g
R2Rs2 L22
M2g
0.5*C12
0.5*C12
0.5*C1g
0.5*C2g
M12
Cutset
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 11
Example - Three Wires & Two Sources• Resulting loop impedance model for three parallel wires
driven by two Thevenin sources
0.5*C12
0.5*C2g
0.5*C1g
R1+RgRs1L11+Lgg-2M1g
0.5*C12
M12-M1g-M2g+Lgg
i2Rg
R2+RgRs2L22+Lgg-2M1g
0.5*C1g
i1Rg
0.5*C1g
v1
v2
mutual resistances
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 12
Transmission Line Behavior• On and off chip signals can always be modeled with lumped
RLC circuits
• Wire segments are modeled with π or t segments
• L, R, C, and G can be frequency dependent
• But inductance is not always important
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 13
Transmission Line Behavior
13
• Inductance is important when– Driver source impedance Rs is low
Rs < Zo where Zo = characteristic impedance of line
– Driver rise time τr is fast
τr < 2.5 τfwhere τf = time of flight
– Line loss is lowR << jωL or (R / 2Zo) << 1
• Can be restated for point to point nets asRsCtot < 1/2 RlineCline < τf
Wave frontdecays exponentially
with this constant
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 14
When Inductance is Important• Nets ring and net delays become unpredictable unless:
– Net topologies are constrained• Point to point nets• Periodically loaded nets • Near and far end clusters
– Nets are driven appropriately• Not to strong and not to weak• Not to fast and not to slow
– Nets are terminated appropriately• Source termination• Far end termination
– Resistance to Vdd or Gnd or any Thevenin Voltage • AC termination = RC circuit• Active hold clamps• Diode or Schottky diode clamps
14
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 15
Transmission Line Behavior• Perfectly source terminated point to point, loss-less net
15
V(t)
Rs = Zoτf
time
far end
near end
Zo = LC
τf = LC
τf
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 16
Transmission Line Behavior• Under driven point to point, loss-less net
16
Rs = 3Zoτf Zo = L
C
τf = LC
V(t)
time
far end
near end
ApproximatesRC step response
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 17
Transmission Line Behavior• Over driven point to point, loss-less net
17
Rs = 1/3 Zoτf Zo = L
C
τf = LC
V(t)
time
far end
near end
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 18
Reflection and Transmission
18
Γv = ZL - ZoZL+ Zo
Γv = 1, ZL= ∞0, ZL= Zo
-1, ZL= 0
Τv = 1 + Γv = 2ZLZL+ Zo
With incident wave Vinc traveling down the line
Voltage reflection coefficient
Voltage transmission coefficient
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 19
Equivalent Circuits Along Line
19
Rs
ZoVs
near end
Zo2Vinc
Zo along line
ZL2Vinc
Zo far end
Vinc+
-
Zdiscontinuity
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 20
Discontinuities Along Line
20
C
L
Vs
Rs = Zo
Vs
Rs = Zo
Vs=11
1/21/2 (1- e-2t/ZoC)
Vs=11
1/21- 1/2(1- e-2Zot/L)
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 21
Well Behaved Net Topologies• Point to Point Nets
21
Rs = Zoτf
Source terminated
Rs << Zoτf
Far end terminated
Rterm ≅ ZoVterm
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 22
Well Behaved Net Topologies• Periodically Loaded Nets
22
Source terminated: Near end switches last
Rs = Zeff
CL CL CL CL
With periodic loadingZeff = L
C + nCL
τf = L(C+nCL)
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 23
Well Behaved Net Topologies• Periodically Loaded Nets
23
Far end terminated: Near end switches first
Rs << Zeff
CL CL CL CL
With periodic loadingZeff = L
C + nCL
τf = L(C+nCL)
Rterm ≅ ZeffVterm
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 24
Well Behaved Net Topologies• Near end (or Star) cluster
24
Rs = Zo/N
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 25
Well Behaved Net Topologies• Far-end cluster
25
Rs = Zo/N Zo/N
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 26
Well Behaved Net Topologies• Double far-end terminated bus
26
CL CL CL CL
Vterm
Rs << Zo
Vterm
CL
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 27
Ideal Transmission Lines
27
γ =Z = LC ω LC
I(z)
V(z)
tiL
zV
tVC
zi
∂∂
−=∂∂
∂∂
−=∂∂
2
2
2
2
tVLC
zV
∂∂
=∂∂
Steady State Solution:])VV[
1(ReI
]VV[ReV
)()(
)()(
eeee
tzjtzj
tzjtzj
Zωγωγ
ωγωγ
+−−−+
+−−−+
+=
+=
where
IdealTelegrapher’s Equation
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 28
Transmission Lines with Loss
28
j γ (ω) = (jωL + R) jωCZ(ω) = jωL + RjωC
≅ LC (1 - j R/2ω L) ≅ (1 - j R/2ω L)jω LC
02)(
ZRLC += ωωγ
00 2
)(ZC
RjZZω
ω −=
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 29
Waveforms Along a Low Loss Line
29
(1- e-R*length/2Zo)1
Vs
Rs << Zo
τfwhere τf = length / velocity
With complex impedance & complex propagation constanthigh speed wavefront decays exponentially & distorts
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 30
Distortionless Transmission Line
30
j γ (ω) = (jωL + R)(jωC + G)Z(ω) = jωL + RjωC + G
= LC
= ( jω + R /L)LC
Oliver Heaviside (1887)
LRCG // =
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 31
Waveforms Along a Distortionless Line
31
(1- e-R*length/Zo)1
Vs
Rs << Zo
τfwhere τf = length / velocity
With real impedance and complex propagation constanthigh speed wavefront decays exponentially but without distortion
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 32
Basic CMOS I/O and Receiver Design
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 33
Bidirection CMOS I/O Buffer
33
data
enable Pad
0 101
data
enable
Hi ZHi Z0
1
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 34
CMOS I/O Receiver• Any two input gate that
– Has good noise immunity– Provides on-chip control when off-chip inputs float
• Example: two input nand
34
0 101
data
enable
X
111 0
X1
data
enablePad
out
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 35
Actual CMOS I/O and Receiver Design
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 36
Actual CMOS I/O Design• Output Impedance Control• Slew Rate Control• Mixed Voltage Designs
– Input Design for Higher Voltages– Output Design for Higher Voltages
• Dual Power Supplies• Floating Well Designs• Open Source Signaling
• Other Circuits– Differential I/O Circuits– Hysteresis Receivers
• ESD Circuits
36
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 37
Output Impedance Control• Device “resistances” are too variable for source
termination– Devices are non-linear – Variations due to Vdd, Temp, and process variations
alone are >2X in linear region!
• Output stages must be designed to reduce this variation– On-chip resistors designs – Logically tunable designs
37
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 38
Impedance Control Using On-Chip Resistors• Given a precise on-chip resistor, this design provides the
best impedance control
38
data
enable
Pad
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 39
Tunable Impedance Control• Stacked device settings can be preset or dynamically
controlled
39
n1 n2 n3
data
enable Pad
p3p2p1
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 40
Slew Rate Control• Output stage slew rate is controlled to reduce noise
– Cross talk noise– Simultaneous switching noise– Reflections at discontinuities
• Slew rate control is accomplished by controlling the pre-driver delay and/or pre-driver strength
40
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 41
Slew Rate Control• Output stage is divided and pre-drive signal is designed to
sequentially arrive at the different sections
41
data
enable Pad
δ δ
δ δ
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 42
Slew Rate Control & Impedance Control• Pre-driver design might even permit crossover currents to
guarantee impedance even during switching
42
data
enable Pad
δ δ
δ δ
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 43
Feedback Slew Rate Control I/O Buffer
data
enablePad
(Motorola 68332, McDermott, Carter)
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 44
Mixed Voltage Designs• Needed when chips have different supply voltages• Low voltage circuits can be damaged by high voltage
inputs• High voltage circuits suffer delay & noise problems when
receiving low voltage signals
44
Vdd1
newertechnology
Vdd2Bi-directionalI/O Buffers
oldertechnology
Vdd1 < Vdd2
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 45
Input Design for Higher Voltages• Modifications for gate oxide & ESD protection
45
ESD Diodes
Pad
ESD Diodes
Pad
Receiving Same Level Receiving Higher Level
change beta ratio
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 46
Pass Gate Behavior• nfet and pfet pass gates can be used to limit voltages
46
0
Vdd
V1
V1 - Vtn
0
Vdd
V1
V1 - Vtp
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 47
Dual Supply Designs• Separately power I/O circuits at a lower voltage
– No additional process steps required– Extra design to avoid performance penalty – ESD & simultaneous switching noise compromised
47
Vdd1
newertechnology
Vdd2Bi-directionalI/O Buffers
oldertechnology
Vdd1
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 48
Output Stage at a Lower Voltage• Slow rising delay due to low overdrive on pfet• Reduced drive = reduced noise immunity on nand receiver
48
ESD Diodes
data
inhibit
Vdd1
Pad
Vdd1 or Vdd2
enable
Vdd2
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 49 49
Output Stage at a Lower Voltage• Improve rising delay with nfet pull up• Change p/n beta ratio on nand to lower switch point
ESD Diodes
data
inhibit
1.2 Volts
Pad
1.2 or 1.8 Volts
enable
1.8 Volts
change beta ratio
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 50
Dual Supply Designs• Separately power the I/O circuits at a higher voltage
– More complicated circuits– ESD & simultaneous switching noise compromised
50
1.8 Volts
oldertechnology
1.2 Volts Bi-directionalI/O Buffers
newertechnology
1.8 Volts
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 51
Output Stage at a Higher Voltage• Slow rising delay due to low overdrive on pfet• Reduced drive = reduced noise immunity on nand receiver
51
data
enable
Vdd1
Pad
Vdd2
Vdd2
Vdd1
LevelShifter
Vbias
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 52
Floating Well Designs • Enabled output stage sends lower voltage - Vdd1• Disabled output stage tolerates higher voltage - Vdd2
52
data
enablePad
Vdd1
Vdd1
Vdd1
Vdd1
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 53
Open Drain Signaling• Avoids complexity of multiple chip power supplies
– Off-chip termination resistors pull net up– On-chip nfet devices pull net down
• Increases transmission line design complexity• Wired OR functionality
53
CL CL CL CL
VttVtt
CL
Driving Chip
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 54
Other Circuits• Differential I/O Circuits
– Reduces simultaneous switching noise– Improves receiver common mode noise immunity– Receives smaller signal levels– “Pseudo” to full differential possible
• Hysteresis Receivers– High noise immunity– Excellent for low-speed asynchronous test & control
signals
• Hold Clamps
54
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 55
Differential Output Buffers
55
Pseudo Differential Outputs
Differential Outputsout
out
out out
Vbias
Vdd
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 56
Differential Transmission Lines
56
Pseudo = two lines
Zo
ZoDifferential = coupled pair
coupled
Zeff < Zo
Zeff < Zo
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 57
Differential Far End Termination
57
Pseudo Differential Termination
Vtt
R = Zo Vtt
R = ZoDifferential Termination
R = 2 Zo
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 58
Differential Receivers
58
Pseudo Differential Receiver
Differential Receiverout
out
out out
Vbias
Vdd
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 59
Self Biased Differential Receiver• Combines best of nfet and pfet differential receivers
59
out out
Nbias
Vdd
out out
Pbias
Vdd
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 60
Self Biased Differential Receiver• Combines best of nfet and pfet differential receivers
– Rail to rail output swing– Excellent common mode noise rejection
60
out
Vdd
(Bazes, JSSC 91)
or reference
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 61
Hysteresis Input Receivers• Separates rising & fall edge dc transfer curves
inhibit
Pad
weak feedback inverter
Vin
Vout
VoutVin
risingfalling
and only
Pad Vin Vout
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 62
Hold Clamps
• Weak clamps hold tri-stated source terminated nets
• Stronger clamps will actively terminate the net– Can be slower than passive termination schemes
Padweak feedback inverter
Vdd
I/O
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 63
ESD Design• Pins subjected to ESD (electrostatic discharge) events
during test & handling• Over-voltages can also occur during functional operation
– System power-on– Hot-plugging
• ESD discharge can occur between any two pins – I/O to I/O– I/O to Vdd or Gnd
• Pins are measured against standard ESD tests– Human body model– Machine model– Charged Device Model
• ESD performance depends on many parameters other circuits don’t care about
63
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 64
ESD Circuits• Non-breakdown based circuits
– Diodes– Bipolar Junction Transistor– MOS FET
• Breakdown based circuits– Thick Field Oxide Device – SCR (silicon controlled rectifier)
64
Primary ESD Circuit inCMOS Designs
Most Diode Circuitsare really Bipolar Circuits
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 65
Dual Diode ESD Circuits
65
ESD Diodes
Pad
ESD Diodes
Pad
Single Supply Design Mixed voltage design
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 66
FET ESD Circuits: non-breakdown mode
66
ESD Diodes
Pad
nfet in “diode”configuration
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 67
FET ESD Circuits: breakdown mode
67
ESD Diodes
Pad
nfet protectsby clamping voltage
after device snapbackV
Isnapback
secondbreakdown
Vgs > Vt
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 68
Diode ESD Circuits• fet devices are parasitic npn & pnp bipolar circuits
68
ESD Diodes
Pad
ESD bipolar devices
Pad
• vertical pnp device to substrate
• horizontal npn device to guard rings (before trench isolation)
• low vdd to gnd impedance to due on-chip capacitance provide additional discharge paths
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 69
Parasitic Bipolar Circuits• fet devices are parasitic npn & pnp bipolar circuits
69
• vertical pnp device to substrate
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 70
ESD Test Models• Human Body Model
– Requirements 2 - 4 kVolts– Positive or negative discharge between any two pins
70
VHBM DUT
C = 100 pF
R = 1.5 KΩ
i(t)
timet = 2-10 nsec
ipeak = VHBM/1500
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 71
ESD Test Models• Machine Model
– Requirements 200 - 400 Volts– Positive or negative discharge between any two pins
71
VMM DUT
C = 200 pF
R < 8.5 Ω
L = 0.5 - 0.75 μH
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 72
ESD Performance Factors• Diode symmetry is important
– Bipolar conduction increases with temperature– Hot spots conduct more, heat up more, conduct more,
… and finally burn out• Layout corners are rounded to reduce electric fields• Decoupling capacitance needed between all supplies• Functional performance requirements impose ESD size &
load capacitance constraints• Parasitic bipolar effects abound• Breakdown clamps don’t scale
72
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 73
Increasing Bandwidth
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 74
Common Clock Transfers
74
• Chip to chip transfers controlled by common bus clock• Equal length card routes to each chip & on-chip PLL’s
minimize clock skew
clocksource
PLL
Chip A
PLL Chip B
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 75
Common Clock Transfers
75
clocksource
PLL
Chip A
PLLChip B
Tclk - A
TtofTdrive
TAclk
Tclk - B
TBclk
Treceive Tsetup
max(Tclk - A+TAclk +Tdrive+ Ttof+ Treceive + Tsetup ) - min(TBclk - Tclk - B) < Tcycle
Cycle time to meet setup time
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 76
Source Synchronous I/O
76
• Send source clock with source data
• Resolve clock phase differences with τ1, τ2, & τ3
clocksource
PLL
Chip A
PLL
Chip Bτ3
τ2τ1
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 77
Bus Pumping
77
• With Ttof > Tcycle, multiple bits are present on the wire
clocksource
PLL
Chip A
PLL
Chip Bτ3
τ2τ1
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 78
Dual Data Rate
78
• Conventional source synchronous design– Data launched & captured on single clock edge – Clock switches at f– Maximium data rate = 1/2 * f
• Dual data rate - if clock can switch at f, why not data?– Data is launched & captured on both clock edges – Clock switches f– Maximum data rate = f
Conventional Dual Data Rate
Data
Clock
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 79
Simultaneous Bidirectional Signaling • Two chips send & receive data simultaneously on a
point to point net• Waveforms superimpose on the transmission line• Each chip selects it’s receiver reference voltage based
on the data it sent• Sending data is subtracted from total waveform
Chip A
3/4 Vdd1/4 Vdd
Chip B
3/4 Vdd1/4 Vdd
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 80
Pattern Based Driver Compensation • Incident waveforms along a long-lossy lines attenuate • Slow “RC” like response to final level
(1- e-R*length/2Zo)1/2
Vs
Rs = Zo
τfwhere τf = length / velocity
With complex impedance and propagation constanthigh speed wavefront decays exponentially
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 81
Pattern Based Driver Compensation
• Adjust driver strength based on bits sent in earlier cycles
• Example: When driving low to high– Drive harder if previous bits sent = 00– Drive weaker if previous bits sent = 10
1 0 0 1 0 0
Without Compensation
1 0 0 1 0 0
Drive harder
With Compensation
ReceiverSwitch Point
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 82
Increasing Bandwidth
• Preceding techniques cannot be achieved through clever circuit design alone
• Requires good packaging technology & net design– Good termination– Minimal capacitive & inductive discontinuities– Low cross-talk– Low simultaneous switching noise
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 83
SerDes Architecture
The University of Texas at AustinEE382M VLSI-II Class Notes Foil # 84
PCI-Express Interface
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