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EE4800 CMOS Digital IC Design & Analysis
Lecture 13 Packaging, Power
and Clock Distributions
Zhuo Feng
13.1
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
■ Packaging
Outline
■ Power Distribution
■ Clock Distribution
13.2
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Packages
■ Package functions
► Electrical connection of signals and power from
chip to board
► Little delay or distortion
► Mechanical connection of chip to board
► Removes heat produced on chip
► Protects chip from mechanical damage
► Compatible with thermal expansion
► Inexpensive to manufacture and test
13.3
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Package Types
■ Through-hole vs. surface mount
13.4
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Multichip Modules
■ Pentium Pro MCM
► Fast connection of CPU to cache
► Expensive, requires known good dice
13.5
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Chip-to-Package Bonding
■ Traditionally, chip is surrounded by pad frame
► Metal pads on 100 – 200 mm pitch
► Gold bond wires attach pads to package
► Lead frame distributes signals in package
► Metal heat spreader helps with cooling
13.6
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Advanced Packages
■ Bond wires contribute parasitic inductance
■ Fancy packages have many signal, power layers
► Like tiny printed circuit boards
■ Flip-chip places connections across surface of
die rather than around periphery
► Top level metal pads covered with solder balls
► Chip flips upside down
► Carefully aligned to package (done blind!)
► Heated to melt balls
► Also called C4 (Controlled Collapse Chip Connection)
13.7
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Package Parasitics
 Use many VDD, GND in parallel
– Inductance, IDD
Package
Signal Pads
Signal Pins
13.8
Chip
VDD
Bond Wire
Lead Frame
Board
VDD
Package
Capacitor
Chip
Chip
GND
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Board
GND
Heat Dissipation
■ 60 W light bulb has surface area of 120 cm2
■ Itanium 2 die dissipates 130 W over 4 cm2
► Chips have enormous power densities
► Cooling is a serious challenge
■ Package spreads heat to larger surface area
► Heat sinks may increase surface area further
► Fans increase airflow rate over surface area
► Liquid cooling used in extreme cases ($$$)
13.9
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Thermal Resistance
■ DT = qjaP
► DT: temperature rise on chip
► qja: thermal resistance of chip junction to ambient
► P: power dissipation on chip
■ Thermal resistances combine like resistors
► Series and parallel
■ qja = qjp + qpa
► Series combination
13.10
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Example
■ Your chip has a heat sink with a thermal
■
■
■
■
resistance to the package of 4.0° C/W.
The resistance from chip to package is 1° C/W.
The system box ambient temperature may reach
55° C.
The chip temperature must not exceed 100° C.
What is the maximum chip power dissipation?
■ (100-55 C) / (4 + 1 C/W) = 9 W
13.11
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Temperature Sensor
■ Monitor die temperature and throttle performance
■ Use a pair of pnp bipolar transistors
► Vertical pnp available in CMOS
qV B E
Ic  Ise
kT
 V BE 
D V BE  V BE 1  V BE 2 
kT
q
ln
Ic
Ic
I c 2  kT 
I c 1  kT
kT  I c 1
ln

ln

ln
ln m




q 
Is
Is 
q  Ic2 
q
■ Voltage difference is proportional to absolute temp
► Measure with on-chip A/D converter
13.12
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Power Distribution
■ Power Distribution Network functions
► Carry current from pads to transistors on chip
► Maintain stable voltage with low noise
► Provide average and peak power demands
► Provide current return paths for signals
► Avoid electromigration & self-heating wearout
► Consume little chip area and wire
► Easy to lay out
13.13
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
IR Drop in Power Distribution Network
 IR drop: voltage drop due to non-ideal resistive wires
VDD
X
X
VDD
VDD
GND
13.14
GND
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Cadence
Power Delivery Network
C4 Bumps
Package
Chip
Socket
PCB
Entire Power Delivery Network
On-Chip Power Delivery Network
13.15
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Power System Model
■ Power comes from regulator on system board
► Board and package add parasitic R and L
► Bypass capacitors help stabilize supply voltage
► But capacitors also have parasitic R and L
■ Simulate system for time and frequency responses
Voltage
Regulator
VDD
Bulk
Capacitor
Board
13.16
Printed Circuit
Board Planes
Ceramic
Capacitor
Package
and Pins
Package
Capacitor
Solder
Bumps
On-Chip
Capacitor
Package
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Chip
On-Chip
Current Demand
Power Delivery Network Modeling
Thousands of nodes
13.17
Multi-million nodes
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
On-Chip Power Grid Modeling & Analysis
■ Multi-layer interconnects are modeled as 3D RC network
► Switching gate effects are modeled by time-varying current loadings
Vdd
Vdd
Vdd
Vdd
■ DC analysis solves linear system
Tens of millions
of unknowns !
G v  b
■ Transient analysis solves
G  v (t )  C 
dv ( t )
dt
13.18
 b (t )
G
n n
C 
n n
v
n 1
b 
n 1
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
:Conductance Matrix
:Capacitance Matrix
:
Node Voltage Vector
: Current Loading Vector
Power Requirements
■ VDD = VDDnominal – Vdroop
■ Want Vdroop < +/- 10% of VDD
■ Sources of Vdroop
► IR drops
► L di/dt noise
■ IDD changes on many time scales
Power
Max
clock gating
Average
Min
Time
13.19
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Important Metrics I
■ Voltage Droop.
► Two sources: IR drops and Ldi/dt noises.
► Large voltage droop leads to circuit timing failure or function
failure.
► Usually only 5% rail is allowed for 90nm or beyond
technologies.
VDDn
Tn
+
V dn  V D D  V D D n  V SSn 
Vn
V dn  5% V DD
-
VDD
VSSn
Vdn
VDDn -VSSn
13.20
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
5%VDD
IR Drop
■ A chip draws 24 W from a 1.2 V supply. The
power supply impedance is 5 mW. What is the IR
drop?
■ IDD = 24 W / 1.2 V = 20 A
■ IR drop = (20 A)(5 mW) = 100 mV
13.21
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
L di/dt Noise
■ A 1.2 V chip switches from an idle mode
consuming 5W to a full-power mode consuming
53 W. The transition takes 10 clock cycles at 1
GHz. The supply inductance is 0.1 nH. What is
the L di/dt droop?
DI = (53 W – 5 W)/(1.2 V) = 40 A
Dt = 10 cycles * (1 ns / cycle) = 10 ns
L di/dt droop = (0.1 nH) * (40 A / 10 ns) = 0.4 V
13.22
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Important Metrics II
■ Current density.
► High direct current density leads to electromigration
phenomenon.
► Electromigration decreases reliability or even breaks
connections.
J avg , m  I avg , m / w m
J avg , m  
Im
Im
0
Iavg,
m
13.23
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Technology Scaling: Challenges
■ Scaled down supply voltage vs. increased
operating frequency and power densities.
► Higher percentage of voltage droops.
► Lower voltage droop tolerance.
■ Shrunk chip area vs. increased gated density.
► Less wiring resources.
► Larger power delivery network dimensions.
■ Fine-grained power management.
► Power gating.
► Multiple power domains.
13.24
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Technology Scaling: Implications
■ Simulation:
► Stricter accuracy requirement.
► More efficient in terms of runtime and memory.
► Full chip simulation with package and integrated
components.
■ Verification:
► Power-gated power delivery network with multiple power
gating configurations.
■ Design:
► Optimization for wire sizing.
► Detailed tradeoff analysis for on-chip voltage regulation.
► Optimization for on-chip voltage regulation.
13.25
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Frequency Response
■ Multiple capacitors in parallel
► Large capacitor near regulator has low impedance at low
frequencies
► But also has a low self-resonant frequency
► Small capacitors near chip and on chip have low impedance at
high frequencies
■ Choose caps to get low impedance at all frequencies
impedance
frequency (Hz)
13.26
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Example: Pentium 4
■ Power supply impedance for Pentium 4
► Spike near 100 MHz caused by package L
■ Step response to sudden supply current chain
► 1st droop: on-chip bypass caps
► 2nd droop: package capacitance
► 3rd droop: board capacitance
[Wong06]
13.27
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Clock Distribution
■ On a small chip, the clock distribution network is just a wire
► And possibly an inverter for clkb
■ On practical chips, the RC delay of the wire resistance and
gate load is very long
► Variations in this delay cause clock to get to different elements at
different times
► This is called clock skew
■ Most chips use repeaters to buffer the clock and equalize the
delay
► Reduces but doesn’t eliminate skew
13.28
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Example
■ Skew comes from differences in gate and wire delay
► With right buffer sizing, clk1 and clk2 could ideally arrive at the
same time.
► But power supply noise changes buffer delays
► clk2 and clk3 will always see RC skew
gclk
3 mm
3.1 mm
clk1
1.3 pF
13.29
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
clk2
0.4 pF
0.5 mm
clk3
0.4 pF
Review: Skew Impact
available for work
■ Skew adds sequencing
Q1
F1
■ Ideally full cycle is
clk
Combinational Logic
Tc
clk
tpcq
overhead
Q1
F1
clk
t pd  T c   t pcq  t setup  t skew 
Q1
CL
clk
D2
t cd  t hold  t ccq  t skew
F2
sequencing overhead
tskew
clk
thold
Q1 tccq
D2
13.30
tskew
tpdq
D2
■ Increases hold time too
D2
F2
clk
tcd
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
tsetup
Solutions
■ Reduce clock skew
► Careful clock distribution network design
► Plenty of metal wiring resources
■ Analyze clock skew
► Only budget actual, not worst case skews
► Local vs. global skew budgets
■ Tolerate clock skew
► Choose circuit structures insensitive to skew
13.31
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Clock Dist. Networks
■ Ad hoc
■ Grids
■ H-tree
■ Hybrid
13.32
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Clock Grids
■ Use grid on two or more levels to carry
clock
■ Make wires wide to reduce RC delay
■ Ensures low skew between nearby points
■ But possibly large skew across die
13.33
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Alpha Clock Grids
Alpha 21064
Alpha 21164
Alpha 21264
PLL
gclk grid
gclk grid
Alpha 21064
13.34
Alpha 21164
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Alpha 21264
■ Fractal structure
H-Trees
► Gets clock arbitrarily close to any point
► Matched delay along all paths
■ Delay variations cause skew
■ A and B might see big skew
A
13.35
B
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Itanium 2 H-Tree
■ Four levels of buffering:
► Primary driver
Repeaters
► Repeater
► Second-level
clock buffer
► Gater
■ Route around
obstructions
13.36
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
Typical SLCB
Locations
Primary Buffer
Hybrid Networks
■ Use H-tree to distribute clock to many points
■ Tie these points together with a grid
■ Ex: IBM Power4, PowerPC
► H-tree drives 16-64 sector buffers
► Buffers drive total of 1024 points
► All points shorted together with grid
13.37
Z. Feng MTU EE4800 CMOS Digital IC Design & Analysis
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