cmpen 411 vlsi digital circuits spring 2012 lecture...
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Sp12 CMPEN 411 L15 S.1
CMPEN 411VLSI Digital Circuits
Spring 2012
Lecture 15:
Dynamic CMOS
[Adapted from Rabaey’s Digital Integrated Circuits, Second Edition, ©2003 J. Rabaey, A. Chandrakasan, B. Nikolic]
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Power and Energy Design Space
Constant Throughput/Latency
Variable Throughput/Latency
Energy Design Time Non-active Modules Run Time
Active
(Dynamic)
Logic design
Reduced Vdd
TSizing
Multi-Vdd
Clock Gating
DFS, DVS
(Dynamic Freq, Voltage Scaling)
Leakage
(Standby)
Multi-VT
Stack effect
Pin ordering
Sleep Transistors
Multi-Vdd
Variable VT
Input control
Variable VT
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Industry Example: IBM Cu11 (0.13 um)
ASIC Cu11 (130nm) Library : Dual-vt library Nominal Vt level (~300mv) Low Vt level (~210mv)
Low-vt version has same physical footprint ~15% improvement in gate delay ~10x increase in leakage power
Dual-VDD (Voltage Island)
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How about Gate Leakage?
multiple gate oxide (Sylvester et.al., DATE-2004)
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Dynamic CMOS
In _________ circuits at every point in time (except when switching) the output is connected to either GND or VDD
via a low resistance path.
fan-in of N requires ______ devices
_________ circuits rely on the temporary storage of signal values on the capacitance of high impedance nodes.
requires only _________ transistors
takes a sequence of ___________ and conditional __________phases to realize logic functions
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Dynamic Gate
In1
In2 PDN
In3
Me
Mp
CLK
CLK
Out
CL
Out
CLK
CLK
A
B
C
Mp
Me
Two phase operation
________ (CLK = 0)
________ (CLK = 1)
on
off
1
off
on
!((A&B)|C)
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Conditions on Output
Once the output of a dynamic gate is discharged, it cannot be charged again until the next precharge operation.
Inputs to the gate can make ________ transition(s) during evaluation.
Output state is stored on CL
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Properties of Dynamic Gates
Logic function is implemented by the PDN only
number of transistors is _____(versus 2N for static complementary CMOS)
should be smaller in area than static complementary CMOS
Full swing outputs (VOL = GND and VOH = VDD)
Non-ratioed - sizing of the devices is not important for proper functioning (only for performance)
Faster switching speeds
reduced load capacitance due to lower number of transistors per gate (Cint) so a reduced logical effort
reduced load capacitance due to smaller fan-out (Cext)
no Isc, so all the current provided by PDN goes into discharging CL
Ignoring the influence of precharge time on the switching speed of the gate, tpLH = 0 but the presence of the evaluation transistor slows down the tpHL
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Properties of Dynamic Gates, con’t
Power dissipation should be lower
no ______________power consumption since the pull-up path is not on when evaluating
lower ____________- both Cint (since there are fewer transistors connected to the drain output) and Cext (since there the output load is one per connected gate, not two)
by construction can have at most one transition per cycle – no _______________
But power dissipation can be significantly higher due to
_______________________
extra load on ____________
Needs a precharge clock
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Dynamic Behavior
-0.5
0.5
1.5
2.5
0 0.5 1
CLK
CLK
In1
In2
In3
In4
Out
In &
CLKOut
Time, ns
#Trns VOH VOL VM NMH NML tpHL tpLH tpre
6 2.5V 0V VTn 2.5-VTn VTn 110ps 0ns 83ps
Evaluate
Precharge
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Power Consumption of Dynamic Gate
In1
In2 PDN
In3
Me
Mp
CLK
CLK
Out
CL
Power only dissipated when previous Out = 0
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Dynamic Power Consumption is Data Dependent
A B Out
0 0 1
0 1 0
1 0 0
1 1 0
Dynamic 2-input NOR Gate
Assume signal probabilities
PA=1 = 1/2
PB=1 = 1/2
Then transition probability
P01 = Pout=0 x Pout=1
= ___________
Switching activity can be higher in dynamic gates!
P01 =__________
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Issues in Dynamic Design : Charge Leakage
CL
CLK
CLK
Out
A=0
Mp
Me
Minimum clock rate of a few kHz
Leakage
CLK
VOut
Precharge
Evaluate
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Impact of Charge Leakage
Output settles to an intermediate voltage determined by a resistive divider of the pull-up and pull-down networks
Once the output drops below the switching threshold of the fan-out logic gate, the output is interpreted as a low voltage.
-0.5
0.5
1.5
2.5
0 20 40
Time (ms)
Vo
ltag
e (
V)
CLK
Out
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A Solution to Charge Leakage
CL
CLK
CLK
Me
Mp
A
B
!Out
Mkp
Same approach as level restorer for pass
transistor logic
Keeper
Keeper compensates for the charge lost due to the pull-down leakage paths.
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Issues in Dynamic Design : Charge Sharing
CL
CLK
CLK
Ca
Cb
B=0
A
OutMp
Me
Charge stored originally on
CL is redistributed (shared)
over CL and CA leading to
static power consumption by
downstream gates and
possible circuit malfunction.
When Vout = - VDD (Ca / (Ca + CL )) the drop in Vout is
large enough to be below the switching threshold of
the gate it drives causing a malfunction.
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Solution to Charge Redistribution
CLK
CLK
Me
Mp
A
B
Out
MkpCLK
Precharge internal nodes using a clock-
driven transistor (at the cost of increased
area and power)
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Issues in Dynamic Design : Cascading Gates
CLK
CLK
Out1
In
Mp
Me
Mp
Me
CLK
CLK
Out2
V
t
CLK
In
Out1
Out2V
VTn
Only a single 0 1 transition allowed at the
inputs during the evaluation period!
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Domino Logic
In1
In2 PDN
In3
Me
Mp
CLK
CLKOut1
In4 PDN
In5
Me
Mp
CLK
CLKOut2
Mkp
1 1
1 00 0
0 1
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Why Domino?
In1
CLK
CLK
Ini PDN
Inj
Ini
Inj
PDN Ini PDN
Inj
Ini PDN
Inj
Like falling dominos!
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Domino Zero Detector
CLK
In7 In6 In5 In4 In3 In2 In0In1
not zero
How would you build it in static CMOS?
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Properties of Domino Logic
Only non-inverting logic can be implemented, fixes include
can reorganize the logic using Boolean transformations
use differential logic (dual rail)
use np-CMOS (zipper)
Very high speed
tpHL = 0
static inverter can be optimized to match fan-out (separation of fan-in and fan-out capacitances)
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Differential (Dual Rail) Domino
A
B
Me
Mp
CLK
CLK
!Out = !(AB)
!A !B
MkpCLK
Out = AB
Mkp Mp
Due to its high-performance, differential domino is
very popular and is used in several commercial
microprocessors!
1 0 1 0
onoff
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Other Domino Variations
Multiple output domino logic – exploits the fact that certain outputs are subsets of other outputs to generate a number of logic functions in a single gate.
Compound domino
A
B
Me
Mp
CLK
CLK
C
D
E
MeCLK
F
MpCLK
G
Me
H
Mp
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np-CMOS (Zipper)
In1
In2 PDN
In3
Me
Mp
CLK
CLKOut1
In4 PUN
In5
Me
Mp!CLK
!CLK
Out2
(to PDN)
1 1
1 0
0 0
0 1
Only 0 1 transitions allowed at inputs of PDN
Only 1 0 transitions allowed at inputs of PUN
to other
PDN’s
to other
PUN’s
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DCVS Logic (Differential Cascade Voltage Switch
In1
In2
PDN1
Out
!In1
!In2
PDN2
!Out
PDN1 and PDN2 are mutually exclusive
1 0 0on off
off on on off
on off 1
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DCVSL Example
Out
!Out
B
A !A
B!B!B
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How to Choose a Logic Style
Must consider ease of design, robustness (noise immunity), area, speed, power, system clocking requirements, fan-out, functionality, ease of testing
Style # Trans Ease Ratioed? Delay Power
Comp Static 8 1 no 3 1
CPL* 12 + 2 2 no 4 3
domino 6 + 2 4 no 2 2 + clk
DCVSL* 10 3 yes 1 4
4-input NAND
* Dual Rail
Current trend is towards an increased use of complementary static CMOS: design support through DA tools, robust, more amenable to voltage scaling.
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Itanium 2 Domino Circuitry
Integer execution unit
Multimedia execution unit
2 Floating point units
Register Files
Out of order control issue logic
– Source: “Advanced Domino Circuit Design” , Intel,
Tom Grutkowski, DATE 2004
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What is Soft Error
Soft errors are circuit errors caused due to excess charge carriers induced primarily by external radiations
These errors cause an upset event but the circuit it self is not damaged.
Same a SEU (single event upset)
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B
p substrate
G
n+n+
n channel
Soft Errors
The Phenomena
+ - + -+ -
+ -+-
+ -+ -
+ -+ -
A particle strikeCurrent
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Soft Errors
The Phenomena
VDD
Vout
CL
Vin
A particle strike
Bit Flip !!!
A particle
strike
!B
LB
L
W
L
0->11->00
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What cause Soft Errors?
At ground level, there are three major contributors to Soft errors.
1. Cosmic Ray induced neutrons
2. Alpha particles emitted by decaying radioactive impurities in packaging or interconnect materials.
3. Neutron induced 10B fission which releases a Alpha particle and 7Li
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Evidence of Cosmic Ray Strikes
Documented strikes in large servers found in error logs
Normand, “Single Event Upset at Ground Level,” IEEE Transactions on Nuclear Science, Vol. 43, No. 6, December 1996.
Sun Microsystems, 2000
Cosmic ray strikes on L2 cache with no error detection or correction
- caused Sun’s flagship servers to suddenly and mysteriously crash!
Companies affected
- Baby Bell (Atlanta), America Online, Ebay, & dozens of other corporations
- Verisign moved to IBM Unix servers (for the most part)
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Reactions from Companies
Fujitsu SPARC in 130 nm technology
80% of 200k latches protected with parity
compare with very few latches protected in Mckinley
ISSCC, 2003
IBM declared 1000 years system MTBF as product goal
very hard to achieve this goal in a cost-effective way
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Space redundancy: Redundant Logic
Logic 1
Logic 2Voter
Logic3
Point of failure!!
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Next Lecture and Reminders
Next lecture
Timing metrics, static sequential circuits
- Reading assignment – Rabaey, et al, 7.1-7.2