1

A High-Speed Variation-TolerantSub-Threshold Interconnect Technique

Using Capacitive Boosting

Jonggab Kil*, Jie Gu, and Chris H. Kim

*Intel Corporation

University of MinnesotaDepartment of Electrical and Computer Engineering

jonggab.kil@intel.com, chriskim@umn.eduwww.umn.edu/~chriskim/

2

Presentation Agenda• Global Interconnect in Sub-threshold

• Proposed Sub-threshold Interconnect Technique

• Application on Sub-threshold Clocking

• Performance and Power Results

• Test Chip Measurements

• Conclusions

3

• Main Benefit– Super-linear power savings

– Minimum energy solution for low-performance designs

Subthreshold Operation

ddleakddtotalVIVCfP ⋅+⋅⋅⋅= 2α

Vgs(V)

I ds(A

/µm

)

operating region

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

0.0 0.2 0.4 0.6 0.8 1.00.E+00

1.E-04

2.E-04

3.E-04

4.E-04

5.E-04log scale

linear scale

Vgs Ids

• Limitations– PVT variation– Interconnect delay– Lack of a systematic design

methodology

4

Interconnect Scaling Problem

0%

20%

40%

60%

80%

100%

Nor

mal

ized

del

ay

Technology node (nm) 90 65 45 32

Logic delay Global interconnect delay

50%56%

64%73%

with repeaters

0%

20%

40%

60%

80%

100%

Nor

mal

ized

del

ay

Technology node (nm) 90 65 45 32

Logic delay Global interconnect delay

50%

66%

81%91%

without repeaters

• Global interconnect delay dictates system performance• Multiple clock cycles required for global signals to

propagate across die

5

Interconnect Delay in Sub-threshold

We need interconnect technique for sub-threshold logic

0

0.2

0.4

0.6

0.8

1

1.2

1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2

Sub-thresholdSuper-threshold

Nor

mal

ized

del

ay

Supply Voltage (V)

Global interconnect delay Logic delay

Globalinterconnect delay

VDD

Logic delay

VDD

0.18µm, 20°C

• Global interconnect problem worsens in sub-threshold– Device resistance dominates over wire resistance– Wire capacitance is constant while MOS capacitance reduces at

lower voltages

6

Proposed Sub-threshold Interconnect Technique

Operating region is shifted

from sub-threshold to super-threshold

Charge pump circuitry to boost

the gate voltage of drivers

100X+ increase in drive current at the expense of leakage

current

7

Circuit Operation: Active Mode

1

2

IN OUT

*Output holders not shown

0V

0.4V

0V

0.4V 0V

0.4V

0.4V0.8V

-0.4V0V

-0.4V

0.8V

-0.4V

0.8V

NMOS boosted

8

Circuit Operation: Active Mode

0V

0.4V

0V

0.4V

0V

0.4V

-0.4V

0.8V

0.4V0.8V

9

Circuit Operation: Standby Mode

• Half cycle start-up time• No start-up required for inactive periods up to 200 cycles

(@ 0.4V, 4MHz, 20°C)• Level holders can be used for restoring output level

10

Boosting Capacitance in Sub-threshold

-Vt

Cgate

DepletionModerate

Strong inversion

Sub-thresholdOperating

region

Vgs

Operates in weak inversion region

Accumulation

11

Boosting Efficiency

12

Operating Waveforms

0.0 0.1 0.2 0.3 0.4Time (µs)

0.0

0.4

0.2

This work ConventionalInput

0.0 0.1 0.2 0.3 0.4

0.0

0.4

0.8

-0.4

0.0

0.4

0.8

-0.4

VBOOST_N

VBOOST_P

VPRESET_P

VPRESET_N

Time (µs) Time (µs)

Volta

ge (V

)

0.18µm, 20°C

VIN_BARVIN_BAR

0.0 0.1 0.2 0.3 0.4

Volta

ge (V

)

Volta

ge (V

)

• 2.6X delay improvement for 1pF load

13

Delay and Power Consumption

0

50

100

150

200

0.0 1.5 3.0 4.5

Fall delay (This work)

Rise delay (This work)

Rise delay (Conv.)

Fall delay (Conv.)

• 2X+ delay improvement for different interconnect loads• Comparable power consumption once the frequencies

are high enough that active and short circuit power dominate over leakage current

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0 4 8 12

14

Sensitivity to Variation

Voltage (V) Temperature(°C)200.38

This work Delay variation

Conv. Delay variation

4060

800.39

0.400.41

0.42

0.1

0.2

Temperature(°C)200.38

This work Delay variation

Conv. Delay variation

4060

800.39

0.400.41

0.42

0.1

0.2

Voltage (V)

0.05

0.15

0.05

0.15

• Delay variation reduced by 2X under supply (0.4V±5%) and temperature (20~80°C) variations

• The driver transistors are no longer in the sub-threshold region

15

Sub-threshold Clocking

......

......

Four clock buffer stages with each buffer driving four clock buffers and the long interconnects

D. Harris, TVLSI 2001

Clock signal paths are symmetrically routed across the chip for

minimum skew

16

Clock Skew Comparison

Proposed driverAverage delay: 52ns

Standard deviation: 5.2ns

Conventional driverAverage delay: 251ns

Standard deviation: 30.7ns

• 8.3X reduction in 3σ clock skew and 22% reduction in σ/µ using the proposed driver

17

Test Chip Photograph

• The number of transistors per driver increased from 4 to 18 leading to 30% area overhead

• 40% of the driver area is occupied by boosting capacitors

18

Test Chip Schematic

Level-down converter : NMOS transistor is

employed to pull down node A1

Level-up converter PMOS transistors (P7

and P8) used to reduce the contention

Boo

stin

gci

rcui

t

19

Performance Measurements

Rise (fall) delay improved by 2.6X (2.9X)

Supply voltage : core(0.4V), level converter (1.0V), IO buffer (1.8V)

20

Performance Measurements

0.4 0.45 0.5

Proposed boosting technique is efficient in reducing the PVT impact at lower supply voltages

21

Variability and Power Measurements

• Temperature sensitivity reduced from 1.6X to 0.7X• Proposed driver operated with 41% less power dissipation

at 4MHz or 49% higher performance at 5.1µW

22

Conclusions• Global interconnects in sub-threshold region

suffer from increased delay and large sensitivity to PVT variations

• Proposed capacitive boosting technique – Effective for high activity long line drivers– 70% boosting efficiency at 0.4V

• Measurements from a 0.18µm test chip show– 1.7-2.9X delay improvement– 2.3X reduced delay variability at 0.4V– 41% power saving at 4MHz– 49% performance improvement at 5.1µW