1

Why Low Power Testing?

台大電子所 李建模

2

Test is HOT! Power consumption in test mode higher than normal operation

2 X higher [Zorian 93] ISCAS Benchmark circuit simulation [Li 04]

CUT Normal (mW) Scan mW( X higher than normal)

s526 89.5231.2

(1.58X)

s1494 220.0438.0

(1.99X)

s5378 979.91,831.7(1.87X)

s9234 1,303.33,026.8(2.32X)

s38417 5,452.711,618.3

(2.13X)

average 1,210.32,092.7(1.73X)

3

Why Test Power is High?

ATPG patterns try to detect as many faults as possible High toggle activities when testing

ATPG patterns less correlated than functional patterns [Wang 97]

Many circuit nodes are only toggling when scan chains shift

Some DFT circuit only activated in test mode

4

Why Low Power Testing?

Avoid power problems (e.g. IR drop) in test mode Ensure correct operation in test mode

Protect CUT from overheating in test mode No expensive package for testing

Enable parallel testing of multiple cores in SOC Save test time

Avoid reliability problems Overheating can shorten CUT lifetime

5

Intel CPU Power (Normal Operation)

Source: Borkar, De Intel

Pow

er

(Watt

s)

Of

Inte

l P

art

s

4004

80088080

8085

8086

286

386

486

0.1

1

10

100

1000

10000

100000

1971 1974 1978 1985 1992 2000 2004 2008

P6

Pentium®

History and projection

6

Types of Power Dissipation

Classified by Circuit Operation Classified by Circuit Type Classified by Time

7

Classified by Circuit Operation Static power dissipation (circuit activity independent)

Leakage power Dynamic power dissipation (circuit activity dependent)

Short circuit power Switch power

8

Classified by Circuit Operation (2)

Source: Synopsys Power Compiler

9

Short Circuit Power

Short Circuit Power Caused by momentary short circuit current

When NMOS PMOS both on Occurs only when circuit switching Can be estimated by the equation

Ei = energy consumed per transition of gate i. usually provided by the ASIC vendor

TRi = toggle rate of output of gate i. TRi is usually obtained by simulation

)( iigateallforiSC TREP

10

Short Circuit Power

Transient short circuit current, ISC

11

Switching Power

Switching Power is Caused by charging and discharging load capacitance

Wire capacitance, parasitic capacitance Occurs only when circuit is switching Can be estimated by the equation

Cload i = total load capacitance connected to net i. extracted from layout or estimated by synthesis tool

TRi = toggle rate of output of gate i. TRi is usually obtained by simulation

)(.2

2

iinetallfor

iloadDD

SW TRCV

P

12

Leakage Power Leakage power is

Caused by static leakage current Such as source to drain, gate tunneling currents

Independent of circuit activities Dependent on technology

Such as transistor Vt , VDD

Not a big concern …. yet Maybe problems in future generations

Can be estimated by Ileak VDD

13

Leakage Power Projection

Source: Borkar, De Intel

Pow

er

(Watt

s)

Of

Inte

l P

art

s

40048008

80808085

8086286

386486

0.1

1

10

100

1000

10000

100000

1971 1974 1978 1985 1992 2000 2004 2008

P6

0

.4

.8

1.2

1.6

2

2000 2002 2004 2006 2008

VD

D a

nd

VT

Pentium®

8KW

1.7KW

400W

88W 12W

0%

10%

20%

30%

40%

50%

2000 2002 2004 2006 2008

Dra

in L

eakag

e P

ow

er

14

Leakage Significant When Burn In

Leakage power dominates burn in power

Source [Intel 02], [Miller 01]

15

Classified by Circuit Types

Logic power Power dissipation in combinational logic

Sequential power Power dissipation in the sequential elements

Such as scan chain, or latches Clock power

Power dissipation in clock tree

Other types of circuits (not covered in this lecture) Memory power

SRAM, DRAM Analog, Mixed Signal Power

16

Breakdown of Power Dissipation

ISCAS Benchmark Circuit (s9234)

0

200

400

600

800

1000

1200

1400

1600

BIST DFT Non-Scan

Pow

er (

uW)

SC.COMB.

SC.SEQ.

SW.

CLK CLK CLK

Source [Li 04]

SC = Short Ckt SW = Switching

17

Average power Peak power

Classified by time

Time (Clock cycle)

Power waveforms: Dividing a scan chain into two sub-chains [Lee 04]

18

My opinion

Low power testing not really needed now But will be needed in future Especially for burn in

Area needs more research Peak power reduction still needs more research Static power is difficult to control