Transcript Pseudo-Random Sequence Generator

Design of VLSI Systems
By
Premananda B.S.
[email protected]
Modules
• Design Economics
• Special-purpose Subsystems
• Testing and Verification
Design Economics
Design Economics
• IC designer should able to predict the cost and
the time to design a particular IC.
• This guides the choice of implementation
strategy.
• Selling price Stotal = Ctotal / (1-m)
Where m = profit margin and Ctotal = total cost
• Cost to produce an IC are divided into:
– Nonrecurring engineering costs (NRE)
– Recurring costs
– Fixed costs
Non-recurring Engineering Costs
• Cost once spent during the design of an IC, they include:
– Engineering design cost
– Prototype manufacturing cost
– i.e., Ftotal = Etotal + Ptotal
• NRE can be viewed as an investment for which there is
a required rate of return.
• Engineering design costs, include:
– Personnel costs
– Support costs
• Prototype manufacturing costs, include:
– Mask cost
– Test fixture costs
– Package tooling
• The personnel cost include labor for:
–
–
–
–
–
–
–
Architectural design
Logic capture
Simulation for functionality
Layout of modules and chip
Timing verification
DRC and tapeout procedures
Test generation
• The support costs are:
– Computer costs
– CAD software costs
– Training
Recurring Costs
• The cost that recurs every time an IC is sold.
• The total cost is Ct = Cprocess + Cpack + Ctest
– Cprocess = W / (N.Yd.Ypack.)
• Fabrication
–
–
–
–
Wafer cost / (Dice per wafer * Yield)
Wafer cost: $500 - $3000
Dice per wafer:
Yield: Y = e-AD
• For small A, Y  1, cost proportional to area
• For large A, Y  0, cost increases exponentially
• Packaging
• Test
Fixed Costs
• Data sheets and application notes
• Marketing and advertising
Schedule
• Estimate the design cost and design time for the
system.
• Selecting the strategy by which the ICs will be
available in the right time and price.
• Experienced person.
• To estimate schedule some idea of the amount
effort required to complete the design.
• Schedule is a function of personpower.
• Methods for improving the schedules:
– Using a high productivity design method
– Improving the productivity of a given technique
– Decreasing the complexity of the design task by
partitioning
Personpower
•
•
•
•
•
•
Tasks required are:
Architectural design
HDL capture
Functional verification
PAR
Timing verification, signal integrity, reliability
verification
• DRC and tapeout procedures
• Test generation
Example
• You want to start a company to build a wireless
communications chip.
• How much venture capital must you raise?
• Because you are smarter than everyone else,
you can get away with a small team in just two
years:
– Seven digital designers
– Three analog designers
– Five support personnel
Solution
• Digital designers:
–
–
–
–
–
salary
overhead
computer
CAD tools
Total:
• Analog designers
–
–
–
–
–
salary
overhead
computer
CAD tools
Total:
• Support staff
–
–
–
–
salary
overhead
computer
Total:
• Fabrication
– Back-end tools:
– Masks:
– Total:
• Summary
• Digital designers:
–
–
–
–
–
• Support staff
$70k salary
– $45k salary
$30k overhead
– $20k overhead
$10k computer
– $5k computer
$10k CAD tools
– Total: $70k * 5 = $350k
Total: $120k * 7 = $840k • Fabrication
• Analog designers
– Back-end tools: $1M
– $100k salary
– Masks: $1M
– $30k overhead
– Total: $2M / year
– $10k computer
• Summary
– $100k CAD tools
– 2 years @ $3.91M / year
– Total: $240k * 3 = $720k
– $8M design & prototype
Cost Breakdown
• New chip design is fairly capital-intensive
• Maybe you can do it for less?
Special-purpose
Subsystems
Agenda
• Packaging
–
–
–
–
Package options
Chip-to-package connections
Package parasitics
Heat dissipation
• Power Distribution
– On-chip power distribution network
– Supply noise
• I/O
– Basic I/O pad circuits
• Clock
– Clock system architecture
– Global clock generation & distribution
– Local clock gaters
Packaging
• Package functions:
– Electrical connection of signals and power from chip to
board, with 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
Package Options
• Through-hole vs. surface mount
Multichip Modules
• Pentium Pro MCM
– Fast connection of CPU to cache
– Expensive, requires known good dice
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
Advanced Packages
• Metal leads contribute parasitic inductance and
coupling capacitors to their neighbors
• 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
Introduces new testing problems
Package Parasitics
• Use many VDD, GND in parallel
– Inductance, IDD
Package
Signal Pads
Signal Pins
Chip
VDD
Bond Wire
Lead Frame
Board
VDD
Package
Capacitor
Chip
Chip
GND
Board
GND
• Bond wires and lead frame contribute parasitic
inductance to the signal traces.
• They also have mutual inductance and
capacitive coupling to nearby signal traces,
causing crosstalk when multiple signal switch.
• VDD & GND wires also have inductance from
both bond wires and lead frame.
• They have nonzero resistance, which becomes
important for chips drawing large supply current
• High performances packages often include
bypass capacitors between VDD & 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
• Advances in heat sinks, fans, packages have
raised the practical limit for heat removal from
about 8 W in 1985 to nearly 100 W today for
affordable packaging.
• 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 ($$$)
Thermal Resistance
• Temperature difference between transistor
junctions and the ambient air is, 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
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?
• Solution is (100-55 C) / (4 + 1 C/W) = 9 W
Power Distribution
• Power Distribution Network functions:
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–
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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
Power Requirements
• VDD = VDDnominal – Vdroop
• Want Vdroop < +/- 10% of VDD
• L di/dt of bond wire and IR drop across on-chip wires are
often a major source of supply noise
• Sources of Vdroop
– IR drops
– L di/dt noise
• IDD changes on many time scales
Power
Max
clock gating
Average
Min
Time
IR Drops:
• Resistance of power supply network includes:
–
–
–
–
resistance of the on-chip wires and vias,
resistance of bond wires or solder bumps to the package,
resistance of the package planes,
resistance of the PCB planes
• IR drops arise from both average and instantaneous
current requirements.
Ldi/dt Noise:
• Inductance of power supply dominated by the inductance
of the bond wires
• Modern packages devote many of their pins to power
and ground to minimize supply inductance
• Two sources of current transients are switching I/O
signals and changes between idle and active mode in
the chip core
On-chip Bypass Capacitance
• The bypass capacitance is distributed across the chip so
that a local spike in current can be supplied from nearby
bypass capacitance rather than through the resistance of
the overall power grid.
• power distribution network doesn’t really need to carry all
of the peak current.
• Much of the difference between peak and average
current may be supplied by local, on-chip bypass
capacitors.
• On-chip bypass capacitors can reduce the amount of
metal needed for distribution.
• It also greatly reduces the di/dt drawn from the package.
Symbiotic Bypass Capacitors
• Where are the bypass capacitors in this picture?
• Gates that are not switching at a given instant in time act
as symbiotic bypass capacitors
• If only one gate in 60 switches at a given instant, the
bypass capacitance is 30 times the switched
capacitance
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 & frequency responses
Voltage
Regulator
VDD
Bulk
Capacitor
Board
Printed Circuit
Board Planes
Ceramic
Capacitor
Package
and Pins
Package
Capacitor
Package
Solder
Bumps
On-Chip
Capacitor
Chip
On-Chip
Current Demand
Bypass Capacitors
• Need low supply impedance at all frequencies
• Ideal capacitors have impedance decreasing with w
• Real capacitors have parasitic R and L
– Leads to resonant frequency of capacitor
2
10
1
10
1 mF
0.25 nH
impedance
0.03 
10
10
10
0
-1
-2
10
4
10
5
10
6
10
7
frequency (Hz)
10
8
10
9
10
10
Frequency Response
• Use 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)
Input / Output
• Input/Output System functions:
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Communicate between chip and external world
Drive large capacitance off chip
Operate at compatible voltage levels
Provide adequate bandwidth
Limit slew rates to control di/dt noise
Protect chip against electrostatic discharge
Use small number of pins (low cost)
I/O Pad Design
• Pad Types:
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–
–
–
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VDD and GND
Output
Input
Bidirectional
Analog
VDD and GND Pads
• High-performance chips devote about half of their pins to
power and ground.
• This large number of pins is required to carry the high
current and to provide low supply inductance.
• Largest sources of noise in many chips is the ground
bounce caused when the output pads switch.
• The pads must rapidly charge the large external
capacitive loads, causing a big current spike and high
Ldi/dt noise.
• The dirty power and ground lines serving the output pads
are separated from the main power grid to reduce the
coupling of I/O-related noise into the core.
Output Pads
• Drive large off-chip loads (2 – 50 pF)
– With suitable rise/fall times
– Requires chain of successively larger buffers
• Output transistors have gates longer than normal to
prevent avalanche breakdown damage and over voltage
is applied to the drains.
• Guard rings to protect against latchup
– Noise below GND injects charge
into substrate
– Large nMOS output transistor
– p+ inner guard ring
– n+ outer guard ring
• In n-well
Input Pads
• Level conversion
– Higher or lower off-chip V
– May need thick oxide gates A
• Noise filtering
– Schmitt trigger
A
– Hysteresis changes VIH, VIL
VDDH
VDDL
Y
VDDL
A
Y
weak
Y
Y
weak
A
• Protection against electrostatic discharge
ESD Protection
• Static electricity builds up on your body
– Shock delivered to a chip can fry thin gates
– Must dissipate this energy in protection circuits before
Diode
it reaches the gates
clamps
• ESD protection circuits
– Current limiting resistor
– Diode clamps
• ESD testing
R
PAD
Current
limiting
resistor
– Human body model
– Views human as charged capacitor
Thin
gate
oxides
1500 
100 pF
Device
Under
Test
Bidirectional Pads
• Need tristate driver on output:
– Use enable signal to set direction
– Optimized tristate avoids huge series transistors
PAD
En
Din
Dout
NAND
Dout
En
Y
Dout
NOR
Improved tri-state
buffer
Analog Pads
• Pass analog voltages directly in or out of chip:
– No buffering
– Protection circuits must not distort voltages
Clocking
• Synchronous systems use a clock to keep
operations in sequence
– Distinguish this from previous or next
– Determine speed at which machine operates
• Clock must be distributed to all the sequencing
elements
– Flip-flops and latches
• Also distribute clock to other elements
– Domino circuits and memories
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
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
clk1
1.3 pF
3.1 mm
clk2
0.4 pF
0.5 mm
clk3
0.4 pF
Review: Skew Impact
• Ideally full cycle is available for work
• Skew adds sequencing overhead
• Increases hold time too
t pd  Tc   t pcq  tsetup  tskew 
sequencing overhead
tcd  thold  tccq  tskew
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
Clock Skew Sources
• Clock Skew Sources are:
–
–
–
–
systematic,
random,
drift, and
jitter
• Note some engineers do not report jitter as part
of the skew.
Clock System Architecture
Synchronous chip interface with PLL
Phase-locked loop block diagram
Global Clock Distribution Networks
•
•
•
•
•
Grids
H-trees
Spines
Ad-hoc
Hybrid
Clock Grids
• A clock grid is a mesh of horizontal and vertical wires
driven from the middle or edges.
• 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.
• Grids compensate for random skew.
• Grids consume a large amount of metal resources and
hence a high switching capacitance and power
consumption.
Alpha Clock Grids
Alpha 21064
Alpha 21164
Alpha 21264
PLL
gclk grid
Alpha 21064
gclk grid
Alpha 21164
Alpha 21264
H-Trees
• Fractal structure
– Gets clock arbitrarily close to
any point
– Matched delay along all paths
• Delay variations cause skew
• Buffers are added to serve
as repeaters
• A and B might see big skew
• High random skew, drift and
jitter between two nearby
points
A
B
Itanium 2 H-Tree
• Four levels of buffering:
– Primary driver
– Repeater
– Second-level
clock buffer
– Gater
Repeaters
• Route around
obstructions
Typical SLCB
Locations
Primary Buffer
Ad-hoc
• The clock is routed haphazardly with some
attempt to equalize wire lengths or add buffers to
equalize delay.
• Have low systematic skews because the buffer
sizes can be adjusted until nominal delays are
nearly equal.
• Subject to random skew.
Clock Spine
• If loads are uniform, the spine avoids the systematic skew
of the grid by matching the length of the clock wires.
• Save power by not switching certain wires.
• System with many clocked elements may require a large
number of serpentine routes, leading to high area and
capacitance for the clock network.
• Clock spines have large skews between nearby elements
driven by different serpentines.
Hybrid Networks
•
•
•
•
Use H-tree to distribute clock to many points.
Tie these points together with a grid.
Hybrid combination of H-tree and grid offers lower skew.
Hybrid approach has lower systematic skew, less
susceptible to skew from non-uniform load distribution.
• Hybrid approach is regular, making layout of wellcontrolled transmission line structures easier.
• Ex: IBM Power4, PowerPC
– H-tree drives 16-64 sector buffers
– Buffers drive total of 1024 points
– All points shorted together with grid
Clock Generation
en clk
1
2
3
4
Local Clock Gaters
• Local Clock Gaters receives the global clock and
produce the physical clocks required by clocked
elements.
• Clock gaters are often used to stop or gate the
clock to unused blocks of logic to save power.
• Different clock gaters are:
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–
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Enabled or Gated clock
Stretched clocks
Nonoverlapping clocks
Complementary clock
Delayed, Pulsed clocks
Clock Doubler
Clock Buffer
Testing
and
Verification
Agenda
• Testing
– Logic Verification
– Silicon Debug
– Manufacturing Test
• Fault Models
• Observability and Controllability
• Design for Test
– Scan
– BIST
• Boundary Scan
Testing
• Testing is one of the most expensive parts of
chips
– Logic verification accounts for > 50% of design effort
for many chips
– Debug time after fabrication has enormous
opportunity cost
– Shipping defective parts can sink a company
• Example: Intel FDIV bug
– Logic error not caught until > 1M units shipped
– Recall cost $450M (!!!)
Main Difficulties in Testing
• Miniaturization
– Physical access difficult or impossible.
• Increasing complexity
– Large amount of test data.
• Number of access ports remains constant
– Long test application time.
• High speed
– High demand on tester’s driver/sensor mechanism.
• More complicated failure mechanism
 The key to successful testing lies in the design
process.
Some Real Defects in Chips
 Processing defects




Missing contact windows
Parasitic transistors
Oxide breakdown
...
 Material defects
 Bulk defects (cracks, crystal imperfections)
 Surface impurities (ion migration)
 ...
 Time-dependent failures
 Dielectric breakdown
 Electromigration
 ...
 Packaging failures
 Contact degradation
 Seal leaks
 ...
Cost of Finding Defects
 Wafer level
 Packaged chip
 Board level
 System level
 Field level
$0.01 - $0.10
$0.10 - $1
$1 - $10
$10 - $100
$100 - $1000
Errors and Faults
Logic Verification
• Does the chip simulate correctly?
– Usually done at HDL level
– Verification engineers write test bench for HDL
• Can’t test all cases
• Look for corner cases
• Try to break logic design
• Ex: 32-bit adder
– Test all combinations of corner cases as inputs:
• 0, 1, 2, 231-1, -1, -231, a few random numbers
• Good tests require ingenuity
Manufacturing Test
• A speck of dust on a wafer is sufficient to kill chip
• Yield of any chip is < 100%
– Must test chips after manufacturing before delivery to
customers to only ship good parts
• Manufacturing testers are
very expensive
– Minimize time on tester
– Careful selection of
test vectors
•
•
•
•
Layer-layer shorts
Discontinuous wires
Missing or damaged vias
Shorts through the thin gate oxide to the
substrate or well
• The above leads to:
– Nodes shorted to power or ground
– Nodes shorted to each other
– Input floating/ output disconnected
• The length of the tests at the wafer level can be
shortened to reduce test time based on
experience with the test sequence
Testing Your Chips
• If you don’t have a multimillion dollar tester:
– Build a breadboard with LED’s and switches
– Hook up a logic analyzer and pattern generator
– Or use a low-cost functional chip tester
Testers, Test Fixtures and Test
Programs
Logic Verification Principles
•
•
•
•
Test benches and harnesses
Regression testing
Version control
Bug tracking
Silicon Debug
• Test the first chips back from fabrication
– If you are lucky, they work the first time
– If not…
• Logic bugs vs. electrical failures
– Most chip failures are logic bugs from inadequate
simulation
– Some are electrical failures
• Crosstalk
• Dynamic nodes: leakage, charge sharing
• Ratio failures
– A few are tool or methodology failures (e.g. DRC)
• Fix the bugs and fabricate a corrected chip
• How to diagnose failures?
– Hard to access chips
•
•
•
•
•
•
Picoprobes
Electron beam
Laser voltage probing
Picosecond imaging circuit analysis
Infrared imgaing
Focused ion beam
Failures can be:
• Manufacturing failures
– Defect or Outside parametric specifications
• Functional failures
– Chip fails under all conditions
• Electrical failures
– Malfunctions under certain conditions
– Shmoo plots can help to debug electrical failures in
silicon
Shmoo Plots
• Shmoo Plot is made with voltage on X-axis and speed as
Y-axis.
• A healthy normal chip should operate at increasing
frequency as the voltages increases.
• The brick wall pattern suggests that the chip may be
randomly initialized in one of the two states, only one of
which is correct.
• A shmoo can also plot operating speed against
temperature.
• In floor, leakage problem where the part fails at low
frequency independent of the voltage.
• A finger indicates coupling problems dependent on the
alignment, where at certain frequencies the alignment
always causes a failure.
• At cold temp. FETs are faster, have lower effective
resistance, and higher threshold voltages.
• Normal shmoo should show speed increasing as
temperature decreases.
• Failures at low temperature could indicate coupling or
charge sharing noise exacerbated by faster edge rates.
• Failures at high temperature could indicate excessive
leakage or noise problems exacerbated by the lower
threshold voltages.
• Walls at either temperature indicate race conditions
where the path that wins the race varies with
temperature.
At 1.8 V the chip works for clock periods of 2.3 ns and greater.
At higher voltages, the chip can operate at shorter periods.
Stuck-At Faults
• How does a chip fail?
– Usually failures are shorts between two
conductors or opens in a conductor
– This can cause very complicated behavior
• A simpler model: Stuck-At
– Assume all failures cause nodes to be “stuckat” 0 or 1, i.e. shorted to GND or VDD
– Not quite true, but works well in practice
Examples
Short-circuit and open-circuit faults
Delay Fault Testing
Observability & Controllability
• Observability: ease of observing a node by
watching external output pins of the chip
• Controllability: ease of forcing a node to 0 or 1
by driving input pins of the chip
• Combinational logic is usually easy to observe
and control
• Finite state machines can be very difficult,
requiring many cycles to enter desired state
– Especially if state transition diagram is not known to
the test engineer
Test Pattern Generation
• Manufacturing test ideally would check every
node in the circuit to prove it is not stuck.
• Apply the smallest sequence of test vectors
necessary to prove each node is not stuck.
• Good observability and controllability reduces
number of test vectors required for
manufacturing test.
– Reduces the cost of testing
– Motivates design-for-test
• Fault coverage
Test Example
SA1
•
•
•
•
•
•
•
•
A3
A2
A1
A0
n1
n2
n3
Y
• Minimum set:
SA0
A3
A2
A1
A0
n1
Y
n2
n3
Test Example
•
•
•
•
•
•
•
•
A3
A2
A1
A0
n1
n2
n3
Y
SA1
{0110}
• Minimum set:
SA0
{1110}
A3
A2
A1
A0
n1
Y
n2
n3
Test Example
•
•
•
•
•
•
•
•
A3
A2
A1
A0
n1
n2
n3
Y
SA1
{0110}
{1010}
• Minimum set:
SA0
{1110}
{1110}
A3
A2
A1
A0
n1
Y
n2
n3
Test Example
•
•
•
•
•
•
•
•
A3
A2
A1
A0
n1
n2
n3
Y
SA1
{0110}
{1010}
{0100}
• Minimum set:
SA0
{1110}
{1110}
{0110}
A3
A2
A1
A0
n1
Y
n2
n3
Test Example
•
•
•
•
•
•
•
•
A3
A2
A1
A0
n1
n2
n3
Y
SA1
{0110}
{1010}
{0100}
{0110}
• Minimum set:
SA0
{1110}
{1110}
{0110}
{0111}
A3
A2
A1
A0
n1
Y
n2
n3
Test Example
•
•
•
•
•
•
•
•
A3
A2
A1
A0
n1
n2
n3
Y
SA1
{0110}
{1010}
{0100}
{0110}
{1110}
• Minimum set:
SA0
{1110}
{1110}
{0110}
{0111}
{0110}
A3
A2
A1
A0
n1
Y
n2
n3
Test Example
•
•
•
•
•
•
•
•
A3
A2
A1
A0
n1
n2
n3
Y
SA1
{0110}
{1010}
{0100}
{0110}
{1110}
{0110}
• Minimum set:
SA0
{1110}
{1110}
{0110}
{0111}
{0110}
{0100}
A3
A2
A1
A0
n1
Y
n2
n3
Test Example
•
•
•
•
•
•
•
•
A3
A2
A1
A0
n1
n2
n3
Y
SA1
{0110}
{1010}
{0100}
{0110}
{1110}
{0110}
{0101}
• Minimum set:
SA0
{1110}
{1110}
{0110}
{0111}
{0110}
{0100}
{0110}
A3
A2
A1
A0
n1
Y
n2
n3
Test Example
SA1
SA0
A
n1
A
A3
{0110}
{1110}
n2
n3
A
A2
{1010}
{1110}
A
A1
{0100}
{0110}
A0
{0110}
{0111}
n1
{1110}
{0110}
n2
{0110}
{0100}
n3
{0101}
{0110}
Y
{0110}
{1110}
Minimum set: {0100, 0101, 0110, 0111, 1010, 1110}
3
•
•
•
•
•
•
•
•
•
2
1
0
Y
Design for Testability
• Design the chip to increase observability and
controllability.
• If each register could be observed and controlled, test
problem reduces to testing combinational logic between
registers.
• Better yet, logic blocks could enter test mode where they
generate test patterns and report the results
automatically.
• Approaches for DFT:
– Ad-hoc testing
– Scan-based approaches
– BIST
Ad-hoc Methods
• Ad-hoc test techniques as their name suggests
are collections of ideas aimed at reducing the
combinational explosion of testing.
• The Ad hoc techniques represent a bag of tricks
developed over the years by designers to avoid
the overhead of a systematic approach to testing.
• The technique classified in this category is the
use of bus in a bus-oriented system for test
purposes.
• Each register has been made loadable from the
bus and capable of being driven onto the bus.
• The internal logic values that exist on a data bus
are enabled onto the bus for testing purposes.
• Frequently multiplexers can be used to provide
alternative signal paths during testing.
• A complete scan-based testing methodology is
recommended for all digital circuits.
• Objective: Adherence to design guidelines and
testability improvement techniques.
• Common techniques for ad-hoc testing involve:
–
–
–
–
Partitioning large sequential circuits
Adding test points
Adding multiplexers
Providing fro easy state reset
• The goal of Ad hoc technique is to increase
controllability, observability and/or predictability.
Disadvantages:
• Ad-hoc techniques are useful for small designs.
• Experts and tools not available.
• Test generation is often manual with no
guarantee of high fault coverage.
• Design iterations may be necessary.
Ad-hoc approaches are still quite valid, process
densities and chip complexities necessitate a
structured approach to testing.
• Test points:
– Employ test points to enhance controllability and observability.
• Initialization:
– Design circuits to be easily initializable.
• Monostable multivibrators:
– Disable internal one-shots during test.
• Oscillators and clocks:
– Disable internal oscillators and clocks during tests.
• Partitioning Counters and Shift Registers:
– Partition large counters and shift registers into smaller units.
• Partitioning of Large Combinational Circuits:
– Partition large circuits into small sub-circuits to reduce test
generation cost.
• Logical Redundancy:
– Avoid the use of redundant logic.
• Global Feedback Paths:
– Provide logic to break global feedback paths.
Scan Design
• Convert each flip-flop to a scan register
– Only costs one extra multiplexer
• Normal mode: flip-flops behave as usual
• Scan mode: flip-flops behave as shift register
• Contents of flops can be scanned out and new
values scanned in
CLK
SI
D
Flop
SCAN
Q
Scan Path
• A simple means of creating a scan path consists of
placing a multiplexer just ahead of each flip-flop.
• One input to the 2:1 MUX is driven by normal operational
data while the other input is driven by the output of
another flip-flop.
• Serial input of MUX is connected to a PI.
• One of the flip-flop outputs is connected to a PO pin.
• The MUX control line is used as mode control:
– It can permit parallel load for normal operation or it can select
serial shift in order to enter scan mode.
– When scan mode is selected there is complete serial shift path
from an input pin to an output pin.
• Scan can be full-scan or partial-scan.
• For a circuit to have the scan capability, first the designer
uses only D type flip-flops (DFF) with one or more clock
signals, all of which are controlled from primary inputs.
• Once the circuit is functionally verified, the DFFs are
replaced by scan flip-flops (SFF).
• One typical SFF is shown below, here a multiplexer and
two new signals, scan-data SD and test control TC, are
added to the DFF. The original data input D is stored in
the flip-flop when TC is 1 and SD is stored when TC is 0.
Parallel Scan
Scannable Flip-flops
SCAN
SCAN
(a)
SI
D
0
Flop
D

CLK
1

Q
X
Q
SI
Q




(b)


d

SCAN
d
s
D

d
s
Q
X
Q

SI


(c)
s


Level-Sensitive Scan Design
• The figure shows the general configuration for of
a level sensitive scan design latch as well as a
NAND/NOT implementation.
• D is the normal data line and C is the normal
clock line. Line L1 is the normal output.
• Lines SI, A, B and L2 form the shift portion of the
latch.
• SI is the shift data in and L2 is the shift data out.
• A and B are the two phase, non overlapping shift
clocks.
Built-in Self-test
• Built-In Self Test (BIST): is an design technique
in which parts of a circuit are used to test the
circuit itself.
– BIST is the capability of a circuit to test itself.
– The goal of BIST is to add devices to a design that will
allow it to test itself.
• Built-in self-test lets blocks test themselves
– Generate pseudo-random inputs to comb. logic
– Combine outputs into a syndrome
– With high probability, block is fault-free if it produces
the expected syndrome
BIST Done
BIST Start
Test Controller
Output
Response
Analyzer (ORA)
Test Pattern
Generator
Pass/Fail
System Outputs
System Inputs
Input
Isolation
Circuitry
Circuit Under
Test
PRSG
• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence Generator
D
Q[1]
D
Flop
Q[0]
Flop
D
Flop
CLK
Q[2]
Step
Q
0
111
1
2
3
4
5
6
7
PRSG
• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence Generator
Step
Q
0
111
1
110
D
Q[1]
D
Flop
Q[0]
Flop
D
Flop
CLK
Q[2]
2
3
4
5
6
7
PRSG
• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence Generator
Step
Q
0
111
1
110
2
101
D
Q[1]
D
Flop
Q[0]
Flop
D
Flop
CLK
Q[2]
3
4
5
6
7
PRSG
• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence Generator
Step
Q
0
111
1
110
2
101
3
010
D
Q[1]
D
Flop
Q[0]
Flop
D
Flop
CLK
Q[2]
4
5
6
7
PRSG
• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence Generator
Step
Q
0
111
1
110
2
101
3
010
4
100
D
Q[1]
D
Flop
Q[0]
Flop
D
Flop
CLK
Q[2]
5
6
7
PRSG
• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence Generator
Step
Q
0
111
1
110
2
101
3
010
4
100
5
001
D
Q[1]
D
Flop
Q[0]
Flop
D
Flop
CLK
Q[2]
6
7
PRSG
• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence Generator
Step
Q
0
111
1
110
2
101
3
010
4
100
5
001
6
011
D
Q[1]
D
Flop
Q[0]
Flop
D
Flop
CLK
Q[2]
7
PRSG
• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence Generator
Step
Q
0
111
1
110
2
101
3
010
4
100
5
001
6
011
7
111
(repeats)
D
Q[1]
D
Flop
Q[0]
Flop
D
Flop
CLK
Q[2]
Built-in Logic Block Observer (BILBO)
• Combine scan with PRSG & signature analysis
D[0]
D[1]
D[2]
Q[0]
0
PRSG
Logic
Cloud
Flop
1
Flop
SI
Flop
C[0]
C[1]
Q[2] / SO
Q[1]
Signature
Analyzer
MODE
Scan
Test
Reset
Normal
C[1]
0
0
1
1
C[0]
0
1
0
1
Boundary Scan
• Testing boards is also difficult
– Need to verify solder joints are good
• Drive a pin to 0, then to 1
• Check that all connected pins get the values
• Through-hold boards used “bed of nails”
• SMT and BGA boards cannot easily contact pins
• Build capability of observing and controlling pins
into each chip to make board test easier
Schematic of system test logic
Boundary Scan Example
PackageInterconnect
CHIP B
CHIP C
Serial Data Out
CHIP A
CHIP D
IO pad and Boundary Scan
Cell
Serial Data In
Boundary Scan Interface
• The Test Acess Port has following connections:
– TCK:
test clock
– TMS:
test mode select
– TDI:
test data in
– TDO:
test data out
– TRST*:
test reset (optional)
• Chips with internal scan chains can access the
chains through boundary scan for unified test
strategy.
Test Logic Architecture &
Access Port
Test
TAP Controller
Instruction Register
• Instruction Register is two bit long.
• Instruction Register specifies which data register
will be placed in the scan when DR is selected.
• Three instructions are required:
– Bypass
– Sample/Preload
– Extest
• Optional Runbist and Intest.
Test Data Registers
Boundary Scan Register
• Normal Mode: Mode_Control = 0, data passes from IN
to OUT.
• Scan Mode: ShiftDR = 1, ClockDR = scan clock, serial
data is shifted in from SIN to out SOUT.
• Capture Mode: ShiftDR = 0, ClockDR = one clock pulse,
data on the IN line is clocked into QA.
• Update Mode: with QA loaded, Mode_Control = 1,
UpdateDR = one clock pulse, data clock into QA is
applied to OUT.
Summary
• Think about testing from the beginning
– Simulate as you go
– Plan for test after fabrication
• “If you don’t test it, it won’t work!
(Guaranteed)”