lecture 31 system test (lecture 22alt in the alternative sequence)
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Lecture 31 System Test (Lecture 22alt in the Alternative Sequence). Definition Functional test Diagnostic test Fault dictionary Diagnostic tree System design-for-testability (DFT) architecture System partitioning Core test-wrapper DFT overhead Summary. A System and Its Testing. - PowerPoint PPT PresentationTRANSCRIPT
Copyright 2001, Agrawal & Bushnell
VLSI Test: Lecture 31/22alt 1
Lecture 31System Test
(Lecture 22alt in the Alternative Sequence)
Lecture 31System Test
(Lecture 22alt in the Alternative Sequence)
Definition Functional test Diagnostic test
Fault dictionary Diagnostic tree
System design-for-testability (DFT) architecture System partitioning Core test-wrapper DFT overhead
Summary
Copyright 2001, Agrawal & Bushnell
VLSI Test: Lecture 31/22alt 2
A System and Its Testing
A System and Its Testing
A system is an organization of components (hardware/software parts and subsystems) with capability to perform useful functions.
Functional test verifies integrity of system: Checks for presence and sanity of subsystems Checks for system specifications Executes selected (critical) functions
Diagnostic test isolates faulty part: For field maintenance isolates lowest replaceable unit (LRU),
e.g., a board, disc drive, or I/O subsystem For shop repair isolates shop replaceable unit (SRU), e.g., a
faulty chip on a board Diagnostic resolution is the number of suspected faulty units
identified by test; fewer suspects mean higher resolution
Copyright 2001, Agrawal & Bushnell
VLSI Test: Lecture 31/22alt 3
System Test ApplicationsSystem Test Applications
A
Application Functional test Diagnostic test Resolution
Manufacturing Yes LRU, SRU
Maintenance Yes
Field repair LRU
Shop repair SRU
LRU: Lowest replaceable unitSRU: Shop replaceable unit
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VLSI Test: Lecture 31/22alt 4
Functional TestFunctional Test
All or selected (critical) operations executed with non-exhaustive data.
Tests are a subset of design verification tests (test-benches).
Software test metrics used: statement, branch and path coverages; provide low (~70%) structural hardware fault coverage.
Examples: Microprocessor test – all instructions with random data
(David, 1998). Instruction-set fault model – wrong instruction is
executed (Thatte and Abraham, IEEETC-1980).
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VLSI Test: Lecture 31/22alt 5
Gate-Level DiagnosisGate-Level Diagnosis
e
da
bc T3
T1T2
T4a
b
cStuck-at fault tests:
T1 = 010T2 = 011T3 = 100T4 = 110
Logic circuitKarnaugh map
(shaded squares are true outputs)
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VLSI Test: Lecture 31/22alt 6
Gate Replacement Fault
Gate Replacement Fault
e
da
bc T3
T1T2
T4a
b
cStuck-at fault tests:T1 = 010 (pass)T2 = 011 (fail)T3 = 100 (pass)T4 = 110 (fail)
Faulty circuit(OR replaced by AND)
Karnaugh map(faulty output:
red sqaure is 1 output)
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VLSI Test: Lecture 31/22alt 7
Bridging FaultBridging Fault
e
da
bc T3
T1T2
T4a
b
cStuck-at fault tests:T1 = 010 (pass)T2 = 011 (pass)T3 = 100 (fail)T4 = 110 (pass)
Faulty circuit(OR bridge: a, c)
Karnaugh map(red squares are faulty 1 outputs)
a+c
a+c
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VLSI Test: Lecture 31/22alt 8
Fault Test syndrome
t1 t2 t3 t4
No fault
a0, b0, d0
a1
b1
c0
c1, d1, e1
e0
Fault DictionaryFault Dictionary
0
0
1
0
0
1
0
0
0
0
0
1
0
1
0
0
0
1
0
1
0
0
1
0
0
0
0
1
a0 : Line a stuck-
at-0
ti = 0, if Ti passes
= 1, if Ti fails
Copyright 2001, Agrawal & Bushnell
VLSI Test: Lecture 31/22alt 9
Diagnosis with Dictionary
Diagnosis with Dictionary
Fault Test syndrome Diagnosis
t1 t2 t3 t4
OR AND 0 1 0 1 e0
OR-bridge (a,c) 0 0 1 0 b1
OR NOR 1 1 1 1 c1, d1, e1, e0
Dictionary look-up with minimum Hamming distance
Copyright 2001, Agrawal & Bushnell
VLSI Test: Lecture 31/22alt 10
Diagnostic TreeDiagnostic Tree
T4
T1
T2
T3
No faultfound
T3
T2
b1
a1
c1, d1, e1
a0, b0, d0
e0
c0
Pass: t4=0
Fail: t4=1
a0, b0, d0, e0
a1, c1, d1, e1
OR AND
OR bridge(a,c)
OR NOR
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VLSI Test: Lecture 31/22alt 11
System Test: A DFT Problem
System Test: A DFT Problem
Given the changing scenario in VLSI: Mixed-signal circuits System-on-a-chip Multi-chip modules Intellectual property (IP) cores
Prepare the engineer for designing testable, i.e., manufacturable, VLSI systems.
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VLSI Test: Lecture 31/22alt 12
Conventional Test:In-Circuit Test (ICT)Conventional Test:In-Circuit Test (ICT)
A bed-of-nails fixture provides direct access to each chip on the board.
Advantages: Thorough test for devices; good interconnect test.
Limitations: Works best when analog and digital functions are
implemented on separate chips. Devices must be designed for backdriving protection. Not applicable to system-on-a-chip (SOC).
Disadvantages: High cost and inflexibility of test fixture. System test must check for timing.
Copyright 2001, Agrawal & Bushnell
VLSI Test: Lecture 31/22alt 13
PCB vs. SOCPCB vs. SOC
Tested parts In-circuit test (ICT) Easy test access Bulky Slow High assembly cost
High reliability Fast interconnects Low cost Untested cores No internal test access Mixed-signal devices
PCB SOC
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VLSI Test: Lecture 31/22alt 14
Core-Based DesignCore-Based Design
Cores are predesigned and verified but untested blocks: Soft core (synthesizable RTL) Firm core (gate-level netlist) Hard core (non-modifiable layout, often called
legacy core) Core is the intellectual property of vendor
(internal details not available to user.) Core-vendor supplied tests must be applied to
embedded cores.
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VLSI Test: Lecture 31/22alt 15
Partitioning for TestPartitioning for Test
Partition according to test methodology: Logic blocks Memory blocks Analog blocks
Provide test access: Boundary scan Analog test bus
Provide test-wrappers (also called collars) for cores.
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VLSI Test: Lecture 31/22alt 16
Test-Wrapper for a Core
Test-Wrapper for a Core
Test-wrapper (or collar) is the logic added around a core to provide test access to the embedded core.
Test-wrapper provides: For each core input terminal
A normal mode – Core terminal driven by host chip An external test mode – Wrapper element observes core input
terminal for interconnect test An internal test mode – Wrapper element controls state of core
input terminal for testing the logic inside core For each core output terminal
A normal mode – Host chip driven by core terminal An external test mode – Host chip is driven by wrapper element
for interconnect test An internal test mode – Wrapper element observes core outputs
for core test
Copyright 2001, Agrawal & Bushnell
VLSI Test: Lecture 31/22alt 17
A Test-WrapperA Test-Wrapper
Wrappertest
controller
Scan chain
Sc
an c
ha
in
Sc
an c
ha
in
to/from TAP
from/toExternalTest pins
Wrapperelements
Core
Fu
nc
tio
nal
co
re in
pu
ts
Fu
nc
tio
nal
co
re o
utp
uts
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VLSI Test: Lecture 31/22alt 18
Overhead of Test AccessOverhead of Test Access
Test access is non-intrusive. Hardware is added to each I/O signal of block to
be tested. Test access interconnects are mostly local. Hardware overhead is proportional to:
(Block area) – 1/2
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VLSI Test: Lecture 31/22alt 19
Overhead EstimateOverhead EstimateRent’s rule: For a logic block the number of gates Gand the number of terminals t are related by
t = K G
where 1 ≤ K ≤ 5, and ~ 0.5.
Assume that block area A is proportional to G, i.e.,
t is proportional to A 0.5. Since test logic is addedto each terminal t,
Test logic added to terminals
Overhead = ──────────────────── ~ A –0.5
A
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VLSI Test: Lecture 31/22alt 20
DFT Architecture for SOC
DFT Architecture for SOC
User defined test access mechanism (TAM)
Module
1Tes
t
wra
pp
er
Testsource
Testsink
Module
NTes
t
wra
pp
er
Test access port (TAP)
Functionalinputs
FunctionaloutputsFunc.
inputs
Func.outputs
SOC inputs SOC outputsTD
I
TC
K
TM
S
TR
ST
TD
O
Instruction register control
Serial instruction data
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VLSI Test: Lecture 31/22alt 21
DFT ComponentsDFT Components
Test source: Provides test vectors via on-chip LFSR, counter, ROM, or off-chip ATE.
Test sink: Provides output verification using on-chip signature analyzer, or off-chip ATE.
Test access mechanism (TAM): User-defined test data communication structure; carries test signals from source to module, and module to sink; tests module interconnects via test-wrappers; TAM may contain bus, boundary-scan and analog test bus components.
Test controller: Boundary-scan test access port (TAP); receives control signals from outside; serially loads test instructions in test-wrappers.
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VLSI Test: Lecture 31/22alt 22
SummarySummary Functional test: verify system hardware, software, function
and performance; pass/fail test with limited diagnosis; high ( ~100%) software coverage metrics; low ( ~70%) structural fault coverage.
Diagnostic test: High structural coverage; high diagnostic resolution; procedures use fault dictionary or diagnostic tree.
SOC design for testability: Partition SOC into blocks of logic, memory and analog
circuitry, often on architectural boundaries. Provide external or built-in tests for blocks. Provide test access via boundary scan and/or analog test
bus. Develop interconnect tests and system functional tests. Develop diagnostic procedures.