ELEC 7770
Advanced VLSI Design
Spring 2008
System Test
Vishwani D. Agrawal
James J. Danaher Professor
ECE Department, Auburn University
Auburn, AL 36849
vagrawal@eng.auburn.edu
http://www.eng.auburn.edu/~vagrawal/COURSE/E7770_Spr08/course.html
Spring 08, Apr 17
ELEC 7770: Advanced VLSI Design (Agrawal)
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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
Spring 08, Apr 17
ELEC 7770: Advanced VLSI Design (Agrawal)
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System Test Applications
Application
Functional test
Diagnostic test
Resolution
Manufacturing
Yes
LRU, SRU
Maintenance
Yes
A
Field repair
LRU
Shop repair
SRU
LRU: Lowest replaceable unit
SRU: Shop replaceable unit
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ELEC 7770: Advanced VLSI Design (Agrawal)
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Functional 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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ELEC 7770: Advanced VLSI Design (Agrawal)
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Gate-Level Diagnosis
Karnaugh map
Logic circuit
(shaded squares are true outputs)
b
a
b
c
d
T2
e
a
Stuck-at fault tests:
T1 = 010
T2 = 011
T3 = 100
T4 = 110
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T1
ELEC 7770: Advanced VLSI Design (Agrawal)
T4
T3
c
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Gate Replacement Fault
Karnaugh map
Faulty circuit
(faulty output:
Only red sqaure is 1 output)
(OR replaced by AND)
a
b
c
b
d
T2
e
a
Stuck-at fault tests:
T1 = 010 (pass)
T2 = 011 (fail)
T3 = 100 (pass)
T4 = 110 (fail)
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T1
ELEC 7770: Advanced VLSI Design (Agrawal)
T4
T3
c
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Bridging Fault
Faulty circuit
Karnaugh map
(OR bridge: a, c)
(all red squares are faulty 1 outputs)
b
a
a+c
b
c
d
T2
e
a+c
a
Stuck-at fault tests:
T1 = 010 (pass)
T2 = 011 (pass)
T3 = 100 (fail)
T4 = 110 (pass)
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T1
ELEC 7770: Advanced VLSI Design (Agrawal)
T4
T3
c
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Fault Dictionary
Fault
Test syndrome
t1
t2
t3
t4
No fault
0
0
0
0
a0, b0, d0
0
0
0
1
a1
1
0
0
0
b1
0
0
1
0
c0
0
1
0
0
c1, d1, e1
1
0
1
0
e0
0
1
0
1
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ELEC 7770: Advanced VLSI Design (Agrawal)
a0 : Line a stuckat-0
ti = 0, if Ti passes
= 1, if Ti fails
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Diagnosis with Dictionary
Dictionary look-up with minimum Hamming distance
Fault
OR
Test syndrome
t1 t2 t3 t4
Diagnosis
AND
0
1
0
1
e0
OR-bridge (a,c)
0
0
1
0
b1
1
1
1
1
c1, d1, e1, e0
OR
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NOR
ELEC 7770: Advanced VLSI Design (Agrawal)
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Diagnostic Tree
T3
b1
T2
Pass: t4=0
Fail: t4=1
a1
T3
a1, c1, d1, e1
OR bridge
(a,c)
c1, d1, e1
a0, b0, d0
T2
a0, b0, d0, e0
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c0
T1
T4
No fault
found
e0
OR
AND
OR
NOR
ELEC 7770: Advanced VLSI Design (Agrawal)
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System Test: A DFT Problem
 Changing scenario in VLSI:




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Mixed-signal (analog and RF) circuits
System-on-a-chip
Multi-chip modules
Intellectual property (IP) cores
ELEC 7770: Advanced VLSI Design (Agrawal)
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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.
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ELEC 7770: Advanced VLSI Design (Agrawal)
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PCB vs. SOC
PCB






SOC
Tested parts
In-circuit test (ICT)
Easy test access
Bulky
Slow
High assembly cost
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





High reliability
Fast interconnects
Low cost
Untested cores
No internal test
access
Mixed-signal devices
ELEC 7770: Advanced VLSI Design (Agrawal)
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Core-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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ELEC 7770: Advanced VLSI Design (Agrawal)
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Partitioning 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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ELEC 7770: Advanced VLSI Design (Agrawal)
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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
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ELEC 7770: Advanced VLSI Design (Agrawal)
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A Test-Wrapper
from/to
External
Test pins
Scan chain
to/from TAP
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ELEC 7770: Advanced VLSI Design (Agrawal)
Functional
core outputs
Core
Scan chain
Scan chain
Functional
core inputs
Wrapper
elements
Wrapper
test
controller
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Overhead 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)
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– 1/2
ELEC 7770: Advanced VLSI Design (Agrawal)
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Overhead Estimate
Rent’s rule: For a logic block the number of gates G
and the number of terminals t are related by
t =KG
a
where 1 ≤ K ≤ 5, and a ~ 0.5.
Assume that block area A is proportional to G, i.e.,
t is proportional to A 0.5. Since test logic is added
to each terminal t,
Test logic added to terminals
–0.5
Overhead = ──────────────────── ~ A
A
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ELEC 7770: Advanced VLSI Design (Agrawal)
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DFT Architecture for SOC
Test
sink
User defined test access mechanism (TAM)
1
Functional
outputs
Module
N
wrapper
Module
Func.
outputs
Func.
inputs
Test
Test
Functional
inputs
wrapper
Test
source
Instruction register control
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TDO
TRST
TMS
TDI
SOC inputs
TCK
Test access port (TAP)
Serial instruction data
ELEC 7770: Advanced VLSI Design (Agrawal)
SOC outputs
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DFT 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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ELEC 7770: Advanced VLSI Design (Agrawal)
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Summary
 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.
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ELEC 7770: Advanced VLSI Design (Agrawal)
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