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ECE 261 Krish Chakrabarty 1
CMOS Testing-2• Design and test
• Design for testability (DFT)– Scan design
• Built-in self-test
• IDDQ testing
ECE 261 Krish Chakrabarty 2
Design and Test Flow: Old View• Test was merely an afterthought
Specification
DesignDesignerrors
Fabrication
Testing
Randomdefects
Synthesis, full-customsimulation, verification,test generation
Accept Reject
Pass Fail
2
ECE 261 Krish Chakrabarty 3
Design and Test Flow: New ViewDesign and test are tightly coupled
Specification
Design fortestability
Designerrors
Fabrication
Testing
Randomdefects
Accept Reject
Pass Fail
Processimprovements
Designimprovements
Diagnosis
ECE 261 Krish Chakrabarty 4
Testing Sequential CircuitsDifficult problem-internal states cannot be directly controlled and observedLong test sequences are necessarySolution: Scan design-simplify to combinational circuit testing
Combinational
logic
Registers
Primary
inputs
Primary outputs
(controllable) (observable)
State outputs (not observable) State inputs
(not controllable)
3
ECE 261 Krish Chakrabarty 5
Design for Test
• 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.
ECE 261 Krish Chakrabarty 6
Scan DesignMake all flip-flops directly controllable and observable by adding multiplexersPopular design-for-test (DFT) technique-circuit is now combinational for testing purposes
Combinational
logic
Primary inputs
Primary outputs
Scan in
Scan out
Scan cells
State inputs State outputs (controllable) (observable)
4
ECE 261 Krish Chakrabarty 7
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 flopscan be scannedout and new values scannedin
ECE 261 Krish Chakrabarty 8
Scan Cell Design0
1D Q
Scan in(Test data)
Data(Functional)
N/T
N/T = 1: Test mode
N/T = 0: Normal mode
Scan in
Clock
D0 D1 D2 D3
Scanout
N/TQ0 Q1 Q2 Q3Clock
5
ECE 261 Krish Chakrabarty 9
Scannable Flip-flops
ECE 261 Krish Chakrabarty 10
Scan Design• Separate input and output 4-bit scan registers
• Test sequence: {01100, 11011}, first 4 bits are for flip-flops
Combinational circuit
Scan chain/Scan path
Test data
Test responses
01101101
1
0
Controllableprimaryinput
N/T
N/T
6
ECE 261 Krish Chakrabarty 11
Steps in Scan Testing• N/T = 1: Scan in test pattern, hold appropriate bit
pattern on controllable primary inputs
• N/T = 0: Apply test pattern to combinational circuit
• N/T=1: Scan out test responses
• Scan provides complete controllability and observability
• Testing time? How many cycles? How to test scan registers?
ECE 261 Krish Chakrabarty 12
Long Scan Chains
Test vectors need to be translated to scan format
Nor
mal
dat
a
Scan chainTest data
7
ECE 261 Krish Chakrabarty 13
Built-in Self-test
• 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
ECE 261 Krish Chakrabarty 14
Built-in Self Testing (BIST)
On-chip test generator and response monitor
Test
generator
(TGC)
(CUT)
Control
Inputs Outputs
Error
0
1
Response
monitor
(RM)
Circuit
under test
8
ECE 261 Krish Chakrabarty 15
BIST: Advantages• Lower cost due to elimination of external tester• In-system, at-system, high-quality testing• Faster fault detection, ease of diagnosis• Overcomes pin limitations and related interfacing problems• Reduces maintenance and repair costs at system level
ECE 261 Krish Chakrabarty 16
BIST: IssuesTest strategy (random, exhaustive, deterministic)Circuit partitioningTest pattern generation
Exhaustive: countersRandom: Linear-feedback shift registers (LFSRs)Deterministic: ROM, other methods?
Response analysisTest control and scheduling
9
ECE 261 Krish Chakrabarty 17
BIST Logic Circuits
Test patterns
• Linear-feedback shift-register (LFSR)
• Multiple-input signature register (MISR) Test responses
Signature
ECE 261 Krish Chakrabarty 18
BIST Pattern Generation• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence GeneratorStep Q
0 111
1
2
3
4
5
6
7
10
ECE 261 Krish Chakrabarty 19
BIST Pattern Generation• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence GeneratorStep Q
0 111
1 110
2
3
4
5
6
7
ECE 261 Krish Chakrabarty 20
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
4
5
6
7
11
ECE 261 Krish Chakrabarty 21
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
5
6
7
ECE 261 Krish Chakrabarty 22
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
6
7
12
ECE 261 Krish Chakrabarty 23
PRSG• Linear Feedback Shift Register
– Shift register with input taken from XOR of state
– Pseudo-Random Sequence GeneratorStep Q
0 111
1 110
2 101
3 010
4 100
5 001
6
7
ECE 261 Krish Chakrabarty 24
PRPG
• Linear Feedback Shift Register– Shift register with input taken from XOR of state
– Pseudo-Random Sequence GeneratorStep Q
0 111
1 110
2 101
3 010
4 100
5 001
6 011
7
13
ECE 261 Krish Chakrabarty 25
PRSG
• Linear Feedback Shift Register– Shift register with input taken from XOR of state
– Pseudo-Random Sequence GeneratorStep Q
0 111
1 110
2 101
3 010
4 100
5 001
6 011
7 111 (repeats)
ECE 261 Krish Chakrabarty 26
BILBO• Built-in Logic Block Observer
– Combine scan with PRSG & signature analysis
14
ECE 261 Krish Chakrabarty 27
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
ECE 261 Krish Chakrabarty 28
Boundary Scan Example
15
ECE 261 Krish Chakrabarty 29
Boundary Scan InterfaceIEEE 1149.1 JTAG standard
• Boundary scan is accessed through five pins– 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.
ECE 261 Krish Chakrabarty 30
BIST in IndustryEarly days: AT&T (Lucent) incorporated BIST in hundreds of commercial chips Intel: 80386, Pentium, Pentium ProHardware overhead typically 15% of self-tested portion (around 5% for entire chip, e.g. 6% for the Pentium Pro)Regular embedded arrays (RAMs, PLAs) almost always tested using BIST: DEC Alpha, PowerPCBIST for irregular logic not so widespread
16
ECE 261 Krish Chakrabarty 31
IDDQ Testing• Based on current measurements, not voltage
– IDDQ = IDD quiescent
• In CMOS technology, quiescent current is very low
• Testing idea: check for faults by detecting current spikes– Advantage: Massive observability, good for detecting shorts
– Disadvantage: slow, leakage current closer to quiescent current for deep submicron