CMP238: Projeto e Teste de Sistemas VLSI Marcelo Lubaszewski Aula 2 - Teste PPGC - UFRGS 2005/I

Lecture 2 - Fault Modeling Defects, Errors, and Faults Why model faults? Some real defects in VLSI and PCB Common fault models Stuck-at faults Single stuck-at faults Fault equivalence Fault dominance and checkpoint theorem Classes of stuck-at faults and multiple faults Other Common Faults Faults in FPGAs

Defects, Faults, and Errors Defect: unintendent difference between the implemented HW and its intendent design –May or may not cause a system failure Fault: representation of a defect at the abstracted function level Defect X Fault Imperfections in the HWImperfections in the function

Defects, Faults, and Errors Error: Manifestation of a fault that results in incorrect circuit (system) outputs or states –Caused by faults Failure: Deviation of a circuit or system from its specified behavior – Fails to do what it should do –Caused by an error Defect --> Fault ---> Error ---> Failure

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 defects  Dielectric breakdown  Electromigration ...  Packaging defects  Contact degradation  Seal leaks... Ref.: M. J. Howes and D. V. Morgan, Reliability and Degradation - Semiconductor Devices and Circuits, Wiley, 1981.

Example Defect: a short to ground Fault: signal b stuck at logic 0 Error: z has the wrong value if a = b = 1 a b 1 1 z 0 1 0

Example Defect: a short to ground Fault: signal b stuck at logic 0 Error: z has the wrong value if a = b = 1 But, if a = 0, fault exists, but no error! a b 0 1 z 1 0

Why Model Faults? Real defects (often mechanical) too numerous and often not analyzable A fault model identifies targets for testing –Model faults most likely to occur Fault model limits the scope of test generation –Create tests only for the modeled faults A fault model makes analysis possible –Associate specific defects with specific test patterns Effectiveness measurable by experiments –Fault coverage can be computed for specific test patterns to reflect its effectiveness

Common Fault Models Single stuck-at faults Transistor open and short faults Memory faults PLA faults (stuck-at, cross-point, bridging) Functional faults (processors) Delay faults (transition, path) Analog faults

Single Stuck-at Fault Three properties define a single stuck-at fault Only one line is faulty The faulty line is permanently set to 0 or 1 The fault can be at an input or output of a gate

Single Stuck-at Fault Three properties define a single stuck-at fault Only one line is faulty The faulty line is permanently set to 0 or 1 The fault can be at an input or output of a gate Example: NAND gate has 3 fault sites ( ) and 6 single stuck-at faults a b 1 1 z s-a-0 fault, s-a-1 fault

Single Stuck-at Fault Three properties define a single stuck-at fault Only one line is faulty The faulty line is permanently set to 0 or 1 The fault can be at an input or output of a gate Example: NAND gate has 3 fault sites ( ) and 6 single stuck-at faults a b 1 1 z 1 (0) 1 Faulty circuit value Good circuit value s-a-0

Single Stuck-at Fault Three properties define a single stuck-at fault Only one line is faulty The faulty line is permanently set to 0 or 1 The fault can be at an input or output of a gate Example: NAND gate has 3 fault sites ( ) and 6 single stuck-at faults a b 1 1 z 1 (0) 1 Test vector for a s-a-0 fault Faulty circuit value Good circuit value s-a-0

Single Stuck-at Fault a b c d e f 1 0 g h i 1 j k z Good circuit value Example: XOR circuit has 12 fault sites and 24 single stuck-at faults

Single Stuck-at Fault a b c d e f 1 0 g h i 1 s-a-0 j k z 0(1) 1(0) 1 Test vector for h s-a-0 fault Good circuit value Faulty circuit value Example: XOR circuit has 12 fault sites and 24 single stuck-at faults

Fault Equivalence Fault equivalence: Two faults f1 and f2 are equivalent if all tests that detect f1 also detect f2. If faults f1 and f2 are equivalent then the corresponding faulty functions are identical. Fault collapsing: All single faults of a logic circuit can be divided into disjoint equivalence subsets, where all faults in a subset are mutually equivalent. A collapsed fault set contains one fault from each equivalence subset.

Equivalence Rules sa0 sa1 sa0 sa1 sa0 sa1 sa0 sa1 AND NAND OR NOR WIRE NOT FANOUT

Equivalence Rules sa0 sa1 sa0 sa1 sa0 sa1 sa0 sa1 sa0 sa1 AND NAND OR NOR WIRE NOT FANOUT

Equivalence Example sa0 sa1

Equivalence Example sa0 sa1 16 Collapse ratio = =

Equivalence Example sa0 sa1

Equivalence Example sa0 sa1 Faults in red removed by equivalence collapsing 20 Collapse ratio = =

Fault Dominance If all tests of some fault F1 detect another fault F2, then F2 is said to dominate F1. Dominance fault collapsing: If fault F2 dominates F1, then F2 is removed from the fault list. When dominance fault collapsing is used, it is sufficient to consider only the input faults of Boolean gates. See the next example.

Dominance Example s-a-1 F1 s-a-1 F2

Dominance Example s-a-1 F1 s-a-1 F All tests of F2 Only test of F1

Dominance Example s-a-1 F1 s-a-1 F All tests of F2 Only test of F1 s-a-1 s-a-0 A dominance collapsed fault set

Dominance Example sa1 sa0 sa1 sa0 sa1 sa0 sa1 sa0 sa1 sa0 sa1

Dominance Example sa1 sa0 sa1 sa0 sa1 sa0 sa1 sa0 sa1 sa0 sa1

Dominance Example sa0 sa1 sa0 sa1 sa1 sa0 sa1 sa1 sa0 sa1 sa0 sa1 sa0 sa1

Dominance Example sa0 sa1 sa0 sa1 sa1 sa0 sa1 sa1 sa0 sa1 sa0 sa1 sa0 sa1 Faults in red removed by equivalence collapsing

Checkpoints Primary inputs and fanout branches of a combinational circuit are called checkpoints. Total fault sites = 16

Checkpoints Primary inputs and fanout branches of a combinational circuit are called checkpoints. Total fault sites = 16 Checkpoints ( ) = 10

Checkpoints Primary inputs and fanout branches of a combinational circuit are called checkpoints. Total fault sites = 16 Checkpoints ( ) = 10 Checkpoint theorem: A test set that detects all single (multiple) stuck-at faults on all checkpoints of a combinational circuit, also detects all single (multiple) stuck-at faults in that circuit.

Why Fault Collapsing? Memory & CPU- Time saving –To ease the burden for test generation and fault simulation in testing

Multiple Stuck-at Faults A multiple stuck-at fault means that any set of lines is stuck-at some combination of (0,1) values. The total number of single and multiple stuck-at faults in a circuit with k single fault sites is 3 k -1. A single fault test can fail to detect the target fault if another fault is also present, however, such masking of one fault by another is rare. Statistically, single fault tests cover a very large number of multiple faults.

Why Single Stuck- At Fault Model? Complexity is greatly reduced. –Many different physical defects may be modeled by the same logical single stuck- at fault. Single stuck- at fault is technology independent –Can be applied to TTL, ECL, CMOS, etc. Single stuck- at fault is design style independent –Gate Arrays, Standard Cell, Custom VLSI Even when single stuck- at fault does not accurately model some physical defects, the tests derived for logic faults are still valid for most defects. Single stuck- at tests cover a large percentage of multiple stuck- at faults.

Bridging Faults Two or more normally distinct points (lines) are shorted together –Logic effect depends on technology –Wired- AND for TTL –Wired- OR for ECL –CMOS?

Transistor (Switch) Faults MOS transistor is considered an ideal switch and two types of faults are modeled: Stuck-open -- a single transistor is permanently stuck in the open state. Stuck-short -- a single transistor is permanently shorted irrespective of its gate voltage. Detection of a stuck-open fault requires two vectors. Detection of a stuck-short fault requires the measurement of quiescent current (I DDQ ).

CMOS Transistor Stuck- Short Transistor stuck- on may cause ambiguous logic level –depends on the relative impedances of the pull- up & pull- down networks When input is low, both P and N transistors are conducting causing increased quiescent current, called IDDQ fault.

CMOS Transistor Stuck- OPEN Transistor stuck- open may cause output floating.

Functional Faults Fault effects modeled at a higher level than logic for function modules, such as –Decoders –Multiplexers –Adders –Counters –RAMs –ROMs

Functional Faults of Decoder f( L i /L j ): Instead of line L i, Line L j is selected f( L i /L i +L j ): In addition to L i, L j is selected f( L i /0): None of the lines are selected

Memory Faults Parametric Faults – Output Levels –Power Consumption –Noise Margin –Data Retention Time Functional Faults –Stuck Faults in Address Register, Data Register, and Address Decoder –Cell Stuck Faults –Adjacent Cell Coupling Faults –Pattern- Sensitive Faults

Memory Faults Pattern- sensitive faults: the presence of a faulty signal depends on the signal values of the nearby points –Most common in DRAMs Adjacent cell coupling faults –Pattern sensitivity between a pair of cells

PLA Faults Stuck Faults Crosspoint Faults –Extra/ Missing Transistors Bridging Faults Break Faults

Missing Crosspoint Faults in PLA Missing crosspoint in AND- array –Growth fault Missing crosspoint in OR- array –Disappearance fault

Extra Crosspoint Faults in PLA Extra crosspoint in AND- array –Shrinkage or disappearance fault Extra crosspoint in OR- array –Appearance fault

Gate- Delay- Fault Slow to rise, slow to fall –x is slow to rise when channel resistance R1 is abnormally high

Gate- Delay- Fault Disadvantage: –Delay faults resulting from the sum of several small incremental delay defects may not be detected.

Path- Delay- Fault Propagation delay of the path exceeds the clock interval. The number of paths grows exponentially with the number of gates.

State Transition Graph Each state transition is associated with a 4- tuple: –source state, input, output, destination state

Single State Transition Fault Model A fault causes a single state transition to a wrong destination state.

Faults in FPGAs ff F1 F2 F3 F4 Configuration Memory Cell M M M M MMMM LUT BlockRAM SEU (Bit flip) clk E1 E2 E3 E1 E2 E1 E3 E2 E3  Permanent faults: same ASIC models apply  But for transients... Virtex (Xilinx) FPGA building blocks:

Effect of Transients in SRAM-based FPGAs ff F1 F2 F3 F4 Configuration Memory Cell M M M M MMMM LUT BlockRAM SEU (Bit flip) clk E1 E2 E3 E1 E2 E1 E3 E2 E3  Possible Bit flip  Transient effect  Corrected at the next load Virtex (Xilinx) CLB Comb. Logic: ~0.5 % of the FPGA sensitive area

Effect of Transients in SRAM-based FPGAs ff F1 F2 F3 F4 Configuration Memory Cell M M M M MMMM LUT BlockRAM SEU (Bit flip) clk E1 E2 E3 E1 E2 E1 E3 E2 E3  Bit flip  Transient effect  Corrected at the next load Virtex (Xilinx) CLB Flip-flops: ~0.5 % of the FPGA sensitive area

Effect of Transients in SRAM-based FPGAs ff F1 F2 F3 F4 Configuration Memory Cell M M M M MMMM LUT BlockRAM SEU (Bit flip) clk E1 E2 E3 E1 E2 E1 E3 E2 E3  Bit flip  Permanent effect  Corrected by reconfiguration Virtex (Xilinx) CLB LUTs: ~8% of the FPGA sensitive area

Effect of Transients in SRAM- based FPGAs ff F1 F2 F3 F4 Configuration Memory Cell M M M M MMMM LUT BlockRAM SEU (Bit flip) clk E1 E2 E3 E1 E2 E1 E3 E2 E3 Virtex (Xilinx)  Short or open circuit  Corrected by reconfiguration Routing and CLB customization: ~91.0 % of the FPGA sensitive area

Summary Fault models are analyzable approximations of defects and are essential for a test methodology. For digital logic single stuck-at fault model offers best advantage of tools and experience. Many other faults (bridging, stuck-open and multiple stuck- at) are largely covered by stuck-at fault tests. Stuck-short and delay faults and technology-dependent faults require special tests. Memory and analog circuits need other specialized fault models and tests. Transient faults may have permanent effects in FPGAs