9 Fault Modeling Reliable System Design 2011 by

9. Fault Modeling Reliable System Design 2011 by: Amir M. Rahmani

Fault Modeling l l l Why fault modeling? Stuck-at 0/1 fault model The single fault assumption Bridging and delay faults MOS transistors and CMOS Switch-level fault model • l l – Stuck-on/open Functional Faults State Transition Fault Model matlab 1. ir Amir M. Rahmani

Fault Modeling l Fault Model: • l Used for: • • l l a set of assumed faults in a system such that testing for them will test for most faults of a specific class. Designing error detection mechanisms Designing test procedure Fault simulation Quality evaluation Hide complexities of actual defects. Infinitely many defects possible. Based on past knowledge of defect modes and modeling experience. matlab 1. ir Amir M. Rahmani

Common Fault Models l l No model: test exhaustively Hardware fault models: • – Gate level: • stuck-at 0/1 • bridging • delay faults • – Transistor level: • stuck on/open • bridging • l – Functional fault models Software fault model • – Is there one? matlab 1. ir Amir M. Rahmani

Fault mechanisms in hardware l l Temporary Permanent • • • l – Opens: broken connection, also near-opens – Shorts: unwanted connection, also near shorts – Can be seen Imperfect devices matlab 1. ir Amir M. Rahmani

Stuck-at 0/1 Model l l Model: any one or more of these may be stuck at 0 or 1: a gate input, a gate output Assumptions: • • Only one line is faulty. • Faulty line permanently set to 0 or 1. • Fault can be at an input or output of a gate. matlab 1. ir Amir M. Rahmani

Stuck-at 0/1 Example matlab 1. ir Amir M. Rahmani

Why Use Single Stuck-At Fault Model? l l l Complexity is greatly reduced. Many different physical defects may be modeled by the same logical stuck-at fault. Single stuck-at Fault is technology independent. it been successfully used on TTL, ECL, CMOS, etc. Single stuck-at tests cover a large percentage of multiple stuck-at faults. Single stuck-at tests cover a large percentage of un-modeled physical defects. matlab 1. ir Amir M. Rahmani

Bridging (Short) Fault Model l l Common assumption: only nearby lines can be bridged Model: Two lines x and y bridged can cause both to take the value • • • – x AND y AND-bridging (0 -dominance) – x OR y OR-bridging (1 -dominance) – Depends on technology, transistor dimensions etc. matlab 1. ir Amir M. Rahmani

Bridging Fault Example matlab 1. ir Amir M. Rahmani

Delay Fault Model matlab 1. ir Amir M. Rahmani

MOS Transistors matlab 1. ir Amir M. Rahmani

CMOS NOR Gate matlab 1. ir Amir M. Rahmani

Switch-level Fault Model (1) matlab 1. ir Amir M. Rahmani

Switch-level Fault Model (2): Stuck-ON matlab 1. ir Amir M. Rahmani

Functional Faults l Fault effects modeled at a higher level than logic for function modules, such as • • • Decoders Multiplexers Adders Counters RAMs ROMs matlab 1. ir Amir M. Rahmani

Functional Faults of Decoder l l l Instead of line Li, Line Lj is selected In addition to Li, Lj (or Lk) is selected None of the lines are selected matlab 1. ir Amir M. Rahmani

State Transition Fault Model l A fault causes a single state transition to a wrong destination state matlab 1. ir Amir M. Rahmani