1. 4/26/05 Kantipudi: ELEC7250 1 CONTROLLABILITY AND OBSERVABILITY KALYANA R KANTIPUDI VLSI TESTING ’05 TERM PAPER TERM PAPER

2. 4/26/05Kantipudi: ELEC72502 DEFINITIONS: Controllability: The difficulty of setting a particular logic signal to 0 or 1 Controllability: The difficulty of setting a particular logic signal to 0 or 1 Observability: The difficulty of observing the state of a logic signal Observability: The difficulty of observing the state of a logic signal Applicable for both combinational and sequential testability measures. Combinational Testability measures: Spatial domain Sequential Testability measures: Temporal domain

3. 4/26/05Kantipudi: ELEC72503 Why we need them? We need testability information of a circuit We need testability information of a circuitWhy?  DFT ( Design-For-Testability ) We need module-level or Register-Transfer-level measures for this Why? There will be no independent gates in VLSI circuits  ATPG ( Automatic Test Pattern Generation ) We need gate-level measures for this Why? We need to know the easily controllable and observable nodes in a ckt Why?

4. 4/26/05Kantipudi: ELEC72504 Gate-level Testability Measures: SCOAP L. H. Goldstein, "Controllability / Observability Analysis of Digital Circuits" combinational zero controllability (CC0) combinational zero controllability (CC0) combinational one controllability (CC1) combinational one controllability (CC1) combinational observability (CO) combinational observability (CO) sequential zero controllability (SC0) sequential zero controllability (SC0) sequential one controllability (SC1) sequential one controllability (SC1) sequential observability (SO) sequential observability (SO) Primary Inputs: The combinational controllabilities(CC0 & CC1) are set to ‘1’ and all sequential controllabilities(SC0 & SC1) are set to ‘0’ Primary Outputs: Both combinational and sequential observabilities are set to ‘0’

5. 4/26/05Kantipudi: ELEC72505 NAND Gate: But every ATMG has a restriction: It has to be linear. It cannot be NP-Complete SCOAP: All inputs to all logic elements are independent What about reconvergent fan-outs? CC1(I) CC0(Z) CC0(Z) = CC1(A) + CC1(B) + 1 = 2 CC1(I) + 1 A B CC0(Z)CC1(I) CC0(Z) = CC1(I) + 1

6. 4/26/05Kantipudi: ELEC72506 High level Testability Measures  Module-level measures: Every module has its own characteristic Function Every module has its own characteristic Function A probability spectrum from the truth-tables of prim. o/ps of modules A probability spectrum from the truth-tables of prim. o/ps of modules C & O values are calculated from the probability spectrum C & O values are calculated from the probability spectrum  Register-Transfer-level measures: (J. E. Stephenson and J. Grason ’76) As network of components interconnected by unidirectional links As network of components interconnected by unidirectional links Two tasks: Two tasks: Control Task: Propagating controllability from the circuit primary inputs, through other components, to the inputs of the component Control Task: Propagating controllability from the circuit primary inputs, through other components, to the inputs of the component Observation Task: Propagating observability from the outputs of the component through other components to the primary outputs of the circuit Observation Task: Propagating observability from the outputs of the component through other components to the primary outputs of the circuit Controllability(CY) and Observability(OY) of each node is calculated using complex formulae. Controllability(CY) and Observability(OY) of each node is calculated using complex formulae.TMEAS

7. 4/26/05Kantipudi: ELEC72507 Detection probability using testability measures: V. D. Agrawal and M. R. Mercer, “Testability Measures-What Do They Tell Us?” Represented detection probability as an exponential function: So, the fault coverage is given by: Gives only a rough estimation of fault coverage. Found that 70% of the hard-to-test faults are actually detectable. Inference: “Testability measures cannot be the only criteria for DFT.”

8. 4/26/05Kantipudi: ELEC72508 Future needs: Need for accurate testability measures Need for accurate testability measures Need more ATMG tools like TMEAS and ITTAP Need more ATMG tools like TMEAS and ITTAP Conclusion:  The present accuracy of testability measures is not enough; For them to be the only criteria in DFT  Dynamic testability measures will be of much more help for an ATPG.