ELEN 468 Lecture 241 ELEN 468 Advanced Logic Design Lecture 24 Design for Testability

ELEN 468 Lecture 242 Test Cost Test pattern generation Fault simulation Generation of fault sites information Test equipment Test process Test cost may overweight design cost

ELEN 468 Lecture 243 Why Design for Testability? Testability is a design characteristic that influences various costs associated with testing It allows for Device status to be determined Isolation of faults Reduce test time and cost

ELEN 468 Lecture 244 Controllability Ability to establish a specific signal value at each node by setting circuit’s inputs Circuits typically difficult to control: decoders, circuits with feedback, oscillators, clock generators …

ELEN 468 Lecture 245 Observability Ability to determine the signal value at any node in a circuit by controlling the circuit’s inputs and observing its output

ELEN 468 Lecture 246 Predictability Ability to obtain known output values in response to given input stimuli Factors affecting predictability Initial state of circuit Races Hazards … …

ELEN 468 Lecture 247 Difficult Test Cases Sequential logic is more difficult to test than combinational logic Control logic is more difficult to test than data-path logic Random logic is more difficult to test than structured bus-oriented designs Asynchronous design is more difficult to test than synchronous design

ELEN 468 Lecture 248 Quantify Testability Need approximate measure of: Difficulty of setting internal circuit lines to 0 or 1 by setting primary circuit inputs Difficulty of observing internal circuit lines by observing primary outputs Uses: Analysis of difficulty of testing internal circuit parts – redesign or add special test hardware Guidance for algorithms computing test patterns – avoid using hard-to-control lines Estimation of fault coverage Estimation of test vector length

ELEN 468 Lecture 249 Types of Measures  SCOAP – Sandia Controllability and Observability Analysis Program  Combinational measures:  CC0 – Difficulty of setting circuit line to logic 0  CC1 – Difficulty of setting circuit line to logic 1  CO – Difficulty of observing a circuit line  Sequential measures – analogous:  SC0  SC1  SO

ELEN 468 Lecture 2410 Range of SCOAP Measures  Controllabilities – 1 (easiest) to infinity (hardest)  Observabilities – 0 (easiest) to infinity (hardest)  Combinational measures: Roughly proportional to # circuit lines that must be set to control or observe given line  Sequential measures: Roughly proportional to # times a flip-flop must be clocked to control or observe given line

ELEN 468 Lecture 2411 Controllability Examples

ELEN 468 Lecture 2412 Observability Examples

ELEN 468 Lecture 2413 Goal of Design for Testability (DFT) Improve Controllability Observability Predictability

ELEN 468 Lecture 2414 Design and Test Trade-off Most DFT ( Design for Testability ) techniques need extra hardware, or modification to circuits that may affect performances DFT need to consider the cost trade-off between design and test

ELEN 468 Lecture 2415 DFT Methods DFT methods for digital circuits: Ad-hoc methods Structured methods:  Scan  Partial Scan  Built-in self-test (BIST)  Boundary scan

ELEN 468 Lecture 2416 Ad-Hoc DFT Methods Good design practices learnt through experience are used as guidelines: Avoid asynchronous (unclocked) feedback Make flip-flops initializable Avoid redundant gates Avoid large fanin gates Provide test control for difficult-to-control signals Avoid gated clocks Design reviews conducted by experts or design auditing tools Disadvantages of ad-hoc DFT methods: Experts and tools not always available Test generation is often manual with no guarantee of high fault coverage Design iterations may be necessary

ELEN 468 Lecture 2417 Scan Design Circuit is designed using pre-specified design rules Test structure (hardware) is added to the verified design: Add a test control (TC) primary input Replace flip-flops by scan flip-flops (SFF) and connect to form one or more shift registers in the test mode Make input/output of each scan shift register controllable/observable from PI/PO

ELEN 468 Lecture 2418 Scan Design Rules Use only clocked D-type of flip-flops for all state variables At least one PI pin must be available for test; more pins, if available, can be used All clocks must be controlled from PIs

ELEN 468 Lecture 2419 Correcting a Rule Violation All clocks must be controlled from PIs Comb. logic Comb. logic D1 D2 CK Q FF Comb. logic D1 D2 CK Q FF Comb. logic

ELEN 468 Lecture 2420 Scan Storage Cell D Si N/T’ Clk Q, So SSC D Q

ELEN 468 Lecture 2421 Scan Flip-Flop (SFF) D N/T’ Si Clk Q Q MUX D flip-flop Master latchSlave latch Logic overhead

ELEN 468 Lecture 2422 Scan Methods C1C2 C1 MUX Si

ELEN 468 Lecture 2423 Boundary Scan MUX

ELEN 468 Lecture 2424 Integrated Serial Scan SFF Combinational logic PI PO SCANOUT SCANIN Control