Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG1 VLSI Testing Lecture 6: Sequential ATPG n Problem of sequential circuit ATPG n Time-frame expansion n Nine-valued logic n ATPG implementation and drivability n Complexity of ATPG n Cycle-free and cyclic circuits n Asynchronous circuits n Summary

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG2 Sequential Circuits n A sequential circuit has memory in addition to combinational logic. n Test for a fault in a sequential circuit is a sequence of vectors, which n Initializes the circuit to a known state n Activates the fault, and n Propagates the fault effect to a primary output n Methods of sequential circuit ATPG n Time-frame expansion methods n Simulation-based methods

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG3 Example: A Serial Adder FF AnAn BnBn CnCn C n+1 SnSn s-a X X X D D Combinational logic

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG4 Time-Frame Expansion AnAn BnBn FF CnCn C n+1 1 X X SnSn s-a D D Combinational logic S n-1 s-a X D D Combinational logic C n D D X A n-1 B n-1 Time-frame -1 Time-frame 0

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG5 Concept of Time-Frames n If the test sequence for a single stuck-at fault contains n vectors, n Replicate combinational logic block n times n Place fault in each block n Generate a test for the multiple stuck-at fault using combinational ATPG with 9-valued logic Comb. block Fault Time- frame 0 Time- frame Time- frame -n+1 Unknown or given Init. state Vector 0Vector -1 Vector -n+1 PO 0 PO -1 PO -n+1 State variables Next state

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG6 Example for Logic Systems FF2 FF1 A B s-a-1

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG7 Five-Valued Logic (Roth) 0,1, D, D, X A B X X X 0 s-a-1 D A B X X X 0 D FF1 FF2 D D Time-frame -1 Time-frame 0

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG8 Nine-Valued Logic (Muth) 0,1, 1/0, 0/1, 1/X, 0/X, X/0, X/1, X A B X X X 0 s-a-1 0/1 A B 0/X 0/1 X s-a-1 X/1 FF1 FF2 0/1 X/1 Time-frame -1 Time-frame 0

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG9 Implementation of ATPG n Select a PO for fault detection based on drivability analysis. n Place a logic value, 1/0 or 0/1, depending on fault type and number of inversions. n Justify the output value from PIs, considering all necessary paths and adding backward time-frames. n If justification is impossible, then use drivability to select another PO and repeat justification. n If the procedure fails for all reachable POs, then the fault is untestable. n If 1/0 or 0/1 cannot be justified at any PO, but 1/X or 0/X can be justified, the the fault is potentially detectable.

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG10 Drivability Example d(0/1) = 4 d(1/0) = (CC0, CC1) = (6, 4) s-a-1 (4, 4) (10, 15) (11, 16) (10, 16) (22, 17) (17, 11) (5, 9) d(0/1) = 9 d(1/0) = d(0/1) = 109 d(1/0) = d(0/1) = 120 d(1/0) = 27 d(0/1) = d(1/0) = 32 (6, 10) FF d(0/1) = d(1/0) = 20 8 CC0 and CC1 are SCOAP combinational controllabilities d(0/1) and d(1/0) of a line are effort measures for driving a specific fault effect to that line

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG11 Complexity of ATPG  Synchronous circuit -- All flip-flops controlled by clocks; PI and PO synchronized with clock:  Cycle-free circuit – No feedback among flip-flops: Test generation for a fault needs no more than dseq + 1 time- frames, where dseq is the sequential depth.  Cyclic circuit – Contains feedback among flip-flops: May need 9 Nff time-frames, where Nff is the number of flip- flops.  Asynchronous circuit – Higher complexity! Time- Frame 0 Time- Frame max-1 Time- Frame max-2 Time- Frame -2 Time- Frame S0S1 S2 S3 Smax max = Number of distinct vectors with 9-valued elements = 9 Nff

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG12 Cycle-Free Circuits n Characterized by absence of cycles among flip- flops and a sequential depth, dseq. n dseq is the maximum number of flip-flops on any path between PI and PO. n Both good and faulty circuits are initializable. n Test sequence length for a fault is bounded by dseq + 1.

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG13 Cycle-Free Example F1 F2 F3 Level = 1 2 F1 F2 F3 Level = dseq = 3 s - graph Circuit All faults are testable in this circuit.

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG14 Cyclic Circuit Example F1 F2 CNT Z Modulo-3 counter s - graph F1 F2

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG15 Modulo-3 Counter n Cyclic structure – Sequential depth is undefined. n Circuit is not initializable. No tests can be generated for any stuck-at fault. n After expanding the circuit to 9 Nff = 81, or fewer, time-frames ATPG program calls any given target fault untestable. n Circuit can only be functionally tested by multiple observations. n Functional tests, when simulated, give no fault coverage.

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG16 Adding Initializing Hardware F1 F2 CNT Z Initializable modulo-3 counter s - graph F1 F2 CLR s-a-0 s-a-1 Untestable fault Potentially detectable fault

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG17 Benchmark Circuits Circuit PI PO FF Gates Structure Seq. depth Total faults Detected faults Potentially detected faults Untestable faults Abandoned faults Fault coverage (%) Fault efficiency (%) Max. sequence length Total test vectors Gentest CPU s (Sparc 2) s Cycle-free s Cycle-free s Cyclic s Cyclic

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG18 Summary n Combinational ATPG algorithms are extended:  Time-frame expansion unrolls time as combinational array  Nine-valued logic system  Justification via backward time n Cycle-free circuits:  Require at most dseq + 1 time-frames  Always initializable n Cyclic circuits:  May need 9 Nff time-frames  Circuit must be initializable  Partial scan can make circuit cycle-free  Asynchronous circuits: Not discussed n See, M. L. Bushnell and V. D. Agrawal, Essentials of Electronic Testing for Digital, Memory and Mixed-Signal VLSI Circuits, Springer, 2000, Chapter 8.

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG19 Problems to Solve 1. Which type of circuit is easier to test? Circle one in each: n Combinational or sequential n Cyclic or cycle-free n Synchronous or asynchronous 2. What is the maximum number of input vectors that may be needed to initialize a cycle-free circuit with k flip-flops?

Copyright 2001, Agrawal & BushnellLecture 6: Sequential ATPG20 Solution 1. Which type of circuit is easier to test? Circle one in each:  Combinational or sequential  Cyclic or cycle-free  Synchronous or asynchronous 2. What is the maximum number of input vectors that may be needed to initialize a cycle-free circuit with k flip-flops? k vectors. Because that is the maximum sequential depth possible. An example is a k bit shift register.