2 1) : WHAT IS BIST ?BIST (Built-In Self-Test) : is a design technique in which parts of a circuit are used to test the circuit itself .Hardcore : Parts of a circuit that must be operational to execute a self testBIST categories :Memory BISTLogic BISTLogic + Embedded memory (ASICs)Applications : Mission-critical sytems, self-diagnostic circuitry (consumer electronics).
4 3 ) BIST TechniquesThe BIST techniques are classified bassed on the operational condition of the circuit under test (CUT):Off-Line BISTOn-Line BIST
5 3-1) : On-Line BISTTesting occures during normal functional operating conditions (No test mode, Real-Time error detection).Concurrent :Occures simultaneously with normal functional operation (Realized by using coding techniques).Nonconcurrent : Carried out while in idle state (Interruptible in any state, realized by executing diagnostic software/firmware routines).
6 3-2) : Off-Line BISTDeals with testing a system when it is not carrying out its normal functions (Test mode, Non-Real-Time error detection).Testing by using either on-board TPG + Output Response Analyzer (ORA) or Microdiagnostic routines.Structural : Execution based on the structure of the CUT(Explicit fault model - LFSR, ...).Functional : Running based on functional description of CUT(Functional fault model - Diagnostic software).
7 4) : Test Pattern Generation Techniques Exhaustive : Applying all 2**n input combinations, generated by binary counters or complete LFSR.Pseudoexhaustive : Circuit is segmented & each segment is tested exhaustively(Less no. of tests required):Logical segmentation : Cone + Sensitized-pathPhysical segmentation
8 4 ) : Test Pattern Generation Techniques (Cont.) Pseudorandom : Not all 2**n input combinations, Random patterns generated deterministically & repeatably, pattern with/without replacement, applicable to both combinational and sequential circuits.weighted : Non-uniform distribution of 0’s & 1’s, improved fault coverage, using LFSR added with combinational circuits.Adaptive : Using intermediate results of fault simulation to modify 0’s & 1’s weights, more efficient,more hard ware complexity.
9 5) : Test Response compression techniques Response compression : A process to form a “signature” from complete output responses.Signature : Compressed form of saved test results.Alias : Errorous output when faulty & fault-free sig. are the same.Compression procedure : Composition of test vector applying, results storing and comparision of the faulty & faultfree signatures.Compression of :Simple hardware implementation.Small performance degradation - No effect on normal circuit behaviour (delay, execution time).High degree of compression - Signature lenghts to be a logarithmic factor of responses lenghts.Small aliasing errors.
10 5) : Test Response compression techniques (cont.) Compression problems :Existing aliasing errors.Calculating the good circuit signature.Calculation of good circuit signatures :Golden Unit : Applying the test to good part of the CUT.Simulation : Simulating the CUT and making sure of having good signature.Fault Tolerant : Producing copies of CUT and conclude the correct signature by finding the subset which generates the same signature.
11 5) : Test Response compression techniques (cont.) One’s count : The no. of times when 1 occurs in each output (counter).Transition count : The no. of transitions(0 =>1,1=>0) in the output (XOR +counter).Parity checking : The parity of response string, 0 if even & 1 if odd (XOR + D-FF).Syndrome checking : the normalized no. of 1’s inoutput string (k/2**n when k is no. of minterms in an n input circuit), (All possible combination tests).Signature analysis : Based on redundancy checking (LFSR).
12 6) : Factors affecting the choice of BIST Degree of test parallelismFault coverageLevel of packagingTest timeComplexity of replaceable unitFactory and field test-and-repair strategyPerformance degradationArea overhead
13 6-1) : Advantages Lower cost of test Better fault coverage Possibly shorter test timesTests can be performed throughout the operational life of the chip
14 6-2) : Disadvantages Silicon area overhead Access time Requires the use of extra pinsCorrectness is not assured
15 7) : BIST key elements Circuit under test (CUT) Test pattern generators (TPG)Output response analyzer (ORA)Distribution system for data transmission between TPG, CUT and ORABIST controller
16 8) : Specimen BIST architecture Chip, Board or System CUTTPGORACUTBISTcontroller
18 9-1) : Fault models Stuck-At Fault (SAF) Transition Fault (TF) The logic value of a cell or a line is always 0 or 1Transition Fault (TF)A cell or a line that fails to undergo a 0=>1 or a 1=>0 transitionCoupling Fault (CF)A write operation to one cell changes the content of a second cell
19 9-1) : Fault models cont. Neighborhood Pattern Sensitive Fault (NPSF) The content of a cell , or the ability to change its content , is influenced by the contents of some other cells in the memoryAddress Decoder Fault (AF)Any fault that affects address decoderWith a certain address , no cell will be accessedA certain cell is never accessedwith a certain address ,multiple cells are accessed simultaneouslyA certain cell can be accessed by multiple addresses.
20 9-2) : Memory test algorithms Traditional testszero-one,checkerboard,GALPAT,Walking 1/0,Sliding Diagonal,ButterflyMarch tests : Tests for stuck-at ,transition and coupling faultsMATS ,Marching1/0 ,March X , ...Tests for neighborhood pattern sensitive faults (NPSF)
21 9-3) : Memory test categories DC : Tests to verify analog parametersOpen/short -Power consumption-leakage,threshold,...AC : Tests to verify timing parametersSignal rise/fall time, Setup/hold time, Delay, Access time, .Dynamic tests : Detects dynamic faults affecting CUT(Recovery, refresh line stuck-at, bit-line precharge voltage imbalance, …)
22 10) : Advanced Topics Built-In Self-Repair(BISR). Programmable memory BISTGeneralized Linear Feedback Shift Registers(GLFSR) for pseudo random memory BISTTransparent BIST for RAMs.And ....