VLSI TESTING 1 WHAT IS BIST n BIST

VLSI TESTING

1) : WHAT IS BIST ? n 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 test n BIST categories : n n Memory BIST Logic + Embedded memory (ASICs) Applications : Mission-critical sytems, self-diagnostic circuitry (consumer electronics).

2) : BIST Concepts n n n BIST Techniques Test Pattern Generation Techniques (TPG) Test Response Compression Techniques

3 ) BIST Techniques n The BIST techniques are classified bassed on the operational condition of the circuit under test (CUT): Off-Line BIST n On-Line BIST n

3 -1) : On-Line BIST n Testing occures during normal functional operating conditions (No test mode, Real-Time error detection). n Concurrent : Occures simultaneously with normal functional operation (Realized by using coding techniques). n Nonconcurrent : Carried out while in idle state (Interruptible in any state, realized by executing diagnostic software/firmware routines).

3 -2) : Off-Line BIST n n Deals 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. n Structural : Execution based on the structure of the CUT(Explicit fault model - LFSR, . . . ). n Functional : Running based on functional description of CUT(Functional fault model Diagnostic software).

4) : Test Pattern Generation Techniques n n 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): n n Logical segmentation : Cone + Sensitized-path Physical segmentation

4 ) : Test Pattern Generation Techniques (Cont. ) n Pseudorandom : Not all 2**n input combinations, Random patterns generated deterministically & repeatably, pattern with/without replacement, applicable to both combinational and sequential circuits. n n 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.

5) : Test Response compression techniques n n n 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.

5) : Test Response compression techniques (cont. ) n Compression problems : – Existing aliasing errors. – Calculating the good circuit signature. n 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.

5) : Test Response compression techniques (cont. ) n n n 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).

6) : Factors affecting the choice of BIST n n n n Degree of test parallelism Fault coverage Level of packaging Test time Complexity of replaceable unit Factory and field test-and-repair strategy Performance degradation Area overhead

6 -1) : Advantages Lower cost of test n Better fault coverage n Possibly shorter test times n Tests can be performed throughout the operational life of the chip n

6 -2) : Disadvantages n n Silicon area overhead Access time Requires the use of extra pins Correctness is not assured

7) : BIST key elements n n n Circuit under test (CUT) Test pattern generators (TPG) Output response analyzer (ORA) Distribution system for data transmission between TPG, CUT and ORA BIST controller

8) : Specimen BIST architecture Chip, Board or System TPG BIST controller D I S T CUT D I S T ORA

9) : Memory BIST n Memory types – SRAM, DRAM, EEPROM, ROM n Fault models – SAF, TF, CF, NPSF, AF n n Test algorithms Test categories

9 -1) : Fault models n Stuck-At Fault (SAF) – The logic value of a cell or a line is always 0 or 1 n Transition Fault (TF) – A cell or a line that fails to undergo a 0=>1 or a 1=>0 transition n Coupling Fault (CF) – A write operation to one cell changes the content of a second cell

9 -1) : Fault models cont. n 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 memory n Address Decoder Fault (AF) – Any fault that affects address decoder – – – With a certain address , no cell will be accessed A certain cell is never accessed with a certain address , multiple cells are accessed simultaneously – A certain cell can be accessed by multiple addresses.

9 -2) : Memory test algorithms n Traditional tests n n March tests : Tests for stuck-at , transition and coupling faults n n zero-one, checkerboard, GALPAT, Walking 1/0, Sliding Diagonal, Butterfly MATS , Marching 1/0 , March X , . . . Tests for neighborhood pattern sensitive faults (NPSF)

9 -3) : Memory test categories n DC : Tests to verify analog parameters n n AC : Tests to verify timing parameters n n Open/short -Power consumption-leakage, threshold, . . . Signal 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, …)

10) : Advanced Topics Built-In Self-Repair(BISR). n Programmable memory BIST n Generalized Linear Feedback Shift Registers(GLFSR) for pseudo random memory BIST n Transparent BIST for RAMs. n And. . n