Chapter 7. Testing of a digital circuit

Failure: any departure of a system or module from its specified correct operation. A failure is a malfunction. Fault: a condition existing in a hardware or software module that may lead to the failure of the module Hardware fault : external disturbances, manufacturing defects. Software fault : design mistake. Error: an incorrect response from a hardware or software module. An error is the manifestation of a fault. The occurrence of an error indicates that a fault is present in the module. Testing: fault detection/ fault location. Test pattern source Circuit under test (C.U.T) Compare Good/bad Reference value

stuck-at fault bridging fault Fault model stuck-at fault bridging fault stuck at fault • open collector or open base  Z is stuck at logic value 1 regardless of x (Z s-a-1 or x s-a-0) • short between collector and emitter  Z is stuck at 0 regardless of x (Z s-a-0) bridging fault Most case: assume single stuck-at fault Test generation x z = x’ Vpp Z C E B x1 x2 bridging  A D F xn exhausting test 2n  (n+1) test

, let  be the fault where the input xi is s-a-0 1 1 1 1 1101 0 0 0 1 1 0 1 1 A/1 B/0 ••• ••• ••• too much A/0 Z x1 x2 Test vector (1,0) Test generation 1. Algebraic algorithm: Boolean difference  difficult to make computer program 2. Standard algorithm: D-algorithm Boolean difference: to determine the complete set of test to detect a stuck-at fault. The Boolean difference of F(x) with respect to the input xi , let  be the fault where the input xi is s-a-0 The test pattern that detects the fault (xi s-a-0)  Fault free value

Using Shannon’s expression theorem The test pattern to detect xi s-a-0 The test pattern to detect xi s-a-1 If F(x) is dependent on xi (i.e. the fault on xi is detectable). Fi(0) will be different from Fi(1) The test pattern that detects the fault xi s-a-0 The test pattern that detects the fault xi s-a-1

Test vector x1, x2, and x3 for x1 s-a-0 is (1,1,0) or (1,1,1) Test set for x1 s-a-0  F=x1x2+x2’x3 x1 x2 x3 G1 G2 F s-a-0 Test vector x1, x2, and x3 for x1 s-a-0 is (1,1,0) or (1,1,1) h Test set for h s-a-0  F=x1x2+h Test vector x1=don’t care x2=0, x3=1 (0,0,1) or (1,0,1)

Example) Find a test vector! Disadv. of Boolean difference; 1. Difficult to manipulate algebraic equation. 2. This method generates all test inputs for a given fault.  require a large quantity of time and memory Test generation algorithm based on path sensitization Two problems in test generation procedure 1. Creation of a change at the faulty line. 2. Propagation of the change to the primary output line. The basic principles of single-path sensitization 1. At the site of the fault, assign a logical value complementary to the fault being tested. 2. Select a path from the primary inputs through the site of the fault to a primary output  forward sensitizing (apply 1’s to all remaining inputs to the AND and NAND gates in the path, and 0’s to the OR and NOR gates in the path). 3. Determine the primary inputs that will produce all the necessary signal values  backward tracing or line justification. Example) Find a test vector! s-a-0 x1 x2 x3 x4

Multiple path sensitization Example) Find a test vector! x1 x2 x4 x3 G1 G2 G3 G4 G5 G6 G7 G8 We can’t find a test vector using a single-path sensitization. Is there no test vector? Ans) we don’t know. We can try to use multiple-path sensitization = D – algorithms. Multiple path sensitization D-algorithm; 1966 Roth(IBM) – five logic values used : Singular cube: The singular cover of a gate is a compact truth table representation in terms of the {0,1,X} variables. Each row of the singular cover is called a singular cube. s-a-1

 : may be possible with another choice of propagation D-cube 1 1 1 0 X 0 X 0 0 1 2 3 Compact truth table 1 2 3 4 0 1 1 (0/1) 1 2 3 4 A primitive D-cube(PDC) of a fault: minimal specification of inputs to set D or at the output of a faulty gate. Propagation D-cube: minimal specification of inputs to the gate such that a D or on an input (inputs) is propagated to its output as D or s-a-1 D 1 1 D 1 D 1 D 1 1 D D D D 1 D D 1 D D 1 D D D D D D D 1 2 3 4 Just replace to D 0  0   1 1   0 1 X D   D D     X D 0 1 X D  : inconsistent  : may be possible with another choice of propagation D-cube

Consider the process of generating a test for line 5/0 in fig 1. 4. 8 Consider the process of generating a test for line 5/0 in fig 1.4.8. We start with the following cube, which includes the primitive D-cube of failure for this fault: The next step is to propagate this D farther toward line 11. Referring to propagation D-cubes in table 1.4.1, cube j shows that the D in cube k is automatically propagated to lines 6 and 7. Consider now the propagation along the path 6, 9, 11. Table 1.4.1 again shows that the D on line 6 can be moved to line 9 by using cube d. In other words, a new cube can be obtained by combining cubes d and k. This process of combination is referred to as D-cube intersection. Lines 7 and 9 now contain the change, which can be propagated further into line 11.

The next step is determine a value for the primary input line 4 since that is the only line that has not been assigned a value. To create a 0 in line 10, both lines 7 and 8 must be a 1; however, in cube m, line 7 is a D, indicating that cube m cannot result in a test vector along 5, 6, 9, 11. Start again from cube k to propagate along the path 5, 7, 10, 11. This leads to the following cube: The values of 1 and 4 are required to complete the operation. The resulting cube are completing lines 1 and 4 is:

D-frontier: The set of all gates whose output values are unspecified, but whose input has some signal D or 1 2 4 3 G1 G2 G3 G4 G5 G6 G7 G8 s-a-1 5 6 7 8 9 10 11 12 G8,G6 1 1 1 x 0 x x D x x x 1 x 0 1 1 1 1 x x x x D x x x 1 1 0 G5,G6 x 1 1 x x x x x x x x 1 D x x x x x x x x x x x x 1 1 1 D-frontier 1 2 3 4 5 6 7 8 9 10 11 12 Out, G6 1 1 1 1 0 0 1 D 1 1 1 1 1 0 1 1 1 x 0 0 1 D 1 1 1 1 1 0 1 1 1 x 0 x 1 D 1 1 1 x 0 1 1 1 1 x 0 x 1 D x x x 1 1 D 1 1 conflict

When conflict occurs, we have to go back to the previous D-frontier G8,G6 Out, G6 1 1 1 x 0 0 1 D D 1 1 1 1 1 1 1 1 x 0 x 1 D D 1 1 x x 0 1 1 1 1 x 0 x 1 D x x x 1 1 D D 1 D-frontier 1 2 3 4 5 6 7 8 9 10 11 12 1 1 1 1 0 0 1 D D 1 1 Test vector D-algorithm will derive a test for any fault if such a test exists

Detection of Faults in PLA PLA characteristic 1. In a circuit structure, PLA essentially has only two levels of gates. 2. A more general fault model is necessary because of the way PLA is fabricated. Fault Model: incorrect logical connections in the AND and OR plane Growth (G) fault: a connection in the AND plane is missing -> causing the implicant to grow Disappearance (D) fault: a connection in the OR plane is missing -> causing the implicant to disappear Shrinkage (S) fault: an intended connection in the AND plane is made -> causing the implicant to shrink Apperance (A) fault: an intended connection in the OR plane is made -> causing the implicant to appear Single fault assumption: Important calsses of multiple faults are detected by any single fault test set.

Growth fault: L’ = (C·Cij →x, D) Shrinkage fault: L’ = (C* Cij → α, D), α=0,1 Disappearnce fault: L’ = (C, D·Dij →0) Appearance fault: L’ = (C, D·Dij →1) S1 # S2 = S1·NOT(S2) = the cube in S1 not in S2 α

Classification of fault-tolerant technique Fault avoidance - Environmental (dust free) - High quality components - Quality control 2. Fault detection: make sure the system is working or not - Duplication - Error detection code: parity bit - Self-checking - Watchdog processor

4. Dynamic redundancy: reconfiguration 3. Fault masking - Voting (triple modular redundancy) – TMR – NMR - Error correcting code 4. Dynamic redundancy: reconfiguration ••• ••• ••• fault If one of DRAM is fault,  DRAM fault Using reconfiguration ••• ••• ••• fault Remove Reconfiguration Add

Triple modular redundancy f = ab’+a’ b V Assuming that voter “V” is fault-free Without Assumption Even if the voter may be faulty. Main disadvantage: Cost We use the critical environment(spaceship)

Hamming Code – single error detecting Hamming code Error detecting code Hamming distance(HD) between two binary n-tuple A&B is the number of position in which A&B differ. A code C is d error detecting iff the minimum HD between any two code words is d+1 ci cj • d d+1 * We can detect, but we can’t know whose error is detected A code C is d error correcting iff the minimum HD between any two code words is at least 2d+1 2d+1 Hamming Code – single error detecting Hamming code 2qm+q+1, m: # of information bits, q: # of checking bits If m=16 then 5(=q) check bits needed

Detection of multiple faults a circuit with r wires single fault assumption : 2•r faults (s-a-0 or s-a-1) multiple fault assumption : 3r-1 faults  consider the transformation of a given circuits due to some faults, rather than faults themselves  # of transformation << 3r-1 For 2-level AND-OR circuits s-a-0 same effect  remove ,where Pi is the ith P.I. Each AND gate: one P.I. The effects of s-a-0 faults on the function A s-a-0 fault at input or output of a AND gate  eliminate one P.I. from the function  test one minterm which is covered by that P.I. and by other P.I.  a complete set of tests for s-a-0 faults for a two-level AND-OR circuits, consists of n tests corresponding to n P.I.’s in f. Test minterm for the jth P.I. :

Example) f(w,x,y,z) = w’y’+y’z+wxy+xyz’ The effects of a s-a-1 fault at input of AND gate  Output of that gate is independent of the input variable  Equivalent to cover an additional subcube that is adjacent to the original one. Example) f(w,x,y,z) = w’y’+y’z+wxy+xyz’ Test for s-a-0 = {0000,0100, 1101,1001, 1111, 0110} y’ z w’ x y z’ w s-a-1 minterm xyz’ xy: K-map extended 01 00 10 11 wx yz 1 1st P.I. 2nd P.I. 3rd P.I. 4th P.I. Test for s-a-1 = {2 or 10, 7, 11, 12}

Sequential Logic – Memory: Problems Generating a Test Sequence ● Initial state is unknown – controllability is required to set initial state. ● Fault signal must be propagated to primary outputs. Observability of final state is required to check faulty state. Scan Design: All flip-flops in a circuit are interconnected into one or more shift registers and the content of the register is shifted in and out. ● Very few additional external connections used to access many internal nodes at the cost of additional internal logic. ● All states completely observed and controlled form primary I/O.

Testing Procedure in scan-path design – LSSP Scan in test vector Yj using Xn and TCK Apply corresponding test vector on Xi inputs After sufficient time for signals to propagate, check output Z. Apply one clock pulse to SCK to enter new values of Yj into corresponding FFs. Scan out with TCK and check Yj values. Level-Sensitive – Level stays for a certain period. Edge-Sensitive – Level only at pulse change.