February_EDFA_Digital

edfas.org 13 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 21 NO. 1 minimum number possible. Figure 1 illustrates the eight tests for an RCA of arbitrary size. The binary values inside each full-adder showthe values applied to the inputs. Each RCA test consists of a repeating pattern of eight full-adder input pattern combinations, each corresponding to a row of the truth table of the full-adder function. The C-testable property of regular circuits is particularly appealing for use in a test chip, as it provides strong guarantees concerning the number of tests and fault testability regardless of the size of the regular circuit. CM-LCV DESIGN The CM-LCV is a collection of one or more functional unit block (FUB) arrays along with their test access mechanisms that can be readily adapted to fit the given constraints (e.g., silicon area and test I/O). A FUB array is a two-dimensional array of FUBs implementing some logic function. A two-dimensional array is selected because such a structure improves diagnosis due to the diverg- ing fan-out for FUB outputs. Although the necessary conditions for two-dimensional array C-testability are unknown, there aremany known sufficient conditions that ensure two-dimensional array C-testability. One simple approach that ensures C-testability relies on the use of a FUB function that is bijective. A bijective Boolean func- tion is a lossless function with equal numbers of inputs and outputs, where each unique set of input values is mapped to a unique output set of values. It can easily be shown that a two-dimensional array that has an n-input bijective function isC-testable for possible IP faultswith the minimum number of test patterns, namely 2^^n. Figure 2 illustrates a test applied to a FUB array with errors propagating from a single defective FUB (high- lighted in red). Further improvements to the array diagnosability can be realized by applying additional constraints on the bijective FUB function. A stricter property called vh-bijectivity is described in an International Test Conference 2014 paper. [2] VH-bijectivity ensures that an error signal that arrives on only one input port of a FUB is propagated to both the vertical and horizontal output ports. An array composed of vh-bijective FUBs results in near ideal, FUB-level diagnostic resolution for defects confined to a single FUB, regardless of the array size. CM-LCV IMPLEMENTATION Having determined the design of the CM-LCV in the previous section, there remains the problem of implementing it in a manner that reflects the proper- ties of actual product designs. This section begins with a description of the objectives used to ensure that the CM-LCV achieves its stated goal, followed by a description of the implementation flow for creating a CM-LCV that meets these objectives. OBJECTIVES The first objective for the CM-LCV is design reflection. As stated before, design reflection is crucial to the success of the CM-LCV as it ensures that any yield issues discov- ered are relevant to actual product designsmanufactured using the same process. However, measuring the similar- ity between two designs remains difficult. Many different Fig. 1 A constant size test set of size eight for a ripple-carry adder of arbitrary size. Fig. 2 Application of a single test to a 5 × 3 array of FUBs. The FUB located at the intersection of the second row and third column produces an erroneous response (representedby an expected/observednotation) that propagates through the array and is observed at its boundary, as shown.