Digital logic testing and simulation phần 8

Vì vậy, trước khi áp dụng các thuật toán SCOAP một mạch, cần thiết phải có cho mỗi nguyên thủy xuất hiện trong một mạch điện, phương trình thể hiện số 0 và 1 năng kiểm soát đầu ra của nó về năng kiểm soát đầu vào của nó | RANDOM PATTERN EFFECTIVENESS 467 a b Figure Enhancing random test. Weighted Random Patterns Another approach to testing random pattern-resistant faults makes use of weighted random patterns WRP . Sensitizing and propagating faults often require that some primary inputs have a disproportionate number of 1s or 0s. One approach developed for sequential circuits determines the frequency with which inputs are required to change. This is done by simulating the circuit and measuring switching activity at the internal nodes as signal changes occur on the individual primary inputs. Inputs that generate the highest amount of internal activity are deemed most important and are assigned higher weights than others that induce less internal Those with the highest weights are then required to switch more often. A test circuit was designed to allocate signal changes based on the weights assigned during simulation. This hardware scheme is illustrated in Figure . An LFSR generates n-bit patterns. These patterns drive a 1 of 2n selector or decoder. A subset jk of the outputs from the selector drive bit-changer k which in turn drives input k of the IC where m 1 jk - 2n and m is the number of inputs to the IC. The number jk is proportional to the weight assigned to input k. The bit-changers are designed so that only one of them changes in response to a change on the selector outputs hence only one primary input changes at the IC on any vector. When generating weights for the inputs special consideration is given to reset and clock inputs to the circuit. Input 1 Input 2 Input m Figure Weighted pattern generator. 468 BUILT-IN SELF-TEST The WRP is also useful for combinational circuits where BIST is employed. Consider for example a circuit made up of a single 12-input AND gate. It has 4096 possible input combinations. Of these only one the all-1s combination will detect a stuck-at-0 at the output. To detect a stuck-at-1 on any input requires a 0 on that .