Masking of X-values by Use of a Hierarchically Configurable Register

In this paper we consider the test of large circuits. Due to the increasing number of scan chains of industrial designs and due to the limited recourses of the ATE-equipment the compression ratio of the test responses increases. A second issue are undefined states (X-values). In the presence of X-values during test a considerable number of the scan cells cannot be observed at the compressed outputs. We present a new hierarchical two-step method of X-masking to achieve a high observability of scan cells and to reduce the overhead for X-masking. For every scan-shift cycle the generated X-mask is either set to be active or not. By this method the number of X-values at the compacted outputs is considerably reduced. The remaining small number of X-values can be tolerated by an X-tolerant compactor. To implement this method a hierarchically configurable register is used.