Scan chain for rapidly identifying first or second objects of selected

Abstract

A circuit includes a sequential entries for storing objects of different types and a scan chain which can identify an object of a first type which follows after an object of a second type. The first and second types can be identical so that the scan chain identifies the second object of a particular type in the sequence. The scan chain includes single-entry 'generate', 'propagate', 'kill', and 'only' terms which control a scan bit. Conceptually, if the 'only' term is not asserted, an entry of the second type generates the scan bit and asserts the 'only' term. After the 'only' term is asserted, further generation of the scan bit is inhibited. Each entry either propagates the scan bit to the next entry or if the entry is of the first type, kills the scan bit and identifies itself as the selected entry. Look-ahead logic determines group terms from single-entry terms to indicate whether a scan bit would be generated, propagated, or killed by a group of entries. Accordingly, the scan bit is not required to propagate through every entry, and scans can be performed quickly.