Circuit for determining window margin for data transitions in a

Abstract

This invention is directed to a method and a circuit for determining a relative phase shift between a data transition and a data separator window which are used to decode data read back from a mass storage device. The data separator windows provide a time interval in which a data transition must occur in order to be properly interpreted. This is accomplished by delaying the occurrence of the data transition, thereby shifting it toward the trailing edge of the data separator window, to determine if the delayed data transition occurs late within the data separator window. At the same time, the occurrence of the data separator window is delayed, thereby shifting the leading edge of the data separator window toward the data transition, to determine if the data transition occurs late within the data separator window. By delaying the data separator window and the data transition by known, predetermined amounts, a phase shift between the data transition and a particular data separator window in which it occurs can be determined. It is also an object of this invention that if the data transition does not occur within the delayed data separator window or the delayed data transition does not occur within the data separator window, the data transition will be dropped from the data stream and a data read error will have occurred.