Test scan cells

A sequential scan cell includes an input port for functional data and an input for scan test data. The input for scan test data is an input to a master scan flip-flop coupled to a slave scan flip-flop defining a scan test circuit. Such a scan test circuit is coupled to the functional circuit of the sequential scan cell such that the path for a functional signal is not through the scan test circuit, imparting no performance penalty to the functional signal. Scan test data is scanned in and out of the sequential cell by two non-overlapping scan clocks that are active only when system functional clocks are in an off state.