Scan test system with a test interface having a clock control unit for stretching a power shift cycle

A method generates scan patterns for testing an electronic device called DUT having a scan path. A scan tester is arranged for executing a scan shift mode and a capture mode. A scan test interface has a clock control unit for stretching a shift cycle of the scan clock in dependence of a scan clock pattern. The method determines at least one power shift cycle which is expected to cause a voltage drop of a supply voltage exceeding a predetermined threshold during respective shift cycles of the scan shift mode, and generates, in addition to the scan pattern, a scan clock pattern indicative of stretching the power shift cycle. Advantageously, a relatively high scan shift frequency may be used while avoiding detrimental effects of said voltage drop by extending the respective power shift cycle, whereby test time and yield loss are reduced.