Device and method for testing a circuit

A device having testing capabilities, the device includes: a tested circuit that includes multiple scan chains; a compactor adapted to compress scan chain test responses; a mask unit, connected between the multiple scan chains and the compactor, wherein the mask unit is adapted to mask scan chain test responses outputted by the multiple scan chains during a masking period; and an mask prevention unit, adapted to prevent masking of scan chain test responses during a mask prevention period that at least partially overlaps a mask unit configuration period.