Method and on chip circuitry for testing integrated circuits, for instance, flip chip integrated circuits. Provided on the integrated circuit in addition to the conventional circuitry is additional circuitry including a photosensitive element such as a photodiode, the output terminal of which is connected via a Schmidt trigger to the clock terminal of an on-chip flip-flop. The node of the integrated circuit to be tested, for instance, the output terminal of a logic gate, is connected to the D input terminal of the same flip-flop. Hence, light incident on the photosensitive element clocks the flip-flop, allowing sampling of the state of the output signal from the logic gate. Advantageously, the photodiode need not be a specially made structure but in one version is the conventional PN junction provided by, e.g., the drain of a standard CMOS transistor.