Ambient light is detected by a photodiode circuit by measuring the time taken for a digital output of the photodiode circuit to change state in response to exposure of a photodiode of the photodiode circuit to that ambient light. A nominal time for state change is calculated based on photodiode circuit characteristics. Furthermore, an effective time for the photodiode circuit digital output to change state is determined in a calibration mode where the photodiode has been disconnected and a reference current is applied to the circuit. An illumination value of the detected ambient light is then calculated as a function of: the measured time, the effective time and the nominal time.