A portable data processing apparatus is provided. It has at least one data processing function which depends on detected motion of the apparatus. The apparatus comprises a video camera operable to capture successive images of a part of the real environment around the apparatus; a video motion detector operable to detect motion of the apparatus by analysis of image motion between pairs of captured images; a hardware motion detector operable to detect motion of the apparatus, whereby the data processing function depends on motion detected by the hardware motion detector; and a controller operable to adjust the operation of the hardware motion detector if the motion detected by the video motion detector and the motion detected by the hardware motion detector differ by at least a threshold difference.