An IC card, i.e., a portable medium, contains a data memory and a CPU functioning as a control element. The IC card has a keyboard and a built-in battery. It also includes a clock controller, a first clock generator always generating a low-frequency clock signal, and a second clock generator having a...http://www.google.de/patents/US4814591?utm_source=gb-gplus-sharePatent US4814591 - Portable medium
