A clock apparatus has a first clock unit, a backup power supply unit, a second clock unit, and a control unit. The first clock unit is used to count time, and the backup power supply unit is used to supply a backup power voltage to the first clock unit, when a general power supply unit does not supply...http://www.google.fr/patents/US5657297?utm_source=gb-gplus-shareBrevet US5657297 - Clock apparatus having high accuracy
