A system for determining a relative amount of usage of a data retaining device are disclosed. A charge storing device is coupled to a data retaining device in a manner that a use of the data retaining device triggers a charging of the charge storing device. In a period that the data retaining device idles, charges in the charge storing device decay due to natural means. As such, a potential of the charge storing device may be used to indicate an amount of usage of the data retaining device. A comparison of the potentials of two charge storing devices coupled one-to-one to two data retaining devices may be used as a basis to determine a relative amount of usage of each of the two data retaining devices comparing to the other.