Patent ID: 7649422

Claim:
A RTC IC, comprising: an adjustable low-power oscillator, for providing a standard clock, wherein the standard clock produces a frequency drift corresponding to a current operation voltage of the adjustable low-power oscillator; a real time counter, coupled to the adjustable low-power oscillator, for counting a first counting number according to the standard clock without the frequency drift so as to obtain a constant counting time, and producing a detection signal timely; and a control unit, coupled to the adjustable low-power oscillator and the real time counter, for detecting the current operation voltage of the adjustable low-power oscillator when the control unit receives the detection signal, and locating the standard clock without the frequency drift corresponding to the current operation voltage of the adjustable low-power oscillator in a lookup table to determine whether or not the frequency drift is produced by a current standard clock of the adjustable low-power oscillator, wherein if the frequency drift is produced by the current standard clock of the adjustable low-power oscillator, the control unit produces a first control signal or a second control signal correspondingly to make the current standard clock of the adjustable low-power oscillator become the standard clock without the frequency drift corresponding to the current operation voltage of the adjustable low-power oscillator, and the real time counter counts the first counting number so as to obtain the constant counting time accordingly, wherein the adjustable low-power oscillator comprises: a first switch, coupled to the lookup unit, having a first terminal, a second terminal and a control terminal, wherein the control terminal of the first switch receives the first control signal or the second control signal correspondingly and turns on or off the first switch accordingly; a first inverter, having an input terminal and an output terminal, wherein the input terminal of the first inverter is coupled to the first terminal of the first switch; a second inverter, having an input terminal and an output terminal, wherein the input terminal of the second inverter is coupled to the output terminal of the first inverter, and the output terminal of the second inverter is coupled to the second terminal of the first switch; and a third inverter, having an input terminal and an output terminal, wherein the input terminal of the third inverter is coupled to the output terminal of the second inverter, and the output terminal of the third inverter is coupled to the input terminal of the first inverter.