A system on a chip includes a real time clock (RTC) module, a crystal oscillation circuit and a voltage supply circuit. The RTC module is coupled to provide timing functions and the crystal oscillation circuit is coupled to produce an oscillation. The voltage supply circuit is coupled to produce a supply voltage for at least a portion of the RTC module and the crystal oscillation circuit. The voltage supply circuit includes: a reference circuit coupled to produce a reference voltage based on the supply voltage; a transistor coupled to the battery IC pin, wherein the transistor produces the supply voltage based on a regulation signal and the battery voltage; an amplifier coupled to produce the regulation signal based on the reference voltage and a feedback representation of the supply voltage; and a start-up circuit coupled to enable the voltage supply circuit at battery connection.