Electronic devices perform tasks in response to the periodic oscillation of an electrically varying digital voltage that acts as a clock timing signal. On the other hand, human time (also known as “real” time) is measured in seconds, minutes, hours, days, months, and years as dictated by the rotational cycles of the earth. For electronic devices, such as a smart watch, to perform their tasks when required by a user, they must store a digital representation of real time, which is then kept in sync by the digital clock timing signal. In an electronic device, clock timing information is maintained by its Real Time Clock (RTC) circuitry. This will usually be located within the device microcontroller or may be a separate IC on the system board
Figure 1 Clock timing can be provided by a discrete RTC IC on a system board
If the main power source for an electronic device fails, then real-time clock timing information will be lost and will need to be reset once power has been restored. While many portable IoT devices have the functionality to reset the time using a wireless connection to a remote master, 2 this type of communication places an extra power drain on the battery (and obviously can only occur if a wireless signal is available). In the absence of a wireless signal, the only (and clearly less desirable) alternative is for the user to manually reset the time, which may not be always straightforward or even possible. An RTC circuit allows a system to keep track of real time long after a main power failure.
Clock timing is important in many different applications, such as industrial (utility meters, point-of-sale equipment, fire alarms, gaming machines, video security), consumer (digital cameras, portable GPS devices, mobile game devices, satellite receivers, TVs) as well as in portable and home medical equipment.
A 32.768kHz quartz tuning-fork crystal oscillator is the standard clock timing reference for most electronic applications. The real-time clock maintains timing and date information by counting seconds, which requires a 1Hz clock signal derived from the 32.768kHz crystal oscillator. The time and date information are stored in a set of registers, which is accessed through a communication interface, such as I2C. The crystal may be external to the RTC or integrated within the same package. For applications requiring higher accuracy, an integrated MEMS (microelectromechanical system) resonator is used as the clock timing reference.