In the preceding recipe, we were able to put our QEMU system to sleep but were not able to wake it up. We need a device that can send an interrupt to the system when most of its internal components are powered off.

The RTC (Real-time clock) is one such device. One of its functions is to keep the internal clock running when the system is off, and to do this, it has its own battery. RTC power consumption is similar to an electronic watch; it uses the same 3 V battery and can function on its power for years. 

RTC can work as an alarm clock, sending an interrupt to the CPU at a given time. This makes it an ideal device for waking up a system on schedule.

In this recipe, we will learn how to wake up a Linux system at a specific time using the built-in RTC.