Reaalikellon

Reaalikellon is a term used to refer to a real-time clock (RTC), a digital integrated circuit or device that keeps track of the current time and date. In computing and electronics, a RTC continues to keep time when the main system is powered down, typically powered by a small battery or a capacitor. RTCs usually maintain time using a 32.768 kHz crystal oscillator and report the value to the main processor via a bus such as I2C, SPI, or a memory-mapped interface. Time information may be stored in Binary-Coded Decimal or binary format, and often includes features such as seconds, minutes, hours, day, date, month, year, and leap year handling. Many RTCs support an alarm function and a periodic interrupt for timed tasks.

Historically, real-time clocks emerged as separate chips to provide timekeeping outside the host CPU, becoming common in personal computers and embedded devices in the 1980s and 1990s. Modern systems may integrate RTC functionality into microcontrollers or system-on-chip designs, sometimes sharing a clock source with other peripherals.

In use, the RTC provides a stable time base for scheduling, logging, and time-stamping events. It can be synchronized with network time protocols or software clocks but remains autonomous when offline. The term "reaalikellon" thus refers to this essential timekeeping component across Finnish-language technical writing.

See also: real-time clock, real-time operating system, I2C, SPI, CMOS, 32.768 kHz crystal.