The CMOS clock and battery

The real time clock (or RTC for short) is a circuit which synchronises system signals. It is made from a crystal which, as it vibrates, gives off pulses (called timer ticks) in order to keep the system elements running on the same time. The timer frequency (expressed in MHz) the number of times the crystal vibrates each second, i.e. the number of timer ticks per second. The higher the frequency, the more information the system can process.

When the computer is turned off, the power supply stops providing electricity to the motherboard. When the computer is turned on again, the system is still on the right time. An electronic circuit, called the CMOS (Complementary Metal-Oxyde Semiconductor, sometimes called the BIOS CMOS), saves some system information, such as the time, the system date, and a few essential system settings.

The CMOS is kept powered by a battery (a button battery), or a battery located on the motherboard. Information on the hardware installed in the computer (such as the number of tracks or sectors on each hard drive) are stored in the CMOS. As the CMOS is a form of slow storage, certain systems sometimes recopy the CMOS's content into the RAM (fast storage); the term "memory shadow" is used to describe this process of copying the data into RAM.

The "complementary metal-oxide semiconductor" is a transistor manufacturing technology, the latest in a long line which includes the TTL ("Transistor-transistor-logic"), the TTLS (TTL Schottky) (faster), or the NMOS (negative channel) and PMOS (positive channel).

The CMOS allows many complementary channels to run on a single chip. Compared with TTL or TTLS, CMOS is much slower, but it consumes far less energy, which is why it is used in computer clocks, which run on batteries. The term CMOS is sometimes incorrectly used to refer to computer clocks.

When the system time keeps getting reset, or the clock runs late, all that is usually necessary is to change the battery.