|Real time clock (RTC)|
|Tuesday, 12 December 2006|
Clock circuits are an essential component of modern computer systems. A typical computer system includes a processor subsystem of one or more microprocessor, a memory subsystem, one or more chipsets provided to support different types of host processors for different platforms such as desktops, personal computers (PC), servers, workstations and mobile platforms, and to provide an interface with a plurality of input/output (I/O) devices. Chipsets may integrate a large amount of I/O bus interface circuitry and other circuitry onto only a few chips. These chipsets may implement the I/O bus interface circuitry, timer, real-time clock (RTC), direct memory access (DMA) controller, and other additional functionality such as integrated power and thermal management with quick resume capabilities and random seed number generation for security applications such as cryptography, digital signatures, and protected communication protocols. Clock circuits generate a regular series of pulses based on a piezoelectric crystal, which governs the frequency of the pulses. The clock signal that is generated is used to synchronize the operation of the other components and circuits in the system. The boot up operation of a computer system is reliant upon the establishment of a reliable system clock. Establishing this reliable signal adds additional time to the boot up process for a system. A typical personal computer includes two time keeping systems: a hardware real time clock, and a software virtual clock maintained by an operating system. The hardware real time clock typically includes a battery backup source of electrical power, and continuously maintains an estimate of the current date and time. The software virtual clock is typically synchronized to the RTC during PC power up and initialization. On a computer mother board, a RTC is provided to maintain the data stored in a CMOS on the mother board. Generally, electrical power is provided to a RTC by a battery arranged on the mother board. The RTC is used to update the current time and date within the computer system without any intervention from the system processor. Usually the RTC and other peripheral devices are coupled to the peripheral bus through an interrupt controller. Such a clock has various uses such as time stamping files and inserting dates into documents. The RTC is used to update a program clock, typically a date/time clock maintained by the operating system (OS) and referred to herein as the OS clock. An operating system may include a system clock to provide a system time for measuring small increments of time. The system clock may update the system clock in response to a periodic interrupt generated by a system timer, or a real time clock event timer. A personal digital assistant (PDA) is a pen-based computer system where the primary method for inputting data includes a pen or stylus. Many of the functions of a PDA require an accurate real-time clock to provide the calendar function, the clock display, and various timer functions of a PDA require an accurate real-time clock.
An electronic apparatus generally has a RTC (real time clock) installed in the circuit board thereof for recording system time. A real time clock is one of the requisite hardware devices of an electronic device. A standard RTC includes an oscillating circuit coupled to a digital counter. Generally, a digital counter has a register whose contents step through a regular series of states, which are usually states indicating consecutive integers. The counter is typically incremented by a clock signal, wherein the rising or falling edge of the clock transition causes the counter to change from one state to another. A counter may also be set or loaded with an initial value by writing data into the counter register, or by activating a reset function in the counter. A real time clock oscillator includes an inverting amplifier, a feedback resistor, a drive resistor, a crystal and capacitors. Inverting amplifier, feedback resistor, and drive resistor are usually incorporated within an integrated circuit device. On the other hand, crystal and capacitors are usually discrete devices external to the integrated circuit device and are mounted on a printed circuit board. The majority of the power consumed by oscillator circuit is through inverting amplifier. A crystal oscillator circuit generates sinusoidal oscillations across an LC tank, which are converted into a square waveform, often using a biased analog inverter. This inverter trips at a designed trip point and the resulting duty cycle of the generated clock signal is highly dependent on the trip point and the amplitude of the input waveform. The performance characteristics of a crystal oscillator generally depend on both the particular cut and the mode of vibration. Each cut-mode combination is considered as a separate piezoelectric element, and the more commonly used elements often are designated with letter symbols. A typical real time clock can be programmed to produce or generate an RTC alarm event at a designated time. The RTC alarm event may cause, for example, the assertion of an interrupt signal to the microprocessor. A real time clock is typically implemented in specific hardware with a dedicated crystal oscillator to insure accuracy and a battery backup power supply to insure preservation of timekeeping data during an interruption of the primary power supply. This is especially important with personal computers which are frequently powered down. It is common to back up a real-time clock (RTC) with a battery, such as a long-life lithium battery, so that when a main power supply for the RTC fails, or has been disconnected, the RTC will continue to indicate the correct time and provide correct time intervals.