Oscillator circuits can be used to provide timing signals. For example, a personal computer motherboard typically has a Real Time Clock (RTC) circuit that provides an accurate 32.768 KHz oscillating signal that is further processed to obtain the second, minute, and hour values used by the computer system to keep time.
The RTC circuit is typically part of an I/O controller hub chip (sometimes referred to as the south-bridge chipset), and is connected to an external crystal resonator that resonates within a narrow range of operating frequencies. Depending on the crystal oscillator topology, one or more discrete external load capacitors may be connected to the RTC circuit to tune the oscillating frequency. The values of the load capacitors are selected according to an initial circuit layout design so that the RTC circuit in conjunction with the external components will oscillate at a predetermined frequency.
However, variation between different motherboard designs may result in placement of the load capacitors at slightly different locations on the motherboard, resulting in the addition of a certain amount of parasitic capacitance associated with the wiring connections. Other factors, such as tolerances in circuit components and minute routing differences, will also affect the oscillating frequency. Because a small variance in the oscillating frequency may significantly affect the accuracy of the system time signal over time, individual tuning of the capacitance value tailored to a specific motherboard design is required to obtain accurate system timing signals.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.
Like reference symbols in the various drawings indicate like elements.
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