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In my Microcontrollers class today, our professor talked about how for the microcontrollers we use in lab, the clock is actually a "crystal" (either his command of English or of chemistry prevented him from talking more about it). He discussed how watches use a "crystal" to keep time (I know the crystal to be quartz).

This led to a discussion in lab about what material the Intel line of microprocessors use. A few of the questions that came up were, is the clock built into the motherboard like our microcontroller, or is it built into the microprocessor? What material is it made of? I've been looking at the the technical specifications, but I haven't found anything about this yet.

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There will likely be a standard quartz resonator of some sort on the motherboard which will feed the CPU a reference frequency. Inside the CPU, there is a device called a Phase Locked Loop (PLL) which is used to lock the output of a voltage controlled oscillator (VCO) into a well-defined relationship with the reference clock. The output of the VCO will drive the clock distribution network on the chip.

A quartz resonator is built from a small piece of quartz with electrodes deposited onto it along with an amplifier. The crystal is cut to vibrate at a very specific frequency. An applied voltage across the crystal causes it to deform via the piezoelectric effect. Only signals at the correct frequency to make the crystal resonate will be passed through, making the crysal act like a very narrow bandpass filter. The amplifier serves to amplify these oscillations to keep the crystal vibrating, as well as driving the signal out of the resonator.

The VCO is simply a circuit that is designed to oscillate so that the frequency can be adjusted with an analog control voltage. There are many ways to build a VCO depending on the process and frequency range. One simple method is to simply vary the power supply voltage on a ring oscillator. This will change the propagation delay of the inverters used to construct it, changing the frequency of oscillation. Other methods involve utilizing the voltage variable capacitance of one or more varactor diodes.

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