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The vast majority of electronic systems operate at some fixed frequency. | The vast majority of electronic systems operate at some fixed frequency. | ||
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Usually the component that fixes that frequency is a quartz crystal, sealed in a metal can. | Usually the component that fixes that frequency is a quartz crystal, sealed in a metal can. | ||
| − | (Some very-low-cost devices might use a "ceramic resonator" (made of high-stability piezoelectric ceramics, generally lead zirconium titanate) or a resistor and a capacitor to fix the frequency | + | (Some very-low-cost devices might use a "ceramic resonator" (made of high-stability piezoelectric ceramics, generally lead zirconium titanate)or a resistor and a capacitor to fix the frequency). |
| − | The entire circuit that generates the frequency is called an "oscillator", includes the resonant part (crystal, resonator, or RC), some capacitors, and a silicon chip and is | + | The entire circuit that generates the frequency is called an "oscillator", includes the resonant part (crystal, resonator, or RC), some capacitors, and a silicon chip and therefore is called a hybrid device. |
(An oscillator that uses a resistor and a capacitor to fix the frequency is called a "RC oscillator". | (An oscillator that uses a resistor and a capacitor to fix the frequency is called a "RC oscillator". | ||
An oscillator that uses a crystal to fix the frequency is called a "crystal oscillator".) | An oscillator that uses a crystal to fix the frequency is called a "crystal oscillator".) | ||
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Oscillators usually come in a metal can, but Epson also encapsulates them in plastic. Typically an oscillator can has 4 pins. Inside the can is all the components of the oscillator. One applies DC power on 2 of the pins, and the oscillating signal (the "CLK OUT") appears on another pin. (The remaining pin is unused). | Oscillators usually come in a metal can, but Epson also encapsulates them in plastic. Typically an oscillator can has 4 pins. Inside the can is all the components of the oscillator. One applies DC power on 2 of the pins, and the oscillating signal (the "CLK OUT") appears on another pin. (The remaining pin is unused). | ||
| − | An oscillator can also be made from scratch using crystal in a (2-pin) metal can, a couple of capacitors, | + | An oscillator can also be made from scratch using crystal in a (2-pin) metal can, a couple of capacitors, an resitor and an inverter. |
In either case, the frequency is printed on the top of the crystal or oscillator. | In either case, the frequency is printed on the top of the crystal or oscillator. | ||
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that are designed to be directly connected to the 2 pins of a crystal. | that are designed to be directly connected to the 2 pins of a crystal. | ||
(Capacitors from those pins to VCC and GND are also part of the recommended circuit). | (Capacitors from those pins to VCC and GND are also part of the recommended circuit). | ||
| − | An inverter inside the microcontroller acts as the amplifier, and the crystal and capacitors make up the rest of the oscillator | + | An inverter inside the microcontroller acts as the amplifier, and the crystal and capacitors make up the rest of the oscillator. |
In systems with multiple CPUs, it is often simpler, cheaper, and more reliable (avoiding metastability problems) to use a single crystal (rather than a dedicated crystal for each CPU). | In systems with multiple CPUs, it is often simpler, cheaper, and more reliable (avoiding metastability problems) to use a single crystal (rather than a dedicated crystal for each CPU). | ||
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Unfortunately, many people confuse the "clock signal" generated by an oscillator (a simple metronome beat, tone, at constant frequency) with far more complicated "clock system"s that keep track of seconds, minutes, hours, and sometimes days, weeks, months, and years. | Unfortunately, many people confuse the "clock signal" generated by an oscillator (a simple metronome beat, tone, at constant frequency) with far more complicated "clock system"s that keep track of seconds, minutes, hours, and sometimes days, weeks, months, and years. | ||
| − | + | == for further reading == | |
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* [http://techref.massmind.org/techref/clocks.htm massmind: clocks] | * [http://techref.massmind.org/techref/clocks.htm massmind: clocks] | ||
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* [http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en021190 "App Note AN949: Making Your Oscillator Work" by Brett Duane] | * [http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en021190 "App Note AN949: Making Your Oscillator Work" by Brett Duane] | ||
* [http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en020706 "App Note AN943: Practical PICmicro® Oscillator Analysis and Design" by Ruan Lourens] | * [http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1824&appnote=en020706 "App Note AN943: Practical PICmicro® Oscillator Analysis and Design" by Ruan Lourens] | ||
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[[Category:Components]] | [[Category:Components]] | ||
