AN2154| Application Note
Maxim/Dallas > App Notes > MICROCONTROLLERS Keywords: crystal, ceramic, resonator, oscillator, rc oscillator, silicon oscillator, microcontroller clock, micros, microcontrollers, ceramic resonators
Jul 02, 2003
APPLICATION NOTE 2154
Microcontroller Clock--Crystal, Resonator, RC Oscillator, or Silicon Oscillator?
Crystals, ceramic resonators, RC (resistor, capacitor) oscillators, and silicon oscillators are four types of clock sources for use with a microcontroller (uC). The optimal clock source for an application depends on many factors including cost, accuracy, and environmental parameters. This application note discusses the determining factors for selecting a microcontroller clock. Oscillator types are compared. Also see: Microcontroller Clock Support Solutions
Introduction
The majority of clock sources for microcontrollers can be grouped into two types: those based on mechanical resonant devices, such as crystals and ceramic resonators, and those based on electrical phase-shift circuits such as RC (resistor, capacitor) oscillators. Silicon oscillators are typically a fully integrated version of the RC oscillator with the added benefits of current sources, matched resistors and capacitors, and temperature-compensation circuits for increased stability. Two examples of clock sources are illustrated in Figure 1. Figure 1a shows a Pierce oscillator configuration suitable for use with mechanical resonant devices like crystals and ceramic resonators, while Figure 1b shows a simple RC feedback oscillator.
Figure 1. Examples of a simple clock source: (a) a Pierce oscillator configuration and (b) an RC feedback oscillator.
Primary Differences Between Mechanical Resonators and RC Oscillators
Crystal and ceramic resonator-based oscillators (mechanical) typically provide very high initial accuracy and a moderately low temperature coefficient. RC oscillators, in contrast, provide fast startup and low cost, but generally suffer from poor accuracy over temperature and supply voltage, and show variations from 5% to 50% of nominal output frequency. While the circuits illustrated in Figure 1 can produce clean reliable clock signals, their performance will be heavily influenced by environmental conditions, circuit component choice, and the layout of the oscillator circuit. Ceramic resonators and their associated load capacitance values must be
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