Crystal Oscillator and PLL Considerations for AT91M42800A and AT91M55800A Ap Note

Crystal oscillators use an external crystal to generate clock signals for designs that require high stability at a precise frequency. Phase-locked-loops are used to multiply the crystal output frequency to higher values. This application note describes the use of Crystal Oscillator and PLLs in the context of the AT91M42800A and AT91M55800A ARM-based microcontrollers. The ZIP file " Automatic_Calculation_xls.zip " should be downloaded from the Software section of the AT91 pages on Atmel's web site and unzipped. It gives a spreadsheet that is programmed to perform the calculations described in this application note. The mechanical impedance of a crystal for one particular series resonant frequency k (the fundamental and its overtones) can be electrically modelled by a series RLC resonant circuit. See Figure 1. The parasitic capacitance of the crystal's electrodes and the package are modelled by the capacitor C 0. Together with the motional inductance Lx(k) this capacitor forms resonant circuits that also produce parallel resonant frequencies fp(k). They cannot be used for oscillation with the type of oscillators presented here.