MC12430

The MC12430 features a fully integrated onboard crystal oscillator to minimize system implementation costs. The oscillator is a series resonant, multivibrator type design as opposed to the more common parallel resonant oscillator design. The series resonant design provides better stability and eliminates the need for large on chip capacitors. The oscillator is totally self contained so that the only external component required is the crystal. As the oscillator is somewhat sensitive to loading on its inputs the user is advised to mount the crystal as close to the MC12430 as possible to avoid any board level parasitics. To facilitate colocation surface mount crystals are recommended, but not required.

The oscillator circuit is a series resonant circuit and thus for optimum performance a series resonant crystal should be
used. Unfortunately, most crystals are characterized in a parallel resonant mode. Fortunately, there is no physical difference between a series resonant and a parallel resonant crystal. The difference is purely in the way the devices are characterized. As a result, a parallel resonant crystal can be used with the MC12430 with only a minor error in the desired frequency. A parallel resonant mode crystal used in a series resonant circuit will exhibit a frequency of oscillation a few hundred ppm lower than specified, a few hundred ppm translates to kHz inaccuracies. In a general computer application, this level of inaccuracy is immaterial. Table 1 below specifies the performance requirements of the crystals to be used with the MC12430.

The MC12430 is a general purpose synthesized clock source. MC12430's internal VCO will operate over a range of frequencies from 400 to 800 MHz. The differential PECL output can be configured to be the VCO frequency divided by 1, 2, 4 or 8. With the output configured to divide the VCO frequency by 2,and with a 16.000 MHz external quartz crystal used to provide the reference frequency, the output frequency can be specified in 1 MHz steps. The PLL loop filter of MC12430 is fully integrated so that no external components are required.The synthesizer output frequency is configured using a parallel or serial interface.