MPC991

The MPC990 features an onboard crystal oscillator to allow for seed clock generation as well as final distribution.

The onboard oscillator is completely 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 MPC990 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 as opposed to the more common parallel resonant circuit, this eliminates the need for large onboard capacitors. Because the design is a series resonant design for the optimum frequency accuracy a series resonant crystal should be used (see specification table below). Unfortunately most of the shelf crystals are characterized in a parallel resonant mode.

However a parallel resonant crystal is physically no different than a series resonant crystal, a parallel resonant crystal is simply a crystal which has been characterized in its parallel resonant mode. Therefore in the majority of cases a parallel specified crystal can be used with the MPC990 with just a minor frequency error due to the actual series resonant frequency of the parallel resonant specified crystal.

Typically a parallel specified crystal used in a series resonant mode will exhibit an oscillatory frequency a few hundred ppm lower than the specified value. For most processor implementations a few hundred ppm translates into kHz inaccuracies, a level which does not represent a major issue.

The MPC990 is a clock driver which was designed to generate outputs with programmable frequency relationships and not a synthesizer with a fixed input frequency. As a result the crystal input frequency is a function of the desired output frequency. For a design which utilizes the external feedback to the PLL the selection of the crystal frequency is straight forward; simply chose a crystal which is equal in frequency to the fed back signal.