MC912DG128ACPVE Datasheet, PDF (219/478 Pages) Freescale Semiconductor, Inc – Upward compatible with M68HC11 instruction set

Freescale Semiconductor, Inc.
Oscillator
MC68HC912Dx128P Pierce Oscillator Specification
13.5.2.3 Optimizing Component Values
The maximum ESR possible (given a worst-case Gain Margin of 2) is not
the optimum operating point. In some cases, the frequency accuracy of
the oscillator is important and in other cases satisfying the traditional
NRM measurement technique is important.
If frequency accuracy is important, the total load capacitance (the
combination of the load capacitors and their associated parasitics)
should be equal (or close to equal) to the rated load capacitance of the
resonator. Provided the resultant load capacitors yield a maximum
allowed ESR greater than the maximum ESR of the crystal (while
maintaining the worst case Gain Margin of 2), this is an acceptable
operating point. If the maximum allowed ESR is not high enough, the
closest possible components with high enough ESR should be chosen.
Similarly, if meeting a traditional NRM optimization criteria is important,
then the components determined by this method are acceptable if the
same components yield a maximum allowed ESR greater than the
maximum ESR of the crystal while maintaining the worst case Gain
Margin of 2. There is no guarantee that components chosen through
traditional NRM optimization techniques will yield acceptable results
across all expected variations.
13.5.2.4 Key Parameters
The following items are of critical importance to the operation of the
oscillator:
• EXTAL–VSS capacitor value — The value of the component
plus external (board) parasitic in excess of 1.0pF between EXTAL
and VSS.
• XTAL–VSS capacitor value — The value of the component plus
external (board) parasitic in excess of 1.0pF between XTAL and
VSS.
• Maximum Shunt Capacitance — The maximum value of the
resonator’s shunt capacitance (C0) plus the external (board)
parasitics in excess of 0.1pF from EXTAL to XTAL.
MC68HC912DT128A — Rev 4.0
MOTOROLA
Oscillator
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Technical Data
219