MC912DG128ACPVE Datasheet, PDF (196/478 Pages) Freescale Semiconductor, Inc – Upward compatible with M68HC11 instruction set

Oscillator
Freescale Semiconductor, Inc.
13.3.2 MC68HC912DT128A Oscillator Design Guidelines
Proper and robust operation of the oscillator circuit requires excellent
board layout design practice. Poor layout of the application board can
contribute to EMC susceptibility, noise generation, slow starting
oscillators, and reaction to noise on the clock input buffer. In addition to
published errata for the MC68HC912DT128A, the following guidelines
must be followed or failure in operation may occur.
• Minimize Capacitance to VSS on EXTAL pin — The Colpitts
oscillator architecture is sensitive to capacitance in parallel with
the resonator (from EXTAL to VSS). Follow these techniques:
i. Remove ground plane from all layers around resonator
and EXTAL route
ii. Observe a minimum spacing from the EXTAL trace to
all other traces of at least three times the design rule
minimum (until the microcontroller’s pin pitch prohibits
this guideline)
iii. Where possible, use XTAL as a shield between EXTAL
and VSS
iv. Keep EXTAL capacitance to less than 1pF (2pF
absolute maximum)
• Shield all oscillator components from all noisy traces (while
observing above guideline).
• Keep the VSSPLL pin and the VSS reference to the oscillator
as identical as possible. Impedance between these signals must
be minimum.
• Observe best practice supply bypassing on all MCU power
pins. The oscillator’s supply reference is VDD, not VDPLL.
• Account for XTAL–VSS and EXTAL–XTAL parasitics in
component values.
NOTE:
An increase in the EXTAL–XTAL parasitic as a result of reducing
EXTAL–VSS parasitic is acceptable provided component value is
reduced by the appropriate value.
• Minimize XTAL and EXTAL routing lengths to reduce EMC
issues.
Technical Data
196
MC68HC912DT128A — Rev 4.0
Oscillator
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