AN2727 Freescale Semiconductor / Motorola, AN2727 Datasheet - Page 16

• Manufacturer Part Number: AN2727
• Description: Designing Hardware for the HCS12 D-Family
• Manufacturer: Freescale Semiconductor / Motorola

Oscillator Options
Start-up time and margin are typical values, measured on carefully laid out PCBs. The current values
have been simulated for typical conditions.
The use of a fundamental resonator or crystal is always recommended in preference to an overtone
resonator or crystal.
If Colpitts mode is selected, only fundamental mode crystals or resonators may be used.
If Pierce mode is selected, fundamental mode crystals or resonators may be used. Overtone crystals or
resonators may be used after careful component selection.
It is recommended that the suitability of a crystal or resonator for use in a particular application be
confirmed with the crystal or resonator manufacturer.
The values of the load capacitors are specific to the crystal (and its use with the HCS12 oscillator). Load
capacitor values should be confirmed with the crystal or resonator manufacturer.
Common Oscillator layout considerations
Good practice is important when laying out a PCB for any oscillator configuration and it is a good idea to
lay out the oscillator first in any design.
The PCB layout is equally as critical when using a ceramic resonator as when using a quartz crystal.
Keep the oscillator components on the same side of the PCB as the MCU and as close to the MCU as
possible (allowing for the fan-out of any I/O used on the oscillator side of the MCU).
Keep the oscillator tracks on the same side of the PCB as the oscillator — avoid vias in the oscillator
circuit.
Avoid routing other signals on any layers in the region of the oscillator components or tracks. Place an
exclusion zone on all layers around the oscillator.
Remove sections of ground or power planes under oscillator components and tracks (to minimize
parasitic loading).
Include a 100 nF decoupling capacitor close to the VDDPLL/VSSPLL pins. Avoid vias in the oscillator
signal and VxxPLL supply tracks from the decoupling capacitor.
Treat the ground signal for the oscillator as a separate ground, connecting to the main digital ground at
one point only, close by VSSR. Where the main digital ground is implemented with a ground plane, it is
recommended to keep the oscillator / PLL ground separate from the ground plane.
Routing the oscillator ground via the TEST pin can simplify the layout; this also surrounds the oscillator
circuit with a ground ring, which helps to minimize cross-talk and defines an exclusion zone for non-
oscillator signals, and reduces the oscillator ground impedance. The TEST pin is a static digital input that
will not disturb the oscillator ground.
Keep the adjacent RESET signal clean. Where it is connected off the board or to a long track in a noisy
environment, consider adding some series resistance.
Avoid routing signals under the PLL components or tracks, to minimize cross talk.
Designing Hardware for the HCS12 D-Family, Rev. 0
16
Freescale Semiconductor