AN2127 Freescale Semiconductor / Motorola, AN2127 Datasheet - Page 13

• Manufacturer Part Number: AN2127
• Description: EMC Guidelines for MPC Based Automotive Powertrain Systems
• Manufacturer: Freescale Semiconductor / Motorola

3.6 Oscillator Circuit
Via
The use of many vias for power and ground connections is stressed throughout this document. Howev-
er, care must be taken to avoid degradation of the power and ground planes by too many via anti-pads
obstructing the flow of current in the plane (via anti-pads are the absence of metal surrounding a via).
A general rule of thumb is to not allow three via anti-pads to merge (see
The oscillator circuit consists of a bias resistor, the crystal, and two capacitors. The oscillator circuit pro-
vides a reference clock signal to the on-chip PLL. Use the lowest frequency crystal possible and set the
multiplication factor bits to obtain the proper system operating frequency which is generated from the
PLL. The oscillator circuit has currents flowing at the crystal’s fundamental frequency. Also, if the oscil-
lator is clipped, then higher order harmonics will be present as well. In order to minimize the amount of
emissions generated from these currents, the oscillator circuit should be kept as compact as possible
(see
can flow easily between V
Figure
EMC Guidelines for MPC500-Based Automotive Powertrain Systems
Good
10). Also, V
Via Anti-Pad
Components
Freescale Semiconductor, Inc.
SSSYN
SSSYN
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should be connected directly to the ground plane so that return currents
Figure 8 Layer Stack-Up Example
Rev. 0, 15 July 2001
and the two capacitors (C
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Acceptable
Figure 9 Via Example
X
and C
Y
).
Figure
Bad
Ground
Signal
Power
Signal
9).
MOTOROLA
13