MPC8315 with XTAL

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MPC8315 with XTAL

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Contributor I

We are experiencing random hardware watchdogs on boards with MPC8315 which clock is obtained through a 33Mhz Xtal. We have build thousands of boards, and this happens randomly in a few of them. If we change the clock to an oscillator, the problem disappears. Are there any considerations that should be taken to use a XTAL for this micro?

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NXP TechSupport
NXP TechSupport

Yes, if you are using crystal oscillator, this circuit is very sensitive to noise.

All considerations are about proper board layout:

1. Place power supply bypass capacitors as close to processor pins as possible. A good approach is to place bypass capacitors directly under processor footprint, at the opposite side of board, with vias embedded in capacitor pads to minimize trace length.

For more detailed description and example figures you can look application note AN3065 "Power Supply Design Guidelines for the MPC8641 and MPC8641D", Section 4.1

http://cache.nxp.com/assets/documents/data/en/application-notes/AN3065.pdf

2. Placing crystal oscillator as close as possible to processor, as it usually recommended for microcontrollers - is not absolutely required. But, two traces for XTAL and EXTAL pins must be routed as close one to another, as possible. This should be done at whole trace length from processor to crystal.

3. Routing for XTAL and EXTAL must be done on layer adjacent to ground layer. Ground layer should be solid, with no cutouts.

A good example of crystal oscillator routing is done on our MPC8315DS board (note - this board is different from MPC8315RDB). I do not see this MPC8315DS board on our public web now, but still - I think you can request a full board collateral, including board layout source, if you contact your local distributor and sign NDA:

Distributor Network|NXP 


Have a great day,
Alexander

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