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Issue about the MC9S12XEP100 CLOCK Circuit

Question asked by xiaochen qiang on Nov 26, 2017
Latest reply on Dec 20, 2017 by xiaochen qiang

Hi all:

     My issue is about the MCU clock circuit,,below is the description of the issue:

===========================Issue Description=============================

My product 's MCU is MC9S12XEP100,MCU circuit and the product can work nomaly,but some products(About 2% of the total, so far) occur the phenomenon of abnormal work after about two years used,The specific phenomenon is:the MCU's PLL can not stable,or occur crashes after run some time.

The MCU's clock circuit design as below:

the picture above is the Crystal circuit,

and the XCLKS pin not connect the pull-down,it is suspended;

the MODA and MODB connect the pull-down;

===========================Troubleshoot=============================

In troubleshooting the process described above, we found that there are two obvious deviations in the design:

1.The Crystal  output the 5V  square wave,Obviously exceed EXTAL maximum 2.16V input requirement;

2.XCLKS pin not pulled down;

Then make the following changes to the clock circuit of the MCU on the circuit board of the failed product (the MCU's PLL can not be locked occasionally):

 

At the same time the XCLKS foot drop pull, MODA, MODB and before, pick up the pull-down;

After the above circuit is modified, the MCU still can not lock the PLL; when the crystal oscillator outputs 5V square wave, the PLL can lock occasionally. After the above modification, basically no PLL lock occurs.

=============================Issue advisory===================================

The above is the specific phenomenon of the problem, then my question is:

1. I suspect the crystal oscillator directly outputs 5V square wave to the EXTAL pin, causing the internal clock module of the MCU to malfunction. As a result, the PLL can not be locked, but I just wonder how to determine that the 5V square wave has caused the MCU internal Clock module failure?

2. Why do the above MCUs have a normal working time of nearly 2 years under the condition that there are 2 parts that do not meet the requirements?

     Thanks!

 

 

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