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