Hi,
Please be note that the MC68376 data sheet for the 20MHz version also applies to the 25MHz version. The only difference is the 25MHz passed production tests at a higher clock speed.
For more clarify, MC68376 20MHz devices are qualified at this freq. The 20MHz have parameterized and guaranteed to work up to this freq. If you are using the 20MHz version exceed 20MHz we can not guarantee proper operation. Actually there is no difference between the 20MHz and 25MHz devices, except the 25MHz have been qualified at this higher freq. The 20MHz devices have only been qualified up to 20MHz, and will probably not work at 25MHz. If you need to use the 25MHz devices working at 20MHz, you will only need to change the freq. of the device in software and maybe the external crystal/ext. osc.
I don't have the complete testing process but in a nutshell the devices are tested over the entire temp. rating of the device. There are no specific reasons why a device would be 20MHz and not 25MHz other than not being able to meet all of the timing requirements over the entire operating environment. They could fail due to things like bus timing, PLL, ADC, etc.
As stated above there are no specific failure modes. If they don't meet any of the electrical specifications for the 25MHz they will be tested at 20MHz. Generally the normal testing procedure would start at 25MHz down to 20MHz. The 25 MHz failures are then tested at 20MHz.
Best Regards,
Fang