TMS320F28075: what's the requirement for external crystal oscillator?

Yuan,

                A heat gun is not a accurate way of doing temperature testing. How do you know the ambient temperature around the crystal is exactly 100'C? The correct way of doing temperature testing is to put the PCB inside a temperature chamber, whose temperature can be precisely controlled. Or use a machine like https://www.intestthermal.com/temptronic/thermostream that provides an accurate thermal environment. You need to find the operating temperature range of the crystal from its datasheet and then ensure the temperature during testing does not exceed that temperature. Our T & S parts are rated for 125'C operation (junction and free-air respectively). 

By "active crystal oscillator", I presume you are referring to a "can" oscillator i.e. one that has its own supply and GND pin. In such an oscillator, the crystal is protected by the casing and may not be subject to the high ambient temperature. 

This is likely a crystal issue, not our device issue.