AMD recommends the clock to be sourced from an oscillator and brought in from a pin directly to the SEM controller. While the likelihood of an SEU event hitting the configuration cells associated with creating the clock internally from a PLL or mixed-mode clock manager (MMCM) is very small, it is best to strive for the highest reliability possible. However, if a PLL or MMCM output or other logic is used to generate the clock, ensure the clock never violates the SEM controller minimum period at any time, including during design start-up or prior to PLL/MMCM lock.
When clock management is used, suppress the clock toggling to the SEM controller until after the clock is stable. For example use a BUFGMUX or BUFGCE to keep the SEM controller clock from toggling until PLL/MMCM lock is achieved.
If an unstable clock is provided to the IP, the SEM controller might behave in the following method:
- Boot and initialization sequences not consistently completed
- IP completes initialization but reports fatal error soon after entering observation