188.8.131.52. Synchronous Resets on I/O Ports
The Compiler does not retime registers driving an output port, or registers that an input port drives. If such an I/O register has a synchronous clear, you cannot retime the register. This restriction is not typical of practical designs that contain logic driving resets. However, this issue may arise in benchmarking a smaller piece of logic, where the reset originates from an I/O port. In this case, you cannot retime any of the registers that the reset drives. Adding some registers to the synchronous reset path corrects this condition.
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