2.6.4. Assigning I/O Registers
For the Stratix series, Cyclone series, and the MAX II device families, the Precision Synthesis software can move an internal register to an I/O register without any restrictions on design hierarchy.
For more mature devices, the Precision Synthesis software can move an internal register to an I/O register only when the register exists in the top-level of the hierarchy. If the register is buried in the hierarchy, you must flatten the hierarchy so that the buried registers are moved to the top-level of the design.
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