When the ETH_RX_DATA register is accessed on the CPU interface, the CPRI MegaCore® function waits until a frame of Ethernet data is received on the Fast C & M interface. If you have not setup the Fast C & M interface or the link partner is not transmitting, then the CPU interface waits indefinitely and the cpu_waitrequest signal remains asserted.
Altera® recommends that you read the CPRI_CM_STATUS register to ensure that an RX Fast C & M channel has been initialized as indicated by the rx_fast_dm_ptr_valid bit prior to accessing the Ethernet Registers.