188.8.131.52.2. RX Multi-lane FIFO Deskew State Machine
Add deskew logic at the receiver side to eliminate the lane-to-lane skew created at the transmitter of the link partner, PCB, medium, and local receiver PMA.
Implement a multi-lane alignment deskew state machine to control the RX FIFO operation based on available RX FIFO status flags and control signals.
Each lane's rx_enh_fifo_rd_en should remain deasserted before the RX FIFO deskew is completed. After frame lock is achieved (indicated by the assertion of rx_enh_frame_lock; this signal is not shown in the above state flow), data is written into the RX FIFO after the first alignment word (SYNC word) is found on that channel. Accordingly, the RX FIFO partially empty flag (rx_enh_fifo_pempty) of that channel is asserted. The state machine monitors the rx_enh_fifo_pempty and rx_enh_fifo_pfull signals of all channels. If the rx_enh_fifo_pempty signals from all channels deassert before any channels rx_enh_fifo_pfull assert, which implies the SYNC word has been found on all lanes of the link, the MAC layer can start reading from all the RX FIFO by asserting rx_enh_fifo_rd_en simultaneously. Otherwise, if the rx_enh_fifo_pfull signal of any channel asserts high before the rx_enh_fifo_pempty signals deassertion on all channels, the state machine needs to flush the RX FIFO by asserting rx_enh_fifo_align_clr high for 4 cycles and repeating the soft deskew process.
The following figure shows one RX deskew scenario. In this scenario, all of the RX FIFO partially empty lanes are deasserted while the pfull lanes are still deasserted. This indicates the deskew is successful and the FPGA fabric starts reading data from the RX FIFO.