126.96.36.199.4. Byte Serializer Serialize x4 Mode
The serialize x4 mode is used in high-speed applications such as the PCIe Gen3 protocol mode, where the FPGA fabric cannot operate as fast as the TX PCS.
In serialize x4 mode, the byte serializer serializes 32-bit data into 8-bit data. As the parallel data width from the TX FIFO is divided four times, the clock rate is quadrupled.
After byte serialization, the byte serializer forwards the least significant word first followed by the most significant word. For example, if the FPGA fabric-to-PCS Interface width is 32, the byte serializer forwards tx_parallel_data[7:0] first, followed by tx_parallel_data[15:8], tx_parallel_data[23:16] and tx_parallel_data[31:24].
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