The PCIe 3.0 base specification specifies a block size of 130 bits, with the exception of the SKP Ordered Sets, which can be of variable length. An implementation of a 130-bit data path takes significant resources, so the PCIe Gen3 PCS data path is implemented as 32-bits wide. Because the TX PMA data width is fixed to 32 bits, and the block size is 130 bits with variations, a gearbox is needed to convert 130 bits to 32 bits.
The gearbox block in the TX PCS converts the 130-bit data ( tx_parallel_data[127:0] + pipe_tx_sync_hdr[1:0]) to 32-bit data required by the TX PMA as the datapath implementation is 32 bits to reduce usage of resources. The 130-bit data is received as follows in the 32-bit datapath: 34 (32 + 2-bit sync header), 32, 32, 32. During the first cycle the gearbox converts the 34-bit input data to 32-bit data. During the next 3 clock cycles the gearbox merges bits from adjacent cycles to form the 32-bit data. In order for the gearbox to work correctly, a gap must be provided in the data for every 16 shifts as each shift is 2 bits for converting the initial 34-bit to 32-bit in the gearbox. After 16 shifts the gearbox has an extra 32-bit data that was transmitted out, and thus a gap is required in the input data stream. This gap is achieved by driving pipe_tx_data_valid low for one cycle after every 16 blocks of input data(tx_parallel_data).