188.8.131.52. RX Gearbox, RX Bitslip, and Polarity Inversion
The RX gearbox adapts the PMA data width to the larger bus width of the PCS channel (Gearbox Expansion). It supports different ratios (PCS-PMA interface width : FPGA fabric–PCS interface width) such as 32:66, 40:66, 32:67, 32:64, 40:40, 32:32, 64:64, 67:64, and 66:64 and a bit slipping feature.
RX bitslip is engaged when the RX block synchronizer or rx_bitslip is enabled to shift the word boundary. On the rising edge of the bitslip signal of the RX block synchronizer or rx_bitslip from the FPGA fabric, the word boundary is shifted by one serial bit or 1UI. Each bit slip removes the earliest received bit from the received data.
The receiver gearbox can invert the polarity of the incoming data. This is useful if the receiver signals are reversed on the board or backplane layout. Enable polarity inversion through the Native PHY IP Parameter Editor.
Data valid generation logic is essential for gearbox operation. Each block of data is accompanied by rx_enh_data_valid data valid signal which “qualifies” the block as valid or not. The data valid toggling pattern is dependent on the data width conversion ratio. For example, if the ratio is 66:40, the data valid signal is high in 20 out of 33 cycles or approximately 2 out of 3 cycles and the pattern repeats every 33 rx_clkout RX low-speed parallel clock cycles.
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