18.104.22.168. Transmitter Gearbox
The transmitter gearbox adapts the PCS data width to a smaller bus width for interfacing with the PMA. Because of the transmitter gearbox, the difference in the bus widths between the PCS and the PMA is transparent to the logic in the FPGA fabric.
In addition to providing bus width adaptation, the transmitter gearbox provides the transmitter polarity inversion, bit reversal, and bit-slip features.
Transmitter Polarity Inversion
Transmitter polarity can be used to reverse the positive and negative differential buffer signals. This is useful if these signals are reversed on the board or backplane layout.
A high value on the tx_invpolarity register, which is accessed via the Avalon-MM PHY management interface, inverts the polarity of every bit of the input data word to the serializer in the transmitter datapath. Because inverting the polarity of each bit has the same effect as swapping the positive and negative signals of the differential link, correct data is sent to the receiver. Dynamically changing the tx_invpolarity register value might cause initial disparity errors at the receiver of an 8B/10B encoded link. The downstream system must be able to tolerate these disparity errors.
If polarity inversion is asserted midway through a serializer word, the word will be corrupted.
Transmitter Bit Reversal
The transmitter gearbox can reverse the order of transmitted bits. By default, the transmitter sends out the LSB of a word first. Some protocols, such as Interlaken, require that the MSB of a word (bit 66 in a word [66:0]) is transmitted first. When you enable the transmitter bit reversal, the parallel input to the gearbox is swapped and the MSB is sent out first. The Quartus II software automatically sets the bit reversal for Interlaken configurations.
The transmitter bit-slip allows you to compensate for the channel-to-channel skew between multiple transmitter channels by slipping the data sent to the PMA. The maximum number of bits slipped is controlled from the FPGA fabric and is equal to the width of the PMA-PCS interface, minus one.
The transmitter bit-slip is not supported for all PHYs. Low latency PHY does not allow this feature.
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