18.104.22.168. Clocking Differential Receivers
The I/O PLL receives the external clock input and generates different phases of the same clock. The DPA block automatically chooses one of the clocks from the I/O PLL and aligns the incoming data on each channel.
The synchronizer circuit is a 1-bit wide by 6-bit deep FIFO buffer that compensates for any phase difference between the DPA clock and the data realignment block. If necessary, the user-controlled data realignment circuitry inserts a single bit of latency in the serial bit stream to align to the word boundary. The deserializer includes shift registers and parallel load registers, and sends a maximum of 10 bits to the internal logic.
The physical medium connecting the transmitter and receiver LVDS channels may introduce skew between the serial data and the source-synchronous clock. The instantaneous skew between each LVDS channel and the clock also varies with the jitter on the data and clock signals as seen by the receiver. The three different modes—non-DPA, DPA, and soft-CDR—provide different options to overcome skew between the source synchronous clock (non-DPA, DPA) /reference clock (soft-CDR) and the serial data.
Non-DPA mode allows you to statically select the optimal phase between the source synchronous clock and the received serial data to compensate skew. In DPA mode, the DPA circuitry automatically chooses the best phase to compensate for the skew between the source synchronous clock and the received serial data. Soft-CDR mode provides opportunities for synchronous and asynchronous applications for chip-to-chip and short reach board-to-board applications for SGMII protocols.