18.104.22.168. DPA Block
The DPA block takes in high-speed serial data from the differential input buffer and selects one of the eight phases that the fractional PLLs generate to sample the data. The DPA chooses a phase closest to the phase of the serial data. The maximum phase offset between the received data and the selected phase is 1/8 UI, which is the maximum quantization error of the DPA. The eight phases of the clock are equally divided, offering a 45° resolution.
The following figure shows the possible phase relationships between the DPA clocks and the incoming serial data.
The DPA block continuously monitors the phase of the incoming serial data and selects a new clock phase if it is required. You can prevent the DPA from selecting a new clock phase by asserting the optional RX_DPLL_HOLD port, which is available for each channel.
DPA circuitry does not require a fixed training pattern to lock to the optimum phase out of the eight phases. After reset or power up, the DPA circuitry requires transitions on the received data to lock to the optimum phase. An optional output port, RX_DPA_LOCKED, is available to indicate an initial DPA lock condition to the optimum phase after power up or reset. This signal is not deasserted if the DPA selects a new phase out of the eight clock phases to sample the received data. Do not use the rx_dpa_locked signal to determine a DPA loss-of-lock condition. Use data checkers such as a cyclic redundancy check (CRC) or diagonal interleaved parity (DIP-4) to validate the data.
An independent reset port, RX_RESET, is available to reset the DPA circuitry. You must retrain the DPA circuitry after reset.
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