18.104.22.168. Soft-CDR Mode
The Stratix® V LVDS channel offers the soft-CDR mode to support the GbE and SGMII protocols. A receiver PLL uses the local clock source for reference.
The following figure shows the soft-CDR mode datapath. In SDR and DDR modes, the data width from the IOE is 1 and 2 bits, respectively.
In soft-CDR mode, the synchronizer block is inactive. The DPA circuitry selects an optimal DPA clock phase to sample the data. Use the selected DPA clock for bit-slip operation and deserialization. The DPA block also forwards the selected DPA clock, divided by the deserialization factor called rx_divfwdclk, to the FPGA fabric, along with the deserialized data. This clock signal is put on the periphery clock (PCLK) network.
If you use the soft-CDR mode, do not assert the rx_reset port after the DPA has trained. The DPA continuously chooses new phase taps from the PLL to track parts per million (PPM) differences between the reference clock and incoming data.
You can use every LVDS channel in soft-CDR mode and drive the FPGA fabric using the PCLK network in the Stratix® V device family. The rx_dpa_locked signal is not valid in soft-CDR mode because the DPA continuously changes its phase to track PPM differences between the upstream transmitter and the local receiver input reference clocks. The parallel clock, rx_outclock, generated by the left and right PLLs, is also forwarded to the FPGA fabric.
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