Cyclone® V Device Handbook: Volume 1: Device Interfaces and Integration

ID 683375
Date 10/18/2023
Document Table of Contents Data Realignment Block (Bit Slip)

Skew in the transmitted data along with skew added by the link causes channel-to-channel skew on the received serial data streams. To compensate for this channel-to-channel skew and establish the correct received word boundary at each channel, each receiver channel has a dedicated data realignment circuit that realigns the data by inserting bit latencies into the serial stream.

An optional RX_CHANNEL_DATA_ALIGN port controls the bit insertion of each receiver independently controlled from the internal logic. The data slips one bit on the rising edge of RX_CHANNEL_DATA_ALIGN. The requirements for the RX_CHANNEL_DATA_ALIGN signal include the following items:

  • The minimum pulse width is one period of the parallel clock in the logic array.
  • The minimum low time between pulses is one period of the parallel clock.
  • The signal is an edge-triggered signal.
  • The valid data is available two parallel clock cycles after the rising edge of RX_CHANNEL_DATA_ALIGN.
Figure 110. Data Realignment TimingThis figure shows receiver output (RX_OUT) after one bit slip pulse with the deserialization factor set to 4.

The data realignment circuit can have up to 11 bit-times of insertion before a rollover occurs. The programmable bit rollover point can be from 1 to 11 bit-times, independent of the deserialization factor. Set the programmable bit rollover point equal to, or greater than, the deserialization factor—allowing enough depth in the word alignment circuit to slip through a full word. You can set the value of the bit rollover point using the IP Catalog. An optional status port, RX_CDA_MAX, is available to the FPGA fabric from each channel to indicate the reaching of the preset rollover point.

Figure 111. Receiver Data Realignment RolloverThis figure shows a preset value of four bit-times before rollover occurs. The rx_cda_max signal pulses for one rx_outclock cycle to indicate that rollover has occurred.