2367 - LatticeECP3: How does the RX loss of signal detection circuitry in the LatticeECP3 SERDES/PCS react to a constant value or a drop in signal condition on the SERDES serial inputs?

2367 - LatticeECP3: How does the RX loss of signal detection circuitry in the LatticeECP3 SERDES/PCS react to a constant value or a drop in signal condition on the SERDES serial inputs?

Description:
The following sections describe how the LatticeECP3 SERDES/PCS RX loss of signal (LOS) detection circuitry works.
  • Operation: The RX LOS detection circuitry detects the voltage swing on the HDINP/N pins as shown in the "Loss of Signal" section of FPGA-TN-02190, "LatticeECP3 SERDES/PCS Usage Guide". The rlos_lset register bits set the voltage threshold to compare the voltage swing using a programmable current source technique. The rx_los_low signal triggers when the voltage swing is smaller than the programmed setting. So the higher the rlos_lset value, the larger the voltage swing needs to be to prevent an RX LOS condition. An RX LOS condition means that the rx_los_low_ch[3:0]_s signal goes high.
  • Constant SERDES Inputs: The input to the RX LOS circuitry is AC coupled with a long time constant, which is why a string of a few 0’s or 1's does not cause the LOS condition to occur. In fact, When the input is constant, then the limitation is with the ability of the receiver to lock to the consecutive identical digits CID (Consecutive Identical Digits). In other words, the Received CDR (Clock and Data Recovery) will lose lock and cause the rx_cdr_lol_ch[3:0]_s signal to trigger before the LOS circuit is able to detect a LOS condition. This is why you should use the maximum CID for the receiver as a worst case condition for LOS. For the LatticeECP3, we publish the maximum CID per data rate in the "Serial Input Data Specifications" table of FPGA-DS-02074, "LatticeECP3 Family Data Sheet". You can refer to this table for the worst case CID depending on the data rate you wish to run.
  • Drop in Signal Condition: When you pull the HDINP/N input cables off, or stop driving the inputs, the HDINP/N serial inputs on the channel would come together. The internal termination used will drain the internal capacitance faster and will cause an RX LOS condition immediately.