Cyclone V Device Handbook: Volume 2: Transceivers

ID 683586
Date 10/24/2018
Public
Document Table of Contents

1.2.3.2.3. CDR PLL in Automatic Lock Mode

In automatic lock mode, the LTR/LTD controller directs the transition between the LTR and LTD modes when a set of conditions are met to ensure proper CDR PLL operation. The mode transitions are indicated by the rx_is_lockedtodata status signal.

After power-up or reset of the receiver PMA, the CDR PLL is directed into LTR mode. The controller transitions the CDR PLL from LTR to LTD mode when all the following conditions are met:

  • The frequency of the CDR PLL output clock and input reference clock is within the configured ppm frequency threshold setting
  • The phase of the CDR PLL output clock and input reference clock is within approximately 0.08 unit interval (UI) of difference
  • In PCIe configurations only—the signal detect circuitry must also detect the presence of the signal level at the receiver input above the threshold voltage specified in the PCI Express Base Specification 2.0. (Signal detect is an optional signal in Custom or Native PHY IP. Use the Assignment Editor to select the threshold voltage.)

The controller transitions the CDR PLL from LTD to LTR mode when either of the following conditions are met:

  • The difference in between frequency of the CDR PLL output clock and input reference clock exceeds the configured ppm frequency threshold setting
  • In PCIe configurations only—the signal detect circuitry detects the signal level at the receiver input below the threshold voltage specified in the PCI Express Base Specification 2.0

After switching to LTD mode, the rx_is_lockedtodata status signal is asserted. Lock to data takes a minimum of 4 μs, however the actual lock time depends on the transition density of the incoming data and the parts per million (PPM) difference between the receiver input reference clock and the upstream transmitter reference clock. The receiver PCS logic must be held in reset until the CDR produces a stable recovered clock.

If there is no transition on the incoming serial data for an extended duration, the CDR output clock may drift to a frequency exceeding the configured PPM threshold when compared with the input reference clock. In such a case, the LTR/LTD controller transitions the CDR PLL from LTD to LTR mode.