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AR# 16700

Virtex-II Pro RocketIO - Can the MGT interoperate with an LVDS when they are DC-coupled?

Description

Is it possible to interoperate between the RocketIO transceiver CML driver and receiver and an LVDS driver and receiver if they are not only AC-coupled but DC-coupled?

Solution


Yes, it is possible to use both AC-coupling and DC-coupling when you operate between an LVDS and an MGT. 

To use AC-coupling, the voltage swing of the RocketIO transceiver must be set to be LVDS-compliant.

Additionally, the de-coupling capacitor must be chosen (generally, 10 nF works well).
 
When you DC-couple a RocketIO transceiver and an LVDS transceiver:

  1. The RocketIO transceiver can receive LVDS signals.
    This assumes an Rload of 100 Ohms.
    The 100 Ohms load is available when the RocketIO-receiver termination voltage (VTRX) pin is left floating.
    See the Virtex-II Pro Data Sheet for information on Vih, Vil, differential swing, and common mode voltages.
     
  2. The MGTs can generate LVDS levels.
    This assumes an Rload of 100 Ohms.
    See the Virtex-II Pro Data Sheet for information on Vih, Vil, differential swing, and common mode voltages. 

 

To DC-couple the RocketIO transceiver to an LVDS transceiver, the RX-termination voltage (VTRX) must be left floating, and the TX-termination voltage must be set to 1.8V to produce a common mode voltage of about 1100 mV.

The voltage swing should be set to 500 mV (for typical LVDS levels).
This will ensure valid LVDS sending and receiving levels.
AR# 16700
Date Created 09/03/2007
Last Updated 05/20/2015
Status Active
Type General Article
Devices
  • Virtex-II Pro
  • Virtex-II Pro X