AR# 16700: Virtex-II Pro RocketIO - Can the MGT interoperate with an LVDS when they are DC-coupled?
Virtex-II Pro RocketIO - Can the MGT interoperate with an LVDS when they are DC-coupled?
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?
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:
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.
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.