880 - The LatticeXP2 device does not have a sub-LVDS output type, can I still drive sub-LVDS inputs with the LatticeXP2? <BR>

880 - The LatticeXP2 device does not have a sub-LVDS output type, can I still drive sub-LVDS inputs with the LatticeXP2? <BR>

Situation:
An application uses the LatticeXP2 device and needs the sub-LVDS output type.

Solution:
Given that sub-LVDS signaling requires Vod from 100mv (min) to 200mv (max) and Vcm from +0.75v (min) to +1.05v (max), the sub-LVDS output type can be emulated using the RSDS output type by changing the network values shown for the RSDS output type in the LatticeXP2 datasheet, as follows:

Rs = 274 ohms
VCCIO = 1.8v

The Rp=121 and RT=100 ohm values remain unchanged as shown in the LatticeXP2 datasheet. This will give Vcm=+0.9v and Vod=150mv at the sub-LVDS receiver. You can also change the Rs value lower to get more Vod signal, or change Rs to higher to get less Vod signal. If you change the Rs value, the Rp value should be recalculated as:

Rp = (Rs*100 + 3300)/(Rs - 17)

The Vcm value is by default, 1/2 the VCCIO voltage. The Rp and Rs resistors should be placed as close as possible to the LatticeXP2 output pins and should be either 0402 body size or surface mount resistor packs with minimal stub length traces to

the resistors.


If you need the lowest common mode output noise, you will get best performance when the original Rp resistor value has been split into 2 resistors of value 1/2 Rp each with their center connection to a localized floating, or 0.9v VTT, plane island that is itself bypassed to the GND plane.

You can read more about Sub-LVDS Signaling Using Lattice Devices here:

http://www.latticesemi.com/documents/TN1210.pdf