Atmel SpW-10X SpaceWire Input Fail Safe Resistors, SpaceWire Router, User Manual, Preliminary

SpW-10X	Ref.:	UoD_SpW-10X_
SpaceWire Router		UserManual
	Issue:	3.4
User Manual
	Date:	11th July 2008

5.2.2 SpaceWire Input Fail Safe Resistors

If a SpaceWire input becomes disconnected then no current flows through the termination resistor. The differential voltage across this resistor is then zero. A small noise current, induced by electro- magnetic interference on PCB tracks or on any part of the SpaceWire cable still attached to the receiver, will cause a small differential voltage across the termination resistor. If this is positive the receiver output will be logic 1 and if it the noise current flows in the other direction, giving a negative differential voltage the receiver output will be logic 0. Just a small amount of noise is sufficient to cause the output of the LVDS receiver to transition from 0 to 1 and back continuously. This noise can sometime start a SpaceWire link erroneously.

To overcome this problem a small bias current can be passed through the termination resistor generating a small positive voltage across the bias resistor and forcing the output of the LVDS receiver to logic 1. Now any noise current smaller than this bias current will not cause the receiver to transition. The bias current can be supplied by a pair of resistors connected to the power and ground rails, as illustrated in Figure 5-1.

3V3

R1

+

Disconnected	In	In	RT	Ib
Inputs

‐

R2

Figure 5-1 LVDS Receiver Fail-Safe Resistors

The current generator, In, represents any noise picked up by the disconnected SpaceWire cable or PCB tracks. In Figure 5-1, In is shown as a negative current which causes a negative differential voltage across the termination resistor RT. The bias resistors R1 and R2 cause a bias current , In, to flow through the termination resistor. Provided that the bias current is greater than any negative noise current the output of the LVDS receiver will be logic 1. If the noise current is positive then the LVDS receiver output is logic 1 anyway.

The bias resistors must be chosen to give a bias current through the termination resistor greater than any expected noise current when the input is disconnected. Note that careful PCB design can reduce electro-magnetic interference and reduce the noise current.

Preliminary 35