Atmel SpaceWire Router SpW-10X - Manual (Page 35)

Ref.: UoD_SpW-10X_
UserManual
Issue: 3.4
SpW-10X
SpaceWire Router
User Manual Date: 11 th
July 2008
Preliminary
35
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.
R
1
R
T
R
2
I
n
I
n
I
b
Disconnected
Inputs
+
‐
3V3
Figure 5-1 LVDS Receiver Fail-Safe Resistors
The current generator, I
n
, represents any noise picked up by the disconnected SpaceWire cable or
PCB tracks. In Figure 5-1, I
n
is shown as a negative current which causes a negative differential
voltage across the termination resistor R
T
. The bias resistors R
1
and R
2
cause a bias current , I
n
, 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.