Atmel SpW-10X Manuel D’Utilisation

Ref.:   UoD_SpW-10X_ 

 

Issue:  3.4 

  

 

Date:  11

 

 

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

 

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.