Atmel SpaceWire Router SpW-10X User Manual

Ref.:   UoD_SpW-10X_ 

 

Issue:  3.4 

  

 

Date:  11

 

 

87

 

dependent on the previous input port which had access to that output port.  The next input port to 
transfer data to an output port is the next highest port number (modulo number of ports) that has data 
to send. Thus the input port which previously had access to the output port will be selected last by the 
router control logic.  For example, if input port 2 is transferring data to output port 1 and input ports 5 
and 7 are waiting to transfer data to port 1, then port 5 will selected next. 

Packets which have high priority are always selected before low priority packets in the router.  If two or 
more packets of the same priority level are attempting to use the same destination port then the 
packets are arbitrated in a fair manner.   

Note that router configuration packets, both commands and replies, are treated the same as any other 
packet as far as arbitration is concerned. The arbitration scheme is equally applicable to all types of 
packets with no exceptions. When sending long packets, arbitration can cause substantial delays in 
transferring information. 

The following sub-sections illustrate the various scenarios where arbitration is necessary. 

8.3.2.1  Arbitration of packets with matching priority (1) 

In the Figure 8-6 an example of arbitrating between packets with the same priority is illustrated. Only 
router ports 1-5 are shown for clarity.   

At stage one input ports 1 and 3 have packets to be routed to output port 5.  The previous input port to 
use output port 5 was input port 3 therefore the next input port to be selected by output port 5 will be 
input port 1 (assuming input ports 6, 7, 8, 9, 10 and 0 are not requesting to use output port 5).  

At stage two the router selects the packet arriving at input port 1 and the packet is routed through 
output port 5.  Input port 3 waits until all of the packet from input port 1 has been transferred. 

At stage three the complete packet has been transferred from input port 1. Now input port 3 is able to 
transfer its packet to output port 5. 

R1

2

5

1

3

5

4

5

R1

2

5

1

3

5

4

Two packets waiting to use port 5 

(Previous port which accessed port 5 = 3)

Packet from port 1 is selected

R1

2

5

1

3

4

Packet from port 1 completes
Packet from port 3 is selected

R1

2

5

1

3

4

Packet from port 3 is completes

1

2

3

4