Atmel SpW-10X user manual Arbitration of three packets with matching priority

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

8.3.2.2 Arbitration of packets with matching priority (2)

In the Figure 8-7 another example of arbitrating between packets with the same priority is illustrated. Again 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 the port).

At stage two the router selects the packet at input port 1 and a packet is routed to output port 5. Input port 3 waits until the complete packet has been transferred. While the packet from input port 1 is being transferred to output port 5 another packet arrives at input port 2 to be routed to output port 5.

At stage three the packet from input port 1 has been forwarded and the packet from input port 2 is selected by the router to be routed through output port 5. Input port two is selected before input port 3 as it is the next input port to be considered by the routing control logic after input port 1.

At stage four p the complete packet has been transferred from input port 2. Now input port 3 is able to transfer its packet to output port 5.

1 2

5	1
	2	5
	R1
5	3
	4

Two packets waiting to use port 5 (Previous port which accessed port 5 = 3)

3	1	4
	2	5
5	R1
	3
	4

1

5 2

R1 5

5 3

4

Packet from port 1 is selected

Packet arrives on port 2

1
2	5
R1
3
4

Packet from port 1 completes Packet from port 2 is selected Packet from port 3 waits

Packet from port 2 completes Packet from port 3 is selected

Figure 8-7 Arbitration of three packets with matching priority

Preliminary 88