TSB14C01A, TSB14C01AI, TSB14C01AM

5-V IEEE 1394-1995 BACKPLANE TRANSCEIVER/ARBITER

SGLS107A – FEBRUARY 1999 – REVISED NOVEMBER 1999

25

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

urgent arbitration

The backplane environment enhances the fair priority algorithm by splitting access opportunities among nodes

based on two priority classes: fair and urgent. Nodes using an urgent priority can use up to three-fourths of the

access opportunities, with the remaining ones equally shared among nodes using the fair priority. All nodes are

required to implement the fair priority class, while the urgent priority class is optional. Packets are labeled as

urgent when that priority class was used.

The fair/urgent allocation uses the same fairness interval described in fair arbitration but accompanies the

arbitration_enable flag with an urgent_count. The fair/urgent method works as follows:

their arbitration_enable flags, while nodes implementing urgent priority set their urgent_count to three.

a subaction gap as long as its arbitration_enable flag is set. When its arbitration_enable flag is cleared, it

waits for an arbitration reset gap before it begins arbitrating. When such a node wins an arbitration contest,

it sends a packet without the urgent label and its arbitration_enable flag is cleared.

if its urgent_count is nonzero. When its urgent_count is zero, it waits for an arbitration reset gap before it

begins arbitrating. Whenever such a node wins an arbitration contest, it sends a packet with the urgent label.

is transmitted or received. This includes received packets that are addressed to other nodes.

includes received packets that are addressed to other nodes. This ensures that there is at most three urgent

packets for every fair packet. This does not ensure that every node using urgent priority obtains the bus

three times for each fairness interval. The node arbitrating with the highest priority always obtains the bus

before other nodes arbitrating with an urgent, but lower, priority.

In the presence of urgent nodes, a fairness interval ends after the final fair node and up to three remaining urgent

nodes have successfully accessed the bus. Since all fair nodes now have their arbitration_enable signals reset

and all urgent nodes have their urgent_count decremented to zero, none of the nodes can access the bus. The

bus remains idle until an arbitration reset gap has occurred, re-enabling arbitration on all nodes and starting the

next fairness interval. This process is illustrated in Figure 13, which illustrates a situation where there are three

nodes arbitrating for the bus with physical_IDs such that A has the highest priority, B is in the middle priority,

and C has the lowest priority, with nodes A and C using fair priority and B using urgent: