TSB14C01A, TSB14C01AI, TSB14C01AM

5-V IEEE 1394-1995 BACKPLANE TRANSCEIVER/ARBITER

SGLS107A – FEBRUARY 1999 – REVISED NOVEMBER 1999

23

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

format of arbitration sequence

The following format for the arbitration sequence should be used:

PRIORITY

ARBITRATION NUMBER

4 bits

6 bits

arbitration sequence

The LSB of the priority field is followed by the MSB of the arbitration number.

is reserved for the identification of the cycle start packet.

arbitration

Unless a node is using immediate arbitration to access the bus (in which case there is no contention for the bus),

it is possible that more than one node can attempt to access the bus at a given time. Consequently, it is

necessary for a node to arbitrate for the bus in order to gain access to the bus.

NOTE

A node uses immediate arbitration to send an acknowledge. Since there is no contention for the

bus in this case, arbitration is not necessary. A node that is transmitting an acknowledge does not

arbitrate for the bus, but merely waits for an acknowledge gap to be detected before it begins

transmission. If a node is attempting to gain access to the bus without using immediate access, it

must first arbitrate for the bus.

Arbitration occurs in response to a phy arbitration request from the link. Nodes begin arbitrating once the bus

has become idle for a predetermined amount of time (the appropriate gap indication occurs). Once this happens,

nodes begin a bit-by-bit transmission of their arbitration sequence.

A node can obtain access to the bus in a limited number of ways. Since some arbitration classes allow nodes

to begin arbitration before others, nodes arbitrating with certain arbitration classes can detect that the bus is

busy before they can begin to arbitrate. In this way, certain arbitration classes can be bypassed. For example,

fair and urgent nodes do not get a chance to arbitrate when another node is sending an acknowledge or when

it is arbitrating for an isochronous transfer.

The backplane environment supports the fair, urgent, cycle_master, isochronous, and immediate arbitration

classes.

fairness intervals

The fairness protocol is based on the concept of a fairness interval. A fairness interval consists of one or more

periods of bus activity separated by short idle periods called subaction gaps and is followed by a longer idle

period known as an arbitration reset gap. At the end of each gap, bus arbitration is used to determine the next

bus owner. This concept is shown in Figure 11.