TSB14C01A, TSB14C01AI, TSB14C01AM

5-V IEEE 1394-1995 BACKPLANE TRANSCEIVER/ARBITER

SGLS107A – FEBRUARY 1999 – REVISED NOVEMBER 1999

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POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

arbitration sequence

The arbitration sequence is the process that the TSB14C01A uses to arbitrate the bus between competing

modules.

arbitration number

The arbitration sequence uses a unique arbitration number for each module. This 6-bit number is the same as

the node physical_ID. When less than 6 bits are provided for the arbitration number, they occupy the MSBs of

the arbitration number. The remaining bits are zero-filled. The MSBs are transmitted first.

NOTE

If the serial bus is contained within a host backplane, it is expected that the arbitration number (i.e.,

physical_ID) is set by the host backplane at power up (e.g., with a built-in slot identifier or

configuration mechanism).

It is recommended that this number be software programmable to facilitate testing and to allow for consistent

system operation and repeatability.

priority

Within the arbitration sequence, the arbitration number is preceded by four bits that define a priority level. The

method by which priority is assigned is to be determined by the system integrator with two exceptions. The

lowest priority (all zeros) is reserved for fair arbitration and the highest priority (all ones) is reserved for cycle

start requests. This allows 14 priority levels to be used for the urgent arbitration process.

The use of an urgent priority class allows nodes to be granted a larger portion of the bandwidth on the bus. High

priority nodes are granted the bus before lower priority nodes during urgent allocation of the bus, allowing such

nodes to be granted more bandwidth.

In order to ensure forward progress, the lowest priority level is reserved for fair arbitration. This allows all nodes

arbitrating with this priority level to be allowed one fair access to the bus for each fairness interval. For fair

arbitration, the value of the arbitration number has a minimal impact on the allocation of the bus. Although nodes

with higher arbitration numbers are granted the bus sooner, there is only a small decrease in latency.

The 4-bit priority field is not used in isochronous arbitration. When arbitrating for an isochronous transfer, the

priority field is zero-filled.