ARM Technical Support Knowledge Articles

Can an arbiter be designed to always allow bursts to complete?

Applies to: AHB

Answer

Applies to: AMBA AHB

A SPLIT, RETRY or ERROR response from a slave can always cause a burst to be early terminated. This is outwith the control of the Arbiter and so must be supported.

Undefined length INCR bursts cannot have their end point predicted, so there is no efficient way that an Arbiter design can allow the burst to complete before granting another master. INCR bursts must be arbitrated on a cycle by cycle basis.

Defined length INCRx and WRAPx bursts can have their beats counted, and so allowed to complete by the Arbiter. However because of the AHB arbitration synchronous timing, there is no way to avoid possibly terminating a burst immediately after the first transfer of the burst has been indicated.

The Arbiter only knows that a defined length burst is in progress by sampling the HBURST bus. However the first point at which HBURST can be sampled is after the first clock cycle of the first burst beat, by which time the Arbiter may already have decided to grant another master and will have changed the HGRANT outputs accordingly. Only a combinatorial path from HBURST to HGRANT would allow the burst to be detected in time to avoid early termination in this scenario, but combinatorial paths in the AHB bus are not allowed.

Article last edited on: 2008-09-09 15:47:39

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