ARM Technical Support Knowledge Articles

How can a slave return a BRESP before knowing the AWADDR value for the transaction ?

Applies to: AXI


The AXI specification requires that a BRESP response is not signalled by the slave until the final WLAST data transfer has been completed, but there is no requirement that the AWADDR transfer has been completed or signalled. How does the slave know what response to return, and what if it subsequently needs to change this response once the AWADDR address is known ?


Although the specification allows the BRESP to be returned before the address transaction has completed, in reality most slaves will need to know the address transfer details before a response can be determined.

If you only have one slave in your system (so no address decoding for routing purposes), and the slave can always accept data regardless of the address (so no possibility of an ERROR), then you could return an OKAY response.

However once this response has been signalled, it cannot subsequently be changed once the address transfer is known.

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