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Can WLAST or RLAST be high when the relevant xVALID signal is low ?

Answer

All AXI signals are undefined when the corresponding xVALID signal is 1'b0. Typically they will either keep their previous value until a new valid transfer is being signalled, or they will revert to a default value.

In the case of xLAST signals, it is perfectly legal for them to be at 1'b1 after a transaction has completed and the xVALID signal for that channel has gone to 1'b0, and all that is required is that they are correctly driven when xVALID next changes to 1'b1 to indicate the first transfer in a new transaction.

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