AXI channel dependencies

The AXI protocol defines dependencies between the different channels.
Three of the main dependencies are as follows:
• WLAST transfer must complete before BVALID is asserted.
◦ The manager must send all the write data before a write response can be seen by the
manager. This dependency does not exist in AXI3 but is introduced for AXI4:
◦ In AXI3, the address does not have to be seen before a write response is sent.
◦ In AXI4, all of the data and the address must have been transferred before the manager can
see a write response.
• RVALID cannot be asserted until ARADDR has been transferred.
◦ The subordinate cannot transfer any read data without it seeing the address first. This is
because the subordinate cannot send data back to the manager if it does not know the
address that the data will be read from.
• WVALID can assert before AWVALID.
◦ A manager could use the Write Data channel to send data to the subordinate, before
communicating the address where the subordinate should write these data.