Chapter 7

Response Signaling

This chapter describes the four slave responses in AXI read and write transactions. It contains the following sections:

•About response signaling on page 7-2

•Response types on page 7-4.

Response Signaling

7.1About response signaling

The AXI protocol allows response signalling for both read and write transactions. For read transactions the response information from the slave is passed alongside the read data itself, however for writes the response information is conveyed along the write response channel.

The AXI protocol responses are:

•OKAY

•EXOKAY

•SLVERR

•DECERR.

Table 7-1 shows the encoding of the RRESP[1:0] and BRESP[1:0] signals.

For a write transaction, there is just one response given for the entire burst and not for each data transfer within the burst.

In a read transaction, the slave can give different responses for different transfers within a burst. In a burst of 16 read transfers, for example, the slave might return an OKAY response for 15 of the transfers and a SLVERR response for one of the transfers.

The protocol defines that the required number of data transfers must be performed, even if an error is reported. For example, if a read of 8 transfers is requested from a slave but the slave has an error condition then the slave must perform 8 data transfers, each with an error response. The remainder of the burst is not cancelled if the slave gives a single error response.

Response Signaling

This protocol places restrictions on masters that can issue multiple outstanding addresses and that must also support precise error signaling. Such masters must be able to handle an error response for an earlier transfer while later transfers are already underway.