Response Signaling

7.2Response types

This section describes the four AXI protocol response types:

•Normal access success

•Exclusive access

•Slave error

•Decode error.

7.2.1Normal access success

The OKAY response indicates:

•the success of a normal access

•the failure of an exclusive access

•an exclusive access to a slave that does not support exclusive access.

OKAY is the response for most transactions.

7.2.2Exclusive access

The EXOKAY response indicates the success of an exclusive access. Chapter 6 Atomic Accesses describes this response.

7.2.3Slave error

The SLVERR response indicates an unsuccessful transaction. Examples of slave error conditions are:

•FIFO/buffer overrun or underrun condition

•unsupported transfer size attempted

•write access attempted to read-only location

•timeout condition in the slave

•access attempted to an address where no registers are present

•access attempted to a disabled or powered-down function.

To simplify system monitoring and debugging, it is recommended that error responses are used only for error conditions and not for signaling normal, expected events.

7.2.4Decode error

In a system without a fully-decoded address map, there can be addresses at which there are no slaves to respond to a transaction. In such a system, the interconnect must provide a suitable error response to flag the access as illegal and also to prevent the system from locking up by trying to access a nonexistent slave.

Response Signaling

When the interconnect cannot successfully decode a slave access, it effectively routes the access to a default slave, and the default slave returns the DECERR response.

An implementation option is to have the default slave also record the details of decode errors for later determination of how the errors occurred. In this way, the default slave can significantly simplify the debugging process.

The AXI protocol requires that all data transfers for a transaction are completed, even if an error condition occurs. Therefore any component giving a DECERR response must meet this requirement.

Response Signaling