-Uncompelled Change to Slave State

12.3 System Interface Protocols

Uncompelled Change to Slave State

An uncompelled change to slave state is the transition of the System interface from master state to slave state, initiated by the processor when a processor read request is pending. Release* is asserted automatically after a read request or cluster (see Clusters, later in this chapter, for a definition of a cluster). An uncompelled change to slave state occurs either during or some number of cycles after the issue cycle of a read request, or either during or some number of cycles after the last cycle of the last request in a cluster.

The uncompelled release latency depends on the state of the cache, the presence or absence of a secondary cache, and the secondary cache parameters (see Release Latency, in this chapter). After an uncompelled change to slave state, the processor returns to master state at the end of the next external request. This can be a read response, or some other type of external request.

An external agent must note that the processor has performed an uncompelled change to slave state and begin driving the SysAD bus along with the SysCmd bus. As long as the System interface is in slave state, the external agent can begin an external request without arbitrating for the System interface; that is, without asserting ExtRqst*.

After the external request, the System interface returns to master state.

Whenever a processor read request is pending, after the issue of a read request or after the issue of all of the requests in a cluster, the processor automatically switches the System interface to slave state, even though the external agent is not arbitrating to issue an external request. This transition to slave state allows the external agent to return read response data.

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