TMPR4955F datasheet(77/168 Pages) TOSHIBA

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Chapter 6 TX4955A System Interface

6-5

6.2.5.1

Master state and slave state

The system interface is placed in the master state when the TX4955A processor is

driving the SysAD bus and SysCmd bus. In contrast, the system interface is in the

slave state when the external agent is driving the SysAD bus and SysCmd bus.

The processor asserts the ValidOut* signal if the SysAD bus and SysCmd bus

become valid when the system interface is in the master state. The external agent

asserts the ValidIn* signal if the SysAD bus and SysCmd bus become valid when the

system interface is in the slave state.

6.2.5.2

Shifting from the master state to the slave state

The system interface remains in the master state unless it enters one of the

following states:

•

The external agent issues a request, then usage of the system interface is

granted (external arbitration).

•

The processor issues a read request and shifts into the slave mode by itself.

6.2.5.3

External arbitration

The external agent cannot issue external requests via the system interface unless

the system interface goes into the slave state. Shifts from the master state to the slave

state are arbitrated by the processor using the system interface handshake signals

ExtRqst* and Release.* This shift is performed as follows below.

1) The external agent sends notification that it would like to issue an external

request by asserting the ExtRqst* signal.

2) The processor releases the system interface and changes its state from the master

state to the slave state by asserting the Release* signal for 1 cycle.

3) The system interface returns to the master state when issuing of the external

request is complete.

6.2.5.4

Shifting to the slave state on its own

Shifting to the slave state on its own means that the shift from the master state to

the slave state is started by the processor when the processor read request is still on

hold. The Release signal is automatically asserted after the read transaction.

Self-invoked shifting to the slave state occurs either during the issue cycle of the read

request or several cycles after that.

After shifting to the slave state on its own, the processor returns to the master state

at the end of the next external request. This is made possible by a read request or

other type of external request.

The SysAd bus and SysCmd bus drives must start after the external agent confirms

that the processor autonomously shifted to the slave state. While the system interface

is in the slave state, the external agent can start making external requests without

requesting access to the system interface (without asserting the ExtRqst* signal).

The system interface returns to the master state when the external request ends.

If a processor read request is on hold after a read request is issued, the processor

automatically changes the system interface into the slave state even if the system

interface access necessary for the system agent to issue the external request has not

been requested. By shifting to the slave state in this manner, the external agent

becomes able to return read response data.

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