TMPR4955AF datasheet(90/168 Pages) TOSHIBA

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Chapter 6 TX4955A System Interface

6-18

The processor continues processing external requests while ExtRqst* is asserted.

However, until the processor completes a request that is currently being processed, it

will not be able to release the system interface and put it into the slave state in

preparation for the next external request. Also, until ExtRqst* is asserted, a series of

external requests cannot be interrupted by a processor request.

6.2.8.7

External arbitration protocol

As previously mentioned, the ExtRqst* signal and Release* signal are used in

system interface arbitration. Figure 6.2.18 illustrates the timing of the arbitration

protocol when the slave state changes to the master state.

The arbitration cycle sequence is as follows.

1. The external agent asserts ExtRqst* when it becomes necessary to issue external

requests.

2. The processor asserts Release* for 1 cycle when it becomes possible to process an

external request.

3. The processor sets the SysAD bus and SysCmd bus to tri-state.

4. The external agent must start transmission to the SysAD bus and SysCmd bus 2

cycles after Release* is asserted.

5. The external agent deasserts ExtRqst* 2 cycles after Release* is asserted. This

does not apply however to situations where an attempt is made to issue another

external request.

6. The external agent sets the SysAD bus and SysCmd bus to tri-state when

processing of the external request is complete.

The processor becomes able to issue processor requests 1 cycle after the external

agent sets the busses to tri-state.

Note: The SysADC bus and SysCmdP bus timing is the same as that for the SysAD bus

and SysCmd bus, respectively.

Figure 6.2.18 Arbitration Protocol Relating to External Requests

Master

Slave

Master

345

Cycle

Master Clock

SysAD Bus

ValidIn*

WxtRqst*

SysCmd Bus

Cmd

Release*

NEOD

Addr

Data0

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