TMPR4955AF datasheet(115/168 Pages) TOSHIBA

115 / 168 page

Chapter 6 TX4955A System Interface

6-43

EReq

EReq*

((((I)

Indicates an external agent is requesting bus ownership of the System interface.

To gain mastership of the bus, an external agent must arbitrate with the

processor as follows:

External agent asserts EReq*

Wait for PMaster* to be deasserted (1 to N cycles).

External agent drives SysAD and SysCmd buses. The external agent is guaranteed to

maintain mastership of the bus as long as EReq

EReq

EReq is asserted.

If at any time EReq

EReq

EReq* is deasserted, the external agent must go back to step A and

re-arbitrate for the bus.

ExtAgent Cmd

ExtAgent Data

SysAD

SCycle

SClock

SysCmd

EReq*

PMaster*

Figure 6.3.10 Sample Cycle with EReq* Asserted

From the time that EReq

EReq

EReq* is asserted, the external agent is guaranteed to gain

mastership of the bus after at most one processor request. However, if EOK

EOK

EOK* is being

deasserted, the external agent will gain mastership of the bus without having to accept any

processor requests.

The external agent relinquishes bus mastership by deasserting EReq

EReq

EReq* as shown below:

ExtAgent Cmd

ExtAgent Data

Proc Data

Data Cmd

SysAD

SCycle

SClock

SysCmd

EReq*

PMaster*

Figure 6.3.11 Sample Cycle with Deassertion of EReq*

External agent deasserts EReq

EReq

EReq* and external agent drives the bus.

Bus is set to a tristate.

Processor regains mastership of bus.

Except for a processor read request (see below), assertion of EReq

EReq

EReq* is the only way the

external agent gets and maintains bus mastership.

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