-Independent Transmissions on the SysAD Bus

12.7 Data Rate Control

Independent Transmissions on the SysAD Bus

In most applications, the SysAD bus is a point-to-point connection, running from the processor to a bidirectional registered transceiver residing in an external agent. For these applications, the SysAD bus has only two possible drivers, the processor or the external agent.

Certain applications may require connection of additional drivers and receivers to the SysAD bus, to allow transmissions over the SysAD bus that the processor is not involved in. These are called independent transmissions. To effect an independent transmission, the external agent must coordinate control of the SysAD bus by using arbitration handshake signals and external null requests.

An independent transmission on the SysAD bus follows this procedure:

1. The external agent requests mastership of the SysAD bus, to issue an external request.

2. The processor releases the System interface to slave state.

3. If the processor is being used with a secondary cache, the external agent issues a secondary cache release external null request to return ownership of the secondary cache to the processor.

4. The external agent then allows the independent transmission to take place on the SysAD bus, making sure that ValidIn* is not asserted while the transmission is occurring.

5. When the transmission is complete, the external agent must issue a System interface release external null request to return the System interface to master state.

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