Intel
®
81341 and 81342 I/O Processors
December 2007
Developer’s Manual
Order Number: 315037-002US
403
Messaging Unit—Intel
®
81341 and 81342
4.0
Messaging Unit
This chapter describes the Messaging Unit (MU) of the Intel
®
81341 and 81342 I/O
Processors (81341 and 81342). The MU is located at the front-end of the DDR SDRAM
Memory Controller.
4.1
Overview
The Messaging Unit (MU) provides a mechanism for data to be transferred between the
PCI bus and the Intel XScale
®
processor and notifying the respective system of the
arrival of new data through an interrupt. The MU can be used to send and receive
messages.
The MU is located on the south internal bus of the 81341 and 81342. The MU is located
on the south internal bus interface of the DDR SDRAM Memory Controller Unit. Refer to
Figure 78, “Memory Controller Block Diagram” on page 565
of the DDR SDRAM Memory
Controller Unit chapter. External PCI agents access the MU via the ATU. For example,
the ATU initiates transactions on the internal bus to the MU on behalf of the external
PCI agents.
The MU has four distinct messaging mechanisms. Each allows a host processor or
external PCI agent and the 81341 and 81342 to communicate through message
passing and interrupt generation. The four mechanisms are:
•
Message Registers
— allow the 81341 and 81342 and external PCI agents to
communicate by passing messages in one of four 32-bit Message Registers. In this
context, a message is any 32-bit data value. Message registers combine aspects of
mailbox registers and doorbell registers. Writes to the message registers may
optionally cause interrupts.
•
Doorbell Registers
— allow the 81341 and 81342 to assert the PCI interrupt
signals and allow external PCI agents to generate an interrupt to the Intel XScale
®
processor.
•
Circular Queues
— support a message passing scheme that uses four circular
queues.
•
Index Registers
— support a message passing scheme that uses a portion of the
81341 and 81342 local DDR memory to implement a large set of message
registers.
Each of the above are available to the system designer at the same time. No special
mode selection is needed.