Intel
®
81341 and 81342—System Controller (SC) and Internal Bus Bridge
Intel
®
81341 and 81342 I/O Processors
Developer’s Manual
December 2007
542
Order Number: 315037-002US
6.3.1
Theory of Operation
The bridge forwards north internal bus address requests that are not claimed on the
north internal bus. For example, the bridge performs subtractive decoding on the north
internal bus interface. The bridge claims and forwards a north internal bus data
transaction when the ID provided with the data transaction matches the ID of a
previously claimed write address request.
On the south internal bus, the XBG defines a Bridge Memory Window. Transactions on
the south internal bus that target the Bridge Memory Window are claimed and
forwarded to the north internal bus. The bridge claims and forwards a south internal
bus data transaction when the ID provided with the data transaction matches the ID of
a previously claimed write address request. The Bridge south interface also claims
transactions that target the Bridge memory-mapped registers. Intel XScale
®
processor
transactions that target the Bridge memory-mapped registers are also propagated to
the south internal bus, and then claimed by the Bridge south interface.
Both the north and south internal busses on 81341 and 81342 support the same bus
protocol. The internal bus operates by performing split transactions on both read and
write address requests. Every address request contains an Address Transfer ID (ATID),
which is the ID of the initiator. Every initiator has a unique ATID. When an initiator
makes a read request, it drives its ATID along with the address request. Once the
Bridge claims the request, it maintains the ATID, which it uses when returning read
completion data in the form of a Data Transfer ID (DTID). The initiator of the read
request claims the read completion transaction by observing that the DTID matches its
ATID. For a write request, the initiator drives its ATID along with the address request.
Once the Bridge claims the request, it maintains the ATID, which it uses to claim a data
transaction. The initiator of the write request drives a Data Transfer ID (DTID) with the
data transaction. The Bridge claims the data transaction by observing that the DTID
matches the stored ATID.
Each of the initiator/requester on the 81341 and 81342 has an assigned unique ID,
which helps identify an initiator when returning read data and for the purpose of
lists the encoded initiator IDs. The bridge uses the
same initiator ID of a transaction it claims when forwarding the transaction on the
opposite internal bus.