3. Data Path > Short Term Caching
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PEB383 User Manual
July 25, 2011
Integrated Device Technology, Inc.
Confidential - NDA Required
3.6
Short Term Caching
This feature provides performance improvements in situations where upstream devices are not able to
stream data continuously to meet the prefetching needs of the PEB383. As defined in the
PCI-to-PCI
Bridge Specification (Revision 1.2)
, when the bus master completes a transaction, the bridge is required
to discard the balance of any data that was prefetched for the master. To prevent performance impacts
when dealing with devices between requester and completer that can only stream data of 128 to 512
bytes due to buffering constraints, the PEB383 uses “Short Term Caching.” This feature provides a
time-limited read data cache in which the PEB383 will not discard prefetched read data after the
request completes on the initiating bus.
To enable Short Term Caching, set the STC_EN bit in the
“PCI Miscellaneous Control and Status
. When enabled, the PEB383 does not discard the additional prefetched data when the read
transaction completes on the initiating bus. The PEB383 then continues to prefetch data up to the
amount specified in the
. If the initiator generates a new transaction that
requests the previously prefetched data, the PEB383 returns that data.
The PEB383 discards data after some of the data for a request is returned to the initiator and one of the
following conditions is met:
•
Short-term discard timer is expired before the initiator has requested additional data (see
“Short-term Caching Period Register”
).
•
An upstream posted transaction is received on the PCI Interface
3.7
Polarity Inversion
The PEB383 supports polarity inversion. For information on how to use this feature, see the
PEB383
Board Design Guidelines
.
Short-term caching should only be used in systems that can ensure the data provided to the
master has not been modified since the initial transaction.