3. Data Path > Flow Control
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PEB383 User Manual
July 25, 2011
Integrated Device Technology, Inc.
Confidential - NDA Required
Memory write transactions can contain any or all invalid payload bytes, where as memory write and
invalidate (MWI) transactions carry all the valid payload bytes. The PEB383 decomposes the received
transactions with non-contiguous byte enables on 32-byte boundaries while writing into the request
FIFO.
The PCI Core makes a request to the PCIe Core if one of the following conditions is met:
•
All data bytes of the transaction are received and are stored in the data buffer
•
Received data bytes count exceeds the programmed threshold value (see UPST_PWR_THRES in
“Upstream Posted Write Threshold Register”
•
Received data bytes count exceeds the PCIe maximum payload size (see MAX_PAY_SIZE in
“PCIe Device Control and Status Register”
•
Address plus received data bytes count exceeds 4 KB
•
Data with non-contiguous byte enables
3.3.3
Downstream Non-posted Buffer
The 512-byte, downstream non-posted buffer stores the data returned for the non-posted requests that
originate on the PCIe Interface and are destined for PCI devices.
A single completion of up to 512-bytes can be stored here. A single outstanding read is issued to the
PCI side.
3.3.4
Downstream Posted Buffer
The 512-byte downstream posted write buffer stores the payload of memory write transactions that
originate on the PCIe Interface and are destined for PCI devices. The amount of space assigned to each
transaction is dynamic.
The PEB383 uses an 8-deep request FIFO to store the request information, including the first and last
Dwords byte enables. The PEB383 initiates a transaction on the PCI Interface only after a complete
packet is stored in the buffer. The PEB383 attempts another outstanding transaction only if the current
transaction is either successfully completed or terminated with either master or target abort.
3.4
Flow Control
The PEB383 handles packet-based protocol on its PCIe Interface, and transaction-based protocol on its
PCI Interface. PCI requesters initiate transactions without prior knowledge on receiver buffer status.
As a result, flow control is managed through retries and disconnects that can waste bus bandwidth. In
comparison, PCIe requesters initiate requests while having prior knowledge on receiver buffer
availability status, and therefore, eliminate the wasteful effects of unnecessary retries and disconnects.
The PEB383 does not issue retries or disconnects on the PCI Interface for completions returned for a
downstream read request, but may issue retries or disconnects for a posted or non-posted transaction on
the PCI Interface based on the buffer space availability.