Altera Stratix V Avalon-ST User Manual
Page 22
Parameter
Value
Description
Reference clock
frequency
100 MHz
The PCI Express Base Specification 3.0 requires a
100 MHz ±300 ppm reference clock. The 125 MHz reference
clock is provided as a convenience for systems that include a
125 MHz clock source. For more information about Gen3
operation, refer to 4.3.8 Refclk Specifications for 8.0 GT/sin the
specification.
For Gen3, Altera recommends using a common reference
clock (0 ppm). For designs with separate reference clocks (non
0 ppm), the PCS occasionally must insert SKP symbols,
potentially causing the PCIe link to go to recovery. Gen1 and
Gen2 modes are not affected by this issue. Systems using the
common reference clock (0 ppm) are not affected by this issue.
The primary repercussion of this is a slight decrease in
bandwidth. On Gen3 x8 systems, this bandwidth impact is
negligible. If non 0 ppm mode is required, so that separate
reference clocks are being used, please contact Altera for
further information and guidance.
RX Buffer credit
allocation -
performance for
received requests
Minimum
Low
Balanced
High
Maximum
Determines the allocation of posted header credits, posted
data credits, non-posted header credits, completion header
credits, and completion data credits in the 16 KByte RX buffer.
The 5 settings allow you to adjust the credit allocation to
optimize your system. The credit allocation for the selected
setting displays in the message pane.
Refer to the Throughput Optimization chapter for more
information about optimizing performance. The Flow Control
chapter explains how the RX credit allocation and the
Maximum payload RX Buffer credit allocation and the
Maximum payload size that you choose affect the allocation
of flow control credits. You can set the Maximum payload
size parameter on the Device tab.
The Message window dynamically updates the number of
credits for Posted, Non-Posted Headers and Data, and
Completion Headers and Data as you change this selection.
3-2
System Settings
UG-01097_sriov
2014.12.15
Altera Corporation
Parameter Settings