Chapter 4: Parameter Settings
4–3
System Settings
November 2012
Altera Corporation
Arria V GZ Hard IP for PCI Express
RX Buffer credit
allocation -
performance for
received requests
(continued)
High
Maximum
■
High
–This setting configures most of the RX Buffer space for
received requests and allocates a slightly larger than minimum
amount of space for received completions. Select this option where
most of the PCIe requests are generated by the other end of the PCIe
link and the local application layer logic only infrequently generates a
small burst of read requests. This option is recommended for typical
root port applications where most of the PCIe traffic is generated by
DMA engines located in the endpoints.
■
Maximum
–This setting configures the minimum PCIe specification
allowed amount of completion space, leaving most of the RX Buffer
space for received requests. Select this option when most of the PCIe
requests are generated by the other end of the PCIe link and the local
application layer logic never or only infrequently generates single
read requests. This option is recommended for control and status
endpoint applications that don't generate any PCIe requests of their
own and only are the target of write and read requests from the root
complex.
Reference clock
frequency
100 MHz
125 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/s” in the specification.
For Gen3, Altera recommends using a common reference clock (0 ppm)
because when using separate reference clocks (non 0 ppm), the PCS
occasionally must insert SKP symbols, potentially causes the PCIe link
to go to recovery. Arria V GZ PCIe Hard IP in Gen1 or 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.
Use 62.5 MHz
application clock
On/Off
This mode is only available only for Gen1 ×1.
Use deprecated RX
Avalon-ST data byte
enable port
(rx_st_be)
On/Off
This parameter is only available for the Avalon-ST Arria V GZ Hard IP for
PCI Express.
Enable byte parity
ports on Avalon-ST
interface
On/Off
When
On
, the RX and TX datapaths are parity protected. Parity is odd.
This parameter is not available for the Avalon-MM Arria V GZ Hard IP for
PCI Express.
Multiple packets per
cycle
On/Off
When
On
, the 256-bit Avalon-ST interface supports the transmission of
TLPs starting at any 128-bit address boundary, allowing support for
multiple packets in a single cycle. To support multiple packets per cycle,
the Avalon-ST interface includes 2 start of packet and end of packet
signals for the 256-bit Avalon-ST interfaces. This feature is only
supported for Gen3 ×8. This parameter is not available for the
Avalon-MM Arria V GZ Hard IP for PCI Express.
For more information refer to
“Multiple Packets per Cycle” on
.
Table 4–2. System Settings for PCI Express (Part 3 of 4)
Parameter
Value
Description