CertusPro-NX SerDes/PCS Usage Guide
Preliminary
Technical Note
100
© 2020-2021 Lattice Semiconductor
FPGA-TN-02245-0.81
All rights reserved. CONFIDENTIAL
Protocol Mode
PCI Express Mode
PCI Express is a high performance, fully scalable, well-defined standard for a wide variety of computing and
communications platforms. Being a packet based serial technology, PCI Express greatly reduces the number of required
pins and simplifies board routing and manufacturing. PCI Express is a point-to-point technology, as opposed to the
multi-drop bus in PCI. Each PCI Express device has the advantage of full duplex communication with its link partner to
greatly increase overall system bandwidth.
All CertusPro-NX SerDes/PCS quads can be configured as PCI Express PIPE mode to work with PCIe Link Layer soft IP.
But only Quad0 integrates PCIe Link Layer hard IP, which can be configured as PCI Express Hard IP mode. Both PIPE
mode and Hard IP mode work with PCI Express PCS, PMA Controller and PMA. The channel cannot be configured as
MPCS mode or EPCS mode when this channel has been configured as PCI Express mode, considering all these modes
share the PMA Controller and PMA channels.
PCIe Base Specification requires external AC-coupling. The recommended Capacitance are shown in
. The
detected state is the initial state after PCIe reset or power up. In this state, a device electrically detects if a receiver
device is present at the far end of the link by observing the rate of change. And the device compares the time the line
voltage to charge-up and the expected time. If a receiver device is attached, the charge time is much longer. The value
and location of this AC-coupling capacitor affects the charge time during PCIe detect state. That is why the
recommended capacitance range is so narrow.
Table 11.1. PCI Express Recommend AC Capacitance
PCIe Generation
Minimum
Typical
Maximum
Gen1
75 nF
—
265 nF
Gen2
75 nF
—
265 nF
Gen3
176 nF
—
265 nF
PCI Express Base Specification defines a beacon signal inside the Link Training and Status State Machine (LTSSM). While
a PCIe link is in L2 power state, its main power source and clock are turned off. An auxiliary voltage (V
aux
) keeps a small
part of the device working, including the wake-up logic. To signal a wake-up event, a downstream device can drive the
beacon signal upstream to start the L2 exit sequence. A switch or bridge receiving a beacon signal on its downstream
port must forward notification upstream by sending the beacon signal on its upstream port or by asserting the WAKE#
pin.
However, CertusPro-NX SerDes does not support the auxiliary power required by PCIe LTSSM L2 state. It is meaningless
to support PCIe beacon signal or WAKE# signal generation and detection for unsupported LTSSM L2 state.
For more detailed information on CertusPro-NX PCI Express features, function descriptions, and IP usage, refer to
CertusPro-NX PCI Express Hardened IP Core User Guide (FPGA-IPUG-
XXXXX
).
Generic 8B/10B Mode
The Generic 8B/10B mode of CertusPro-NX SerDes/PCS is intended for applications requiring 8B/10B encoding and
decoding without the need for additional protocol-specific data manipulation. CertusPro-NX SerDes/PCS can support
Generic 8B/10B applications from 625Mbps to 8.1Gbps per channel.
In Generic 8B/10B mode, 8B/10B Encoder module and 8B/10B Decoder module are enabled. Tx Lane-to-lane Deskew
module, Tx FIFO module, Word Aligner module, Rx Lane Aligner module, Elastic Buffer module and Rx FIFO module can
be enabled or bypassed per user design. Refer to the
section for detailed information about these modules
inside CertusPro-NX SerDes/PCS 8B/10B Channel PCS.