IDT 89HPES12T3G2, Preliminary User'S Manual

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IDT Link Operation
PES12T3G2 User Manual 3 - 5 January 28, 2013
Notes
When operating at 5.0 Gbps, the PES12T3G2 port initiates a link speed downgrade in the following
cases:
– Link speed downgrade triggered when the PHY layer cannot achieve reliable operation at the
higher speed. In this case, the PES12T3G2 port continues to support the higher speed in the
training-sets it transmits during link training.
– Software sets the target link speed to 2.5 Gbps and sets the LRET bit in the PCIELCTL register.
In this case, the PES12T3G2 port removes support for the higher speed in the training-sets it
transmits during link training.
Additionally, the PES12T3G2 ports always respond to link partner requests to change speed. In this
case, the speed change is only successful when both components in the link advertise support the target
speed. When a link speed upgrade operation fails, the PHY LTSSM reverts back to the speed before the
upgrade (i.e., 2.5 Gbps) and does not autonomously initiate a subsequent link speed upgrade. In this case,
the PHY continues to support Gen1 and Gen2 data rates and therefore responds to link partner requests for
link speed upgrade, or to link speed upgrades triggered by software setting the LRET bit in the PCIELCTL
register.
A system designer may limit the maximum speed at which each port operates by changing the target
link speed via software or EEPROM and forcing link retraining. For additional information, refer to section
Software Management of Link Speed below.
Software Management of Link Speed
Software can interact with the link control and status registers of each port to set the link speed, as well
as receive notification of link speed changes. This gives software the capability to choose the desired link
speed based on system specific criteria. For example, depending on the traffic load expected on a link, soft-
ware can choose to downgrade link speed to 2.5 Gbps in order to reduce power on a low-traffic link, and
later upgrade the link to 5.0 Gbps when the bandwidth is required. Software may also choose to change the
link speed due to link reliability reasons (i.e., a link that has reliability problems at 5.0 Gbps may be down-
graded to 2.5 Gbps).
As mentioned above, the Target Link Speed (TLS) field of the Link Control 2 Register (PCIELCTL2) sets
the preferred link speed. By default, the Target Link Speed of each PES12T3G2 port is set to 5.0 Gbps.
In order to change link speed, software must write to the TLS field of the port’s PCIELCTL2 register and
subsequently force a link retrain by writing to the Link Retrain (LRET) bit of the Link Control (PCIELCTL)
register. The port will only initiate the link speed change if it has recorded support for the higher speed by its
link partner since exit from the Detect state. Else, the port will transition through the Recovery state without
setting the ‘Speed_Change’ bit in the transmitted training-sets.
Software is notified of link speed changes via the link bandwidth notification mechanism described in the
PCIe 2.0 specification. This mechanism is enabled by setting the Link Bandwidth Management Interrupt
Enable (LBWINTEN) bit in the PCIELCTL register of switch downstream ports. For downstream ports, when
the link speed is changed due to the following reasons, the Link Bandwidth Management Status (LBWSTS)
bit in the PCIELSTS register is set.
– Link speed downgrade initiated by a the PES12T3G2 port when the PHY layer cannot achieve
reliable operation at the higher speed. Note that this does not include link speed downgrading due
to failure to achieve symbol lock while upgrading link speed via the Recovery states.
– Link speed change initiated by the link partner that was not indicated as an autonomous change.
Also, the LBWSTS bit is set whenever software sets the LRET bit in the PCIELCTL register, even if the
link speed is not changed. Note that the LBWSTS bit is not set during the initial link speed change (i.e., the
speed change from Gen1 to Gen2 after fundamental reset or a full-link-retrain via the ‘Detect’ state). Addi-
tionally, for downstream ports, the Link Autonomous Bandwidth Status (LABWSTS) is set when the link
partner initiates a link speed change that was indicated as an autonomous change.