Table of Contents
Advertisement
IDT Link Operation
Notes
PES48H12G2 User Manual
The current link speed of each port is reported via the Current Link Speed (CLS) field of the port's Link
Status Register (PCIELSTS). The above behavior applies after full link retrain (i.e., when the LTSSM transi-
tions through the 'Detect' state). Assuming the target link speed is set to 5.0 GT/s, a PES48H12G2 port initi-
ates a link speed upgrade in the following cases:
– Link speed upgrade after initial link train (i.e., from the Detect state) to L0 at 2.5 GT/s, when the
link partner advertised support for the higher speed.
– Link speed upgrade after full link retrain (i.e., via the Detect state) to L0 at 2.5 GT/s, when the link
partner advertised support for the higher speed.
– When software sets the Link Retrain (LRET) bit in the PCIELCTL register and the PES48H12G2
port has recorded support for the higher speed by its link partner.
When operating at 5.0 GT/s, a PES48H12G2 port initiates a link speed downgrade in the following
cases:
– When the PHY layer cannot achieve reliable operation at the higher speed. In this case, the
switch's port continues to support the higher speed in the training-sets it transmits during link
training.
– When software sets the target link speed to 2.5 GT/s and sets the LRET bit in the PCIELCTL
register. In this case, the switch's port removes support for the higher speed in the training-sets it
transmits during link training.
Additionally, the PES48H12G2 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 GT/s) 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 port's
PCIELCTL register.
The PES48H12G2 ports do not have a mechanism to autonomously regulate link speed. As a result, the
Hardware Autonomous Speed Disable (HASD) bit in the PCIELCTL2 register has no effect and is hardwired
to 0x0. Additionally, PES48H12G2 ports never set the 'Autonomous Change' bit in the training sets
exchanged with the link partner during link training
stream port may autonomously change link speed. When this occurs, the PES48H12G2 downstream port
sets the Link Autonomous Bandwidth Status (LABWSTS) bit in the PCIELSTS 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. Refer to section Link Retraining on page 7-9
for further details.
Software Management of Link Speed
Software can interact with the link control and status registers of downstream ports 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, software can choose to downgrade link speed to 2.5 GT/s in order to reduce power on a low-traffic
link, and later upgrade the link to 5.0 GT/s 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 GT/s may
be downgraded to 2.5 GT/s).
1.
The speed advertisement of the link partner is noted by PES48H12G2 in the latest LTSSM entry to the Configu-
ration.Complete or Recovery.RcvrCfg sub-states.
2.
Note that the 'Autonomous Change' bit is located in bit 6 of the fourth symbol in the training sets. This bit has
multiple meanings depending on the LTSSM state in which it is issued. PES48H12G2 never sets this bit in LTSSM
states in which this bit carries the 'autonomous change' meaning.
2
. Still, a link partner connected to a PES48H12G2 down-
7 - 8
1
April 5, 2013
Advertisement
Table of Contents
