
IDT Link Operation
PES32NT24xG2 User Manual
7 - 7
January 30, 2013
Notes
It is the responsibility of the upstream component of the link (i.e., switch downstream switch ports) to
keep the link at the target link speed or at the highest common speed supported by both components of the
link, whichever is lower. In addition, the upstream component must initiate a link speed upgrade if it has
recorded support for the higher speed by its link partner and software sets the Link Retrain bit in the
PCIELCTL register with a target link speed which is not equal to the current link speed.
The upstream component (i.e., switch downstream switch port) is capable of notifying software of link
speed changes via the Link Bandwidth Notification mechanism described in the PCI Express Base Specifi-
cation.
Link Speed Negotiation in the PES32NT24xG2
PES32NT24xG2 ports support data rates of 5.0 GT/s and 2.5 GT/s. The highest data rate of each link is
determined dynamically, and depends on the following factors:
–
Maximum link data rate supported by both components of the link.
–
The Target Link Speed set via the Link Control 2 Register (PCIELCTL2) in function 0 of the port.
–
The reliability of the link at 5.0 GT/s.
By default, the Target Link Speed (TLS) of each port is set to 5.0 GT/s. Therefore, the PES32NT24xG2
ports advertise support for 2.5 GT/s and 5.0 GT/s during the link training process via training-sets. During
normal operation, the TLS field should not be modified in an upstream port.
After a fundamental reset, each port link trains to the L0 state at 2.5 GT/s (Gen 1). Once the data-link
layer reaches the DL_Active state, if the Target Link Speed indicates 5.0 GT/s (default value), the
PES32NT24xG2 downstream switch ports automatically initiate link speed upgrade to 5.0 GT/s (Gen 2)
using the link speed change mechanism described in the PCI Express Base Specification. Upstream ports
do not automatically initiate link speed upgrade to Gen 2.
–
The Initial Link Speed Change Control (ILSCC) bit in a port’s PHYLCFG0 register controls whether
the port automatically initiates a speed upgrade to Gen 2. If the ILSCC bit is set, the port does not
automatically initiate a speed change to Gen 2. Software may modify this bit to change the default
behavior.
–
The Link Bandwidth Management Status (LBWSTS) bit in the PCIELSTS register of downstream
switch ports is not set since the initial link speed upgrade is not caused by a software directed link
retrain or due to link reliability issues.
The current link speed of each port is reported via the Current Link Speed (CLS) field of the port’s Link
Status Register (PCIELSTS). If the port operates in a multi-function mode, all functions of the port report the
same value in this field.
The above behavior also applies after full link retrain (i.e., when the LTSSM transitions through the
‘Detect’ state).
Assuming the target link speed is set to 5.0 GT/s, a switch port initiates 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.
–
For downstream ports, when the link operates at 2.5 GT/s data rate and the link partner advertises
support for the higher speed via the Recovery states. PES32NT24xG2 downstream ports auto-
matically initiate a link speed upgrade to 5.0 GT/s to keep the link operating at the highest speed
as required by the PCI Express Base Specification.
–
When software sets the Link Retrain (LRET) bit in the port’s PCIELCTL register and the port has
recorded support for the higher speed by its link partner.
1
1.
The speed advertisement of the link partner is noted by the switch in the latest LTSSM entry to the Configura-
tion.Complete or Recovery.RcvrCfg sub-states.
Summary of Contents for PCI Express 89HPES32NT24xG2
Page 20: ...IDT Table of Contents PES32NT24xG2 User Manual x January 30 2013 Notes...
Page 24: ...IDT List of Tables PES32NT24xG2 User Manual xiv January 30 2013 Notes...
Page 28: ...IDT List of Figures PES32NT24xG2 User Manual xviii January 30 2013 Notes...
Page 56: ...IDT PES32NT24xG2 Device Overview PES32NT24xG2 User Manual 1 20 January 30 2013 Notes...
Page 100: ...IDT Switch Core PES32NT24xG2 User Manual 4 22 January 30 2013 Notes...
Page 128: ...IDT Failover PES32NT24xG2 User Manual 6 4 January 30 2013 Notes...
Page 148: ...IDT Link Operation PES32NT24xG2 User Manual 7 20 January 30 2013 Notes...
Page 164: ...IDT SerDes PES32NT24xG2 User Manual 8 16 January 30 2013 Notes...
Page 170: ...IDT Power Management PES32NT24xG2 User Manual 9 6 January 30 2013 Notes...
Page 196: ...IDT Transparent Switch Operation PES32NT24xG2 User Manual 10 26 January 30 2013 Notes...
Page 244: ...IDT SMBus Interfaces PES32NT24xG2 User Manual 12 40 January 30 2013 Notes...
Page 247: ...IDT General Purpose I O PES32NT24xG2 User Manual 13 3 January 30 2013 Notes...
Page 248: ...IDT General Purpose I O PES32NT24xG2 User Manual 13 4 January 30 2013 Notes...
Page 330: ...IDT Switch Events PES32NT24xG2 User Manual 16 6 January 30 2013 Notes...
Page 342: ...IDT Multicast PES32NT24xG2 User Manual 17 12 January 30 2013 Notes...
Page 344: ...IDT Temperature Sensor PES32NT24xG2 User Manual 18 2 January 30 2013 Notes...
Page 384: ...IDT Register Organization PES32NT24xG2 User Manual 19 40 January 30 2013...
Page 492: ...IDT Proprietary Port Specific Registers PES32NT24xG2 User Manual 21 44 January 30 2013 Notes...
Page 588: ...IDT NT Endpoint Registers PES32NT24xG2 User Manual 22 96 January 30 2013 Notes...
Page 710: ...IDT JTAG Boundary Scan PES32NT24xG2 User Manual 25 12 January 30 2013 Notes...
Page 743: ...IDT Usage Models PES32NT24xG2 User Manual 26 33 January 30 2013 Notes...
Page 744: ...IDT Usage Models PES32NT24xG2 User Manual 26 34 January 30 2013 Notes...