
IDT Link Operation
PES32NT24xG2 User Manual
7 - 6
January 30, 2013
Notes
Software may be notified of link width reconfiguration via the link bandwidth notification mechanism
described in the PCI Express Base Specification. This mechanism is enabled by setting the Link Bandwidth
Management Interrupt Enable (LBWINTEN) bit in the PCIELCTL register of switch downstream switch
ports.
Dynamic Link Width Reconfiguration in the PES32NT24xG2
PES32NT24xG2 ports support dynamic link width upconfiguration and downconfiguration in response to
link partner requests. This capability is honored for regular links and crosslinks.
The switch’s ports do not initiate autonomous link width upconfiguration and downconfiguration of links,
except for downconfiguration due to link reliability reasons. Therefore, the Hardware Autonomous Width
Disable (HAWD) bit in the port’s PCIELCTL register has no effect and is hardwired to 0x0. Additionally, the
switch’s ports never set the ‘Autonomous Change’ bit in the training sets exchanged with the link partner
during link training.
1
A downstream switch port’s link partner may autonomously change link width. When this occurs, the
PES32NT24xG2 downstream switch port sets the Link Autonomous Bandwidth Status (LABWSTS) bit in
the PCIELSTS register.
Link Speed Negotiation
The PCI Express Base Specification introduces support for 5.0 GT/s data rate (i.e., Gen 2), in addition to
the 2.5 GT/s data rates (i.e., Gen 1) mandated in previous versions of the specification. Per this specifica-
tion, all lanes of a link must operate at the same data rate. During full link training (i.e., from the Detect
state), links initially operate at 2.5 GT/s. Once the LTSSM on both components of the link reach the L0 state
and the data-link layer enters the DL_Active state, the link speed may be upgraded to 5.0 GT/s if this capa-
bility is advertised by both components. The process of upgrading the link speed does not result in a
link_down state.
A component advertises its supported speeds via the Data Rate Identifier bits in the TS1 and TS2
training sets transmitted to its link partner during link training. The PCI Express Base Specification permits
a component to change its supported speeds dynamically. It is allowed for a component to advertise
supported link speeds without necessarily changing the link speed, via the Recovery LTSSM state.
A component determines the supported speeds of its link partner by examining the Data Rate Identifier
bits in the TS1/TS2 training sets received during link training, specifically in the Configuration.Complete and
Recovery.RcvrCfg states. The last advertisement received overrides any previously recorded value.
Either link component may request a link speed change due to software requests or link reliability
reasons (i.e., speed downgrade). Downstream components are further permitted to request link speed
changes due to autonomous hardware initiated mechanisms. A component must only initiate a link speed
change when it knows that its link partner supports the target speed via prior exchange of Training Sets.
Gen 2 support is optional while Gen 1 support is mandatory.
If neither component in the link advertises support for Gen 2, then the link remains operating in Gen 1
speed. If one component has advertised support for Gen 1 and Gen 2, and the other has advertised support
for Gen 1 only, then the link will remain operating in Gen 1 speed until the lesser speed component decides
to:
–
Advertise support for Gen 2 via the Recovery state without modifying the link speed. The link
remains operating at Gen 1 speed.
–
Transition the link speed to Gen 2 via the Recovery.Speed state. The link will operate at Gen 2
speed. In this case, the advertisement of Gen 2 speed by both components is done implicitly in
the Recovery substates entered while modifying the link speed.
1.
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. The switch never sets this bit in LTSSM
states in which this bit carries the ‘autonomous change’ meaning.
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...