IDT Link Operation
PES24T3G2 User Manual
3 - 7
February 22, 2012
Notes
Once the link speed is downgraded, the link speed will remain at 2.5 Gbps until the link fully retrains (i.e.,
the PHY LTSSM transitions through the Detect state) or the LRET bit is set in the PCIELCTL register, with a
target link speed of 5.0 Gbps. If the link partner requests to upgrade the link speed (i.e., via the Recovery
state), the PHY LTSSM enters the Recovery state but the link speed remains at 2.5 Gbps.
The user may determine the current error number and monitoring period counts by reading the Error
Number Count (ENCNT) and Monitoring Period Count (MPCNT) fields in the Autonomous Link Reliability
Count (ALRCNT) register
1
. The MPCNT value is in units of micro-seconds. When the monitoring period
count (MPCNT) reaches the monitoring period (PERIOD field in the ALRERT register), hardware resets the
ENCNT and MPCNT fields to their initial value and re-starts both counts. These counts are also reset when
a full-link retrain occurs or when the LRET bit in the PCIELCTL register is set.
When a link is determined to be unreliable (i.e., ULD bit set in the ALRSTS register), the error number
count and monitor period counts stop (ENCNT and MPCNT fields are not reset and keep their value
unchanged). The user may read these fields to determine the error count and the monitoring period count at
which the link was determined to be unreliable.
2
To re-enable the mechanism, the user must clear the
enable bit (EN) in the ALRCTL register, then clear the ULD bit in the ALRSTS register, and then set the EN
bit again.
The Autonomous Link Reliability mechanism is not affected by the state of the Hardware Autonomous
Speed Disable (HASD) bit in the PCI Express Link Control 2 (PCIELCTL2) register, since this bit does not
apply to speed changes caused by link reliability issues. Additionally, note that when the link speed is down-
graded by the ALR mechanism, the Link Bandwidth Management Status (LBWSTS) bit is set in the PCI
Express Link Status (PCIELSTS) register of downstream ports. This may in turn cause an interrupt to be
sent upstream when the Link Bandwidth Management Interrupt Enable (LBWINTEN) bit is set in the PCI
Express Link Control (PCIELCTL) register.
Link Retraining
Per the PCIe 2.0 specification, link retraining can be done autonomously in response to link problems
(i.e., repeated TLP replay attempts) or as a result of software setting the link retrain (LRET) bit in the PCI
Express Link Control (PCIELCTL) register. Writing a one to the Link Retrain (LRET) bit in the upstream
port’s PCI Express Link Control (PCIELCTL) register when the REGUNLOCK bit is set in the SWCTL
register forces the upstream PCIe to retrain. When this occurs the LTSSM transitions directly to the
Recovery state.
Writing a one to the Link Retrain (LRET) bit in a downstream port’s PCI Express Link Control
(PCIELCTL) register regardless of the REGUNLOCK bit state in the SWCTL register forces the down-
stream PCIe link to retrain. When this occurs, the LTSSM transitions directly to the Recovery state. Writing
a one to the Full Link Retrain (FLRET) bit in the Phy Link State 0 (PHYLSTSE 0) register of any port forces
that port’s PCIe link to retrain. When this occurs, the LTSSM transitions directly to the Detect state.
Link retraining does not result in the link going down, unless the LTSSM transitions through the Detect
state in its retraining attempt. The speed of the link is not necessarily changed as a result of link retraining.
A link that operates at 5.0 Gbps will continue to operate at that speed if the link retraining attempt is
successful at that speed. Otherwise, the link speed is changed to 2.5 Gbps.
When link retraining results in the speed of the link being downgraded from 5.0 Gbps to 2.5 Gbps, the
Link Bandwidth Management Status (LBWSTS) bit is set in the PCI Express Link Status (PCIELSTS)
register (for downstream ports only). Also, the PHY LTSSM remains at the downgraded speed until the link
partner requests a link speed upgrade
3
, software sets the LRET bit in the PCIELCTL register, or the link is
fully retained via the FLRET bit in the PHYLSTATE0 register. Refer to section Link Speed Negotiation in the
4.
Note that per the PCIe 2.0 specification, the LBWSTS bit is not set if the link transitions through the DL_Down
state.
1.
Note that these counts are active even when the ALR mechanism is disabled. A user may read these counts to
monitor link reliability, without enabling the ALR mechanism to reduce link speed. Finally, note that the ALR mech-
anism must be enabled in order for the ULD bit to get set.
2.
When a link is determined to be unreliable, the error count (ENCNT) field will match the value of the error
threshold (ERRT).
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Страница 8: ...IDT PES24T3G2 User Manual 6 February 22 2012 Notes...
Страница 12: ...IDT Table of Contents PES24T3G2 User Manual iv February 22 2012 Notes...
Страница 14: ...IDT List of Tables PES24T3G2 User Manual vi February 22 2012 Notes...
Страница 16: ...IDT List of Figures PES24T3G2 User Manual viii February 22 2012 Notes...
Страница 20: ...IDT Register List PES24T3G2 User Manual xii February 22 2012 Notes...
Страница 32: ...IDT PES24T3G2 Device Overview PES24T3G2 User Manual 1 12 February 22 2012 Notes...
Страница 72: ...IDT SMBus Interfaces PES24T3G2 User Manual 5 20 February 22 2012 Notes...
Страница 76: ...IDT Power Management PES24T3G2 User Manual 6 4 February 22 2012 Notes...
Страница 156: ...IDT Configuration Registers PES24T3G2 User Manual 8 74 February 22 2012 Notes...