
IDT SerDes
PES32NT24xG2 User Manual
8 - 12
January 30, 2013
Notes
Transmit Margining Using the PCI Express Link Control 2 Register
When the Transmit Margin (TM) field in the port’s PCIELCTL2 register is set to a value other than
‘Normal Operating Range’, the transmitter voltage levels are controlled by hardware based on the setting of
the TM field, and not by the S[x]TXLCTL0 and S[x]TXLCTL1 registers
1
. Per the PCI Express Base Specifi-
cation, transmit margining may be done in full-swing mode or in low-swing mode. Table 8.9 shows the
transmit margining settings supported by the switch.
Note that in compliance mode (i.e., when the associated port’s PHY LTSSM is in the Polling.Compliance
state), the SerDes transmit level is controlled by the TM field in the associated port’s PCIELCTL2 register,
and the de-emphasis setting is controlled by the LTSSM based on the rules described in Section 4.2.6.2.2
of the PCI Express Base Specification.
–
When the LTSSM enters the Polling.Compliance state in full-swing mode, the values for full-swing
margining are applied.
–
When the LTSSM enters the Polling.Compliance state in low-swing mode, the values for low-
swing margining are applied.
Finally, when the TM field is modified, the newly selected value is not applied until the PHY LTSSM tran-
sitions through the states in which it is allowed to modify the transmit margin setting on the line (e.g.,
Recovery.RcvrLock). Therefore, after modifying this field, it is recommended that the link be retrained by
setting the LRET bit in the port’s PCIELCTL register.
Low-Swing Transmitter Voltage Mode
PES32NT24xG2 ports support the optional low-swing transmit voltage mode defined in the PCI Express
Base Specification. In this mode, the port’s transmitter voltage level is set to approximately half the value of
the full-swing (default) mode, reducing power consumption in the SerDes. This mode is enabled by setting
the Low-Swing Enable (LSE) bit in the port’s SerDes Configuration (SERDESCFG) register.
–
The LSE bit in the port’s SERDESCFG register affects all SerDes lanes associated with the port.
When Low-Swing mode is enabled, the transmitter drive level is reduced and de-emphasis is automati-
cally turned off. Therefore, the Selectable De-emphasis (SDE) and Compliance De-emphasis (CDE) fields
in the PCIELCTL2 register have no effect. Additionally, the Current De-emphasis (CDE) field in the
PCIELSTS2 register becomes invalid. The low-swing mode transmitter voltage swing may be adjusted via
the TDVL_LSG1 (when operating in Gen 1 mode) and TDVL_LSG2 (when operating in Gen 2 mode) fields
in the S[x]TXLCTL1 register.
Table 8.10 shows the transmitter’s drive swing for different values of TDVL_LSG1, when the port oper-
ates in low swing mode at Gen 1 speed
2
. Table 8.11 shows the transmitter’s drive swing for different values
of TDVL_LSG2, when the port operates in low swing mode at Gen 2 speed. The default setting is high-
lighted.
1.
The TX_AMPBOOST field in the S[x]TXLCTL0 register does have an effect during transmit margining.
Full Swing
Mode
(mV)
Low Swing
Mode
(mV)
900
500
700
400
500
300
300
200
200
100
Table 8.9 PCI Express Transmit Margining Levels Supported by the PES32NT24xG2
Содержание PCI Express 89HPES32NT24xG2
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