IDT PCI Express 89HPES32NT24xG2, User Manual, Page: 159 / 744

IDT SerDes
PES32NT24xG2 User Manual 8 - 11 January 30, 2013
Notes
When using the de-emphasis controls, it is important to understand that the actual de-emphasis applied
on the link is a function of the de-emphasis controls, the transmit drive swing controls, and the data rate of
the SerDes. The coarse de-emphasis controls should generally be set as shown in Tables 8.6, 8.7, and 8.8.
Note that there is a coarse de-emphasis control per PHY operating mode. The coarse de-emphasis settings
shown in the above tables ensure that the de-emphasis falls within the nominal range mandated by the PCI
Express Base Specification. As shown in the tables, the coarse de-emphasis setting is dependent on the
transmit drive swing setting. Therefore, modifying the transmit drive swing must be done in conjunction with
modifying the coarse de-emphasis setting.
The fine de-emphasis registers allow modification of the de-emphasis in fine steps. There is a fine de-
emphasis control per PHY operating mode.
– When the PHY operates in Gen 1 data rate with -3.5 dB de-emphasis, the fine de-emphasis is
controlled by the FDC_FS3DBG1 field in the S[x]TXLCTL0 register.
– When the PHY operates in Gen 2 data rate with -3.5 dB de-emphasis, the fine de-emphasis is
controlled by the FDC_FS3DBG2 field in the S[x]TXLCTL0 register.
– When the PHY operates in Gen 2 data rate with -6.0 dB de-emphasis, the fine de-emphasis is
controlled by the FDC_FS6DBG2 field in the S[x]TXLCTL0 register.
Figure 8.2 shows a plot of the de-emphasis seen at the SerDes transmitter pins as a function of the fine
de-emphasis control and transmit drive level controls, when the PHY operates in Gen 2 data rate with -6.0
dB de-emphasis. In the figure, chX_tx_lev[4:0] refers to the TDVL_FS6DBG2 control.
Figure 8.2 Effect of Fine de-emphasis Control at Gen 2 with -6.0 dB Nominal de-emphasis
As shown in Figure 8.2, the de-emphasis applied on the line varies depending on the setting of the
transmit drive level field. Thus, when modifying the transmit drive level of the SerDes, the fine de-emphasis
control must be adjusted appropriately to ensure that the de-emphasis on the line falls within the range
mandated by the PCI Express Base Specification.
Finally, note that it is possible to turn off the de-emphasis (i.e., 0 db de-emphasis) by setting both the
coarse and fine de-emphasis settings to a value of 0x0.