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IDT SerDes
Notes
PES48H12G2 User Manual
In addition to this, the PES48H12G2 offers proprietary fine grain controllability of the SerDes transmitter
voltage level, across a wide range of settings. The PES48H12G2 places no restrictions on the time at which
these settings can be modified (e.g., they can be modified during normal operation of the link or while the
link is being tested).
By default, the SerDes transmit level can be programmed in the range from 950 mV to 120 mV, at steps
1
of ~40 mV each
. In addition, there is an "amplitude-boost" control that monotonically increases the drive
level by ~5% for each of four settings. By default, this control is set to the second level (out of four),
meaning that the range shown above can be increased by up to 10% when the maximum boost setting is
used, or decreased by 5% when the minimum boost setting is used.
Together, the controls for drive-level and amplitude boost allow the system designer to select, on a per-
lane basis if desired, the appropriate drive strength for the channel. For power sensitive applications, the
drive level can be reduced with fine granularity to the desired level, without compromising link reliability.
Note that the PCI Express specification requires that, at 5.0 GT/s, receivers accept incoming signals in
the range 1.2 V to 0.120 V. Thus, the transmitter voltage settings may be modified without requiring any
modification of the link partner's receiver settings. Refer to section Programming of SerDes Controls on
page 8-3 for procedural details on modifying the default SerDes settings and to section SerDes Transmitter
Control Registers on page 8-4 for details on programming the transmitter voltage level and amplitude boost
controls.
De-emphasis
The PCI Express 2.0 specification supports three de-emphasis levels: -3.5 dB (at 2.5 GT/s or 5.0 GT/s
speeds), -6.0 dB (only for 5.0 GT/s), and 0 dB (low-swing mode). The de-emphasis selected for the link is
controlled by the Selectable De-emphasis bit in each port's PCI Express Link Control 2 register
is set by hardware or firmware (e.g., EEPROM) during boot time and remains unchanged during normal
system operation.
To allow the de-emphasis setting to be modified and customized on the link, the PES48H12G2 contains
proprietary per-lane coarse and fine de-emphasis adjustment controls. Together, these controls allow the
nominal de-emphasis setting (i.e., -3.5 dB or -6.0 dB) to be modified with a granularity of ~0.3 dB per
3
setting
. The desired de-emphasis setting can be achieved across the range of driver level settings
described in the previous section. The PES48H12G2 places no restrictions on the time at which the de-
emphasis setting can be modified.
Refer to section SerDes Transmitter Control Registers on page 8-4 for details on programming the
transmitter de-emphasis.
Slew Rate
In addition to transmitter drive level and de-emphasis controls, the PES48H12G2 ports also contain
proprietary fine and coarse slew rate controls. These provide controllability over the rise and fall times of the
transmitted differential signal, and are useful in scenarios where slow rise and fall times negatively affect
the signal eye. The controls for the slew rate are provided on a per-lane basis, and the PES48H12G2
places no restrictions on the time at which they can be modified. Refer to section SerDes Transmitter
Control Registers on page 8-4 for details on programming the slew rate controls.
PCI Express Low-Swing Mode
The PES48H12G2 ports support the optional low-swing transmit voltage mode defined in the PCI
Express 2.0 specification. In this mode, the port's transmitter voltage level is set to approximately half the
value of the full-swing (default) mode, which results in reduced power consumption in the SerDes. In addi-
1.
Values are based on HSPICE simulation results, assuming typical conditions, as measured at the pin of the
device.
2.
In low-swing mode, de-emphasis is automatically set to 0 dB and the Selectable De-emphasis bit in the port's
PCI Express Link Control 2 register is ignored.
3.
Note that the PCI Express specification allows a deviation of +/- 0.5 dB from the nominal setting.
8 - 2
2
. This field
April 5, 2013
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