
Notes
PES34H16 User Manual
4 - 1
October 30, 2008
Chapter 4
Link Operation
Introduction
The PES34H16 contains six x4 ports which may be merged in pairs to form up to three x8 ports. The
remaining 10 ports are x1. The default link width for ports zero through five is x4 and the SerDes lanes are
statically assigned to a port.
Polarity Inversion
Each port of the PES34H16 supports automatic polarity inversion as required by the PCIe specification.
Polarity inversion is a function of the receiver and not the transmitter. The transmitter never inverts its data.
During link training, the receiver examines symbols six through 16 of the TS1 and TS2 ordered sets for
inversion of the PExAP[n] and PExAN[n] signals. If an inversion is detected, then logic for the receiving lane
automatically inverts received data. Polarity inversion is a lane and not a link function. Therefore, it is
possible for some lanes of link to be inverted and for others to not be inverted.
Link Width Negotiation
The PES34H16 supports the optional link variable width negotiation feature outlined in the PCIe specifi-
cation. During link training, Each merged x8 port is capable of negotiating to a x8, x4, x2 or x1 link width and
each unmerged x4 port is capable of negotiating to a x4, x2 or x1 link width.
The negotiated width of each link may be determined from the Link Width (LW) field in the corresponding
port’s PCI Express Link Status (PCIELSTS) register. The Maximum Link Width (MAXLNKWDTH) field in a
port’s PCI Express Link Capabilities (PCIELCAP) register contains the maximum link width of the port. This
field is of RWL type and may be modified when the REGUNLOCK bit is set in the SWCTL register. Modifica-
tion of this field allows the maximum link width of the port to be configured. The new link width takes effect
the next time link training occurs.
The initial value of the MAXLNKWDTH field defaults to x4 mode. To force a link width to a smaller width
than the default value, the MAXLNKWDTH field could be configured through Serial EEPROM initialization
and full link retraining forced. When merged port link negotiates to a width less than x8, then the unused
group of four lanes are powered down to save power. In addition, unused SerDes in a four lane group are
put in a low power state (i.e. L1 state).
When an unmerged port negotiates to a width less than x4, the unused SerDes lanes are put in a low
power state (i.e., L1 state). When a merged or unmerged port is disabled, all SerDes lanes associated with
that port are powered down.
Lane Reversal
The PCIe specification describes an optional lane reversal feature. The PES34H16 supports the auto-
matic lane reversal feature outlined in the PCIe specification. The operation of lane reversal is dependant
on the maximum link width selected by the MAXLNKWDTH field. Lane reversal mapping for the various
non-trivial x4 non-merged port maximum link width configurations supported by the PES34H16 are illus-
trated in Figures 4.1 and 4.2. Lane reversal mapping for the various non-trivial x8 merged port maximum
link width configurations supported by the PES34H16 are illustrated in Figures 4.3 through 4.5.
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