Altera Corporation
2–173
October 2007
Stratix II GX Device Handbook, Volume 2
Stratix II GX Transceiver Architecture Overview
Channel Alignment ||A|| (Deskew)
It is possible for ordered sets to be misaligned with respect to one another
because of board skew or differences between the independent clock
recoveries per serial lane. Channel alignment, also referred to as deskew
or channel bonding, realigns the ordered sets by using the alignment code
group, referred to as /A/. The /A/ code group is transmitted
simultaneously on all four lanes, constituting an ||A|| ordered set,
during idles or IPG. XAUI receivers use these code groups to resolve any
lane-to-lane skew. Skew between the lanes can be up to 40 UI (12.8 ns) as
specified in the standard, which relaxes the board design constraints.
Figure 2–124
shows lane skew at the receiver input and how the deskew
circuitry uses the /A/ code group to deskew the channels.
Figure 2–124. Lane Deskew With the /A/ Code Group
Stratix II GX devices manage XAUI channel alignment with a dedicated
deskew macro that consists of a 16-word-deep FIFO buffer that is
controlled by a XAUI deskew state machine. The XAUI deskew state
machine first looks for the /A/ code group within each channel. When
the XAUI deskew state machine detects /A/ in each channel, the deskew
FIFO buffer is enabled. The deskew state machine now monitors the
reception of /A/ code groups. When four aligned /A/ code groups have
been received the
rx_channelaligned
is asserted. The deskew state
machine continues to monitor the reception of /A/ code groups and
de-asserts the
rx_channelaligned
signal if alignment conditions are
lost. This built-in deskew macro is only enabled for the XAUI protocol.
Figure 2–125
shows the PCS deskew state diagram specified in clause 48
of the IEEE P802.3ae.
Lanes are
Deskewed by
Lining up
the "Align"/A/,
Code Groups
Lanes Skew at
Receiver Input
A
Lane 0
K
K
R
A
K
R
R
K
K
K
R
R
Lane 1
K
K
R
A
K
R
R
K
K
K
R
R
Lane 0
K
K
R
K
R
R
K
K
K
R
R
Lane 1
K
K
R
A
K
R
R
K
K
K
R
R
Lane 2
K
K
R
A
K
R
R
K
K
K
R
R
Lane 3
K
K
R
A
K
R
R
K
K
K
R
R
Lane 2
K
K
R
A
K
R
R
K
K
K
R
R
Lane 3
K
K
R
A
K
R
R
K
K
K
R
R