Altera Stratix GX Transceiver - Manual (Page 123)

Altera Corporation 5–11
January 2005 Stratix GX Transceiver User Guide
XAUI Mode
Stratix GX transceivers handle XAUI channel alignment with a dedicated
deskew macro consisting of a 16-word-deep FIFO module 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
/A/ is detected in each channel, the deskew FIFO module is enabled. The
deskew state machine then monitors the reception of /A/ code-groups.
When four aligned /A/ code-groups are received, the
rx_channelaligned[]
signal is asserted. The deskew state machine
continues to monitor the reception of /A/ code-groups and deasserts the
rx_channelaligned[]
signal if alignment conditions are lost. This
built-in deskew macro is only enabled for the XAUI protocol. For
reference, the PCS deskew state diagram specified in clause 48 of the
IEEE P802.3ae is shown in
Figure 5–8 .
Rate Matcher
XAUI can operate in multi-crystal environments, which can tolerate a
frequency variation of ±100 ppm between crystals. Stratix GX
transceivers have embedded circuitry to perform clock rate
compensation. This is achieved by the insertion or removal of the PCS
SKIP code-group (/R/) from the inter packet gap (IPG) or idle stream.
This process is called rate matching and is sometimes referred to as clock
rate compensation.
The rate matcher in Stratix GX transceivers consists of a 12-word-deep
FIFO module along with control logic. In XAUI mode, the controller
begins to write data into the FIFO module whenever the
rx_channelaligned
signal is asserted. Within the control logic, there
is a FIFO module counter that keeps track of the read and write
executions. When the FIFO module is near an overflow condition, the
receivers delete the /R/ code-group simultaneously across all channels
during IPG or idle conditions. If the FIFO counter is near an underflow
condition, the receivers insert the /R/ code-group simultaneously across
all channels during IPG or idle conditions. This circuitry compensates for
±100 ppm frequency variations.
8B/10B Decoder
The 8B/10B decoder is part of the Stratix GX transceiver blocks. The
purpose of the 8B/10B decoder is to restore the 8-bit data + 1-bit control
identifier from the 10-bit code.