ESE
VITESSE
SEMICONDUCTOR CORPORATION
SEMICONDUCTOR CORPORATION
Preliminary Data Sheet
Gigabit Interconnect Chip
VSC7212
Within the receiver there are elastic buffers used to deskew multiple VSC7212s and/or VSC7216s in order
to align them to a common word clock. The receiver’s elastic buffer allows the chips’ input to be skewed up to
+/-7 bit times in order to accommodate circuit imperfections, differences in transmission delay and jitter.
Multiple devices can be used in synchronous operation if the skew between all serial input pairs is maintained
less than +/-7 serial clock bit times. This allows easy implementation of robust systems.
Chip-to-Chip word alignment is enabled by connecting the WSI input of all devices to the WSO output of
an arbitrarily selected “Master” device. The FLOCK input state and WSI input source determine whether or not
rate matching (IDLE deletion or duplication) will be performed. Chip-to-chip alignment is disabled when WSI
is not connected to a WSO output. Rate matching is disabled when either FLOCK is HIGH or WSI is held LOW
(see Table 6).
In order to perform word alignment, a synchronization point must be seen across all receivers to be aligned
within the +/-7 bit time window. The VSC7212 receiver recognizes the first four characters of the Word Sync
Sequence (either K28.5+ K28.5+ K28.5- K28.5- or K28.5- K28.5- K28.5+ K28.5+) as the synchronization
point. As a model for understanding, consider the case where two VSC7212 transmitters send 16 bits of data to
two receivers via copper media which has small cable length differences causing chip-to-chip skew. Both
transmitters must be word aligned by simultaneously sending the Word Sync Sequence (within the +/-7 bit
window). On detection of the synchronization point, the receivers will reposition the recovered data within their
elastic buffers in order to align both devices and remove any chip-to-chip skew. All normal data characters
following the Word Sync Sequence will be properly chip-to-chip word aligned. In the process of alignment, one
or two of the final twelve K28.5 characters in the Word Sync Sequence may be deleted or duplicated.
The VSC7212 is capable of performing rate matching in multiple device applications by inserting or
deleting IDLEs in parallel across all receivers. This requires that the chip-to-chip aligned data streams contain
IDLEs inserted simultaneously on all transmitters according to the IDLE density requirement previously
described.
Table 6: Word Alignment and Rate Matching Control
FLOCK
WSI Source
Chip-to-Chip Alignment
Rate Matching
0
0
Off
Off
It’s own WSO
0
Off
Enabled within chip
or 1
0
1
1
Another chips’ WSO
Enabled
Off
Enabled between chips
0
1
Off
Off
Off
Another chips’
1
Enabled
Off
WSO
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