VSC7123
Datasheet
If serial input data is not present or does not meet the required baud rate, the VSC7123 continues to produce a
recovered clock, allowing downstream logic functionality to continue. Under these circumstances, the
RCLK/RCLKN output frequency differ from its expected frequency by no more than +1%.
Word Alignment
The VSC7123 provides 7-bit comma character recognition and data word alignment. Word synchronization is
enabled by asserting ENCDET HIGH. When synchronization is enabled, the receiver examines the recovered serial
data for the presence of the “Comma” character. This pattern is “0011111XXX”, where the leading zero corresponds
to the first bit received. The comma sequence is not contained in any normal 8b/10b coded data character or pair of
adjacent characters. It occurs only within special characters, known as K28.1, K28.5, and K28.7, which are defined
for synchronization purposes. Improper alignment of the comma character is defined as any of the following
conditions.
1. The comma is not aligned within the 10-bit transmission character such that R0...R6 = “0011111.”
2. The comma straddles the boundary between two 10-bit transmission characters.
3. The comma is properly aligned but occurs in the received character presented during the rising edge of RFCLK
rather than RCLKN.
When ENCDET is HIGH and an improperly aligned comma is encountered, the recovered clock is stretched (never
slivered) so that the comma character and recovered clocks are properly aligned to R(0:9). This results in proper
character and word alignment. When the parallel data alignment changes in response to a improperly aligned comma
pattern, some data which would have been presented on the parallel output port may be lost. Additionally, the first
Comma pattern may also be lost or corrupted. Subsequent data will be output correctly and properly aligned. When
ENCDET is LOW, the current alignment of the serial data is maintained indefinitely, regardless of data pattern.
When encountering a comma character, COMDET is driven HIGH. The COMDET pulse is presented simultaneously
with the comma character and has a duration equal to the data, or half of an RCLK period. The COMDET signal is
timed so that it can be captured by the adjoining protocol logic on the rising edge of RCLKN. Functional waveforms
for synchronization are given in Figure 2 and Figure 3. Figure 2 shows the case when a comma character is detected
and no phase adjustment is necessary. This figure illustrates the position of the COMDET pulse in relation to the
comma character on R(0:9). Figure 3 shows the case where the K28.5 is detected, but it is misaligned so a change in
the output data alignment is required. Note that up to three characters prior to the comma character may be corrupted
by the realignment process.
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G52212-0 Revision 4.7
March 14, 2008
VITESSE [ VITESSE SEMICONDUCTOR CORPORATION ]
