VITESSE
SEMICONDUCTOR CORPORATION
Advance Product Information
Quad Transceiver
for Gigabit Ethernet
VSC7186
Table 1: Transmission Order and Mapping of a 10b Character
T9
j
T8
h
T7
g
T6
f
T5
i
T4
e
T3
d
T2
c
T1
b
T0
a
Data Bit
10B Bit Position
Comma Character
x
x
x
1
1
1
1
1
0
0
Clock Recovery
The VSC7186 accepts differential high speed serial input from the selected source (either the PECL SIi+/-
pins or the internal SOi+/- data), extracts the clock and retimes the data. Equalizers are included in the receiver
to open the data eye and compensate for Intersymbol Interference (ISI) which may be present in the incoming
data. The serial bit stream should be encoded so as to provide DC balance and limited run length by an 8b/10b
encoding scheme. The digital Clock Recovery Unit (CRU) is completely monolithic and requires no external
components. For proper operation, the baud rate of the data stream to be recovered should be within +200 ppm
of ten times the REF frequency. For example, Gigabit Ethernet systems use 125 MHz oscillators with a +/-
100ppm accuracy resulting in +/-200 ppm between VSC7186 pairs.
Deserializer
The recovered serial bit stream is converted into a 10-bit parallel output character. The VSC7186 provides
complementary TTL recovered clocks, RCi0 and RCi1, at one-twentieth of the serial baud rate if RCM=LOW,
or a single clock at one-tenth the serial baud rate, on RCi1 only, if RCM=HIGH. The clocks are generated by
dividing down the high-speed recovered clock which is phase locked to the serial data. The serial data is
retimed, deserialized and output on Ri(0:9).
If serial input data is not present, or does not meet the required baud rate, the VSC7186 will continue to
produce a recovered clock so that downstream logic may continue to function. The RCi0/RCi1 output frequency
under these circumstances will differ from its expected frequency by no more than +1%.
Word Alignment
The VSC7186 provides 7-bit comma character recognition and data word alignment. Word synchronization
is enabled on all channels by asserting SYNC HIGH. When synchronization is enabled, the receiver examines
the recovered serial data for the presence of the “Comma” pattern. 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 comma alignment is defined as
any of the following conditions:
1) The comma is not aligned within the 10-bit transmission character such that Ri(0..6) = “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
RCi0 rather than RCi1.
When an improperly aligned comma is encountered, the recovered clock is stretched, never slivered, so that
the comma character and recovered clocks are aligned properly to Ri(0:9). This results in proper character and
word alignment. When the parallel data alignment changes in response to a improperly aligned comma pattern,
G52306-0, Rev. 2.0
3/27/00
VITESSE SEMICONDUCTOR CORPORATION
741 Calle Plano, Camarillo, CA 93012 • 805/388-3700 • FAX: 805/987-5896
Page 3
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