S2066
RECEIVER DESCRIPTION
Each receiver channel is designed to implement the
IEEE 802.3z Gigabit Ethernet receiver function
through the physical layer. A block diagram showing
the basic function is provided in Figure 4.
Whenever a signal is present, the receiver attempts
to recover the serial clock from the received data
stream. After acquiring bit synchronization, the
S2066 searches the serial bit stream for the occur-
rence of a K28.5 character on which to perform word
synchronization. Once synchronization on both bit
and word boundaries is achieved, the receiver pro-
vides the word-aligned data on its parallel outputs.
QUAD GIGABIT ETHERNET TRANSCEIVER
The run-length requirements insure that the S2066
will respond appropriately and quickly to a loss of
signal. The run-length checker looks for a minimum
of 120 consecutive ones or zeros. The checking is
done in parallel, thus 12 parallel words are examined.
An off-frequency detection circuit in the S2066 moni-
tors the receiver VCO frequency to ensure that the
input signal is at a valid data rate. The recovered
data clock must be within 200 ppm of the reference
clock for reliable locking of the CRU to the data
stream.
If both the off-frequency test and the run-length test
are satisfied, the CRU will attempt to lock to the
incoming data. Note that if the run length test is sat-
isfied due to noise on the inputs, and no signal is
present, the receiver VCO will maintain frequency
accuracy to within 100 ppm of the target rate as
determined by the REFCLK.
In any transfer of PLL control from the serial data to
the reference clock, the RBC1/0x outputs remain
phase continuous and glitch free, assuring the integ-
rity of downstream clocking.
If at any time, the frequency or run length checks are
violated, the state machine forces the VCO to lock to
the reference clock. This is required to guarantee
that the VCO maintains the correct frequency in the
absence of data.
Data Input
A differential input receiver is provided for each
channel of the S2066. Each channel has a loopback
mode in which the serial data from the transmitter
replaces external serial data. The loopback function
for all four channels is controlled by the loopback
enable signal, LPEN.
The high speed serial inputs to the S2066 are inter-
nally biased to VDD-1.3V. This facilities AC-coupling
of the differential inputs and termination with a single
differential termination.
Clock Recovery Function
Clock recovery is provided for each channel of the
S2066. The receiver PLL has been optimized for the
needs of Gigabit Ethernet systems. A simple state
machine in the clock recovery macro decides whether
to acquire lock from the serial data input or from the
reference clock. The decision is based upon the fre-
quency and run length of the serial data inputs.
Reference Clock Input
The reference clock must be provided from a low
jitter clock source. The frequency of the recovered
data clock (divided by 10 or 20) must be within 200
ppm of the reference clock to insure reliable locking
of the receiver PLL. A single reference clock is pro-
vided to both the transmitter and the receiver of the
S2066.
8
October 13, 2000 / Revision C
AMCC [ APPLIED MICRO CIRCUITS CORPORATION ]
