Draft 6/5/00
2-30
Functional Description
Copyright 2000 by LSI Logic Corporation. All rights reserved.
2.2.10 Clock and Data Recovery
This section describes clock and data recovery methods implemented in
the device for both the 100 Mbits/s and 10 Mbits/s modes.
2.2.10.1 100 Mbits/s Clock and Data Recovery
Clock recovery is accomplished with a phase-locked-loop (PLL). If valid
data is not present on the receive inputs, the PLL is locked to the
25-MHz TX_CLK signal. When the squelch circuit detects valid data on
the receive TP input, and if the device is in the Link Pass state, the PLL
input is switched to the incoming data on the receive inputs. The PLL
then locks on to the transitions in the incoming signal to recover the
clock. The recovered data clock is then used to generate the 25 MHz
nibble clock, RX_CLK, which clocks data into the controller interface
section.
The recovered clock extracted by the PLL latches in data from the TP
receiver to perform data recovery. The data is then converted from a
single bit stream into nibble wide data words according to the format
shown in
Figure 2.3
2.2.10.2 10 Mbits/s Clock and Data Recovery
The clock recovery process for 10 Mbits/s mode is identical to the 100
Mbits/s mode except:
The recovered clock frequency is a 2.5 MHz nibble clock
The PLL is switched from TX_CLK to the TP input when the squelch
indicates valid data
The PLL takes up to 12 transitions (bit times) to lock onto the
preamble, so some of the preamble data symbols are lost. However,
the clock recovery block recovers enough preamble symbols to pass
at least six nibbles of preamble to the receive controller interface as
shown in
Figure 2.3
.
The data recovery process for 10 Mbits/s mode is identical to that of the
100 Mbits/s mode. As mentioned in the Manchester Decoder section, the
data recovery process inherently performs decoding of Manchester
encoded data from the TP inputs.