KS8721B/BT
Micrel
Introduction
100BaseTX Transmit
The 100BaseTX transmit function performs parallel to serial conversion, NRZ to NRZI conversion, MLT-3 encoding and
transmission. The circuitry starts with a parallel to serial conversion, which converts the 25MHz, 4-bit nibbles into a 125 MHz
serial bit stream. The incoming data is clocked in at the positive edge of the TXC signal. The serialized data is further converted
from NRZ to NRZI format, and then transmitted in MLT3 current output. The output current is set by an external 1% 6.49kΩ
resistorforthe1:1transformerratio. Ithasatypicalrise/falltimesof4nsandcompliestotheANSITP-PMDstandardregarding
amplitude balance, overshoot and timing jitters. The wave-shaped 10BaseT output driver is also incorporated into the
100BaseTX driver.
100BaseTX Receive
The 100BaseTX receive function performs adaptive equalization, DC restoration, MLT-3 to NRZI conversion, data and clock
recovery, NRZI to NRZ conversion, and serial to parallel conversion. The receiving side starts with the equalization filter to
compensate inter-symbol interference (ISI) over the twisted pair cable. Since the amplitude loss and phase distortion are a
function of the length of the cable, the equalizer has to adjust its characteristic to optimize the performance. In this design, the
variable equalizer will make an initial estimation based on comparisons of incoming signal strength against some known cable
characteristics, then tunes itself for optimization. This is an ongoing process and can self adjust against the environmental
changes such as temperature variations.
The equalized signal then goes through a DC restoration and data conversion block. The DC restoration circuit is used to
compensate effect of base line wander and improve the dynamic range. The differential data conversion circuit converts the
MLT3 format back to NRZI. The slicing threshold is also adaptive.
The clock recovery circuit extracts the 125MHz clock from the edges of the NRZI signal. This recovered clock is then used to
converttheNRZIsignalintotheNRZformat.Finally,theNRZserialdataisconvertedto4-bitparallel4Bnibbles.Asynchronized
25MHz RXC is generated so that the 4B nibbles is clocked out at the negative edge of RCK25 and is valid for the receiver at
the positive edge. When no valid data is present, the clock recovery circuit is locked to the 25MHz reference clock and both
TXC and RXC clocks continue to run.
PLL Clock Synthesizer
TheKS8721B/BTgenerates125MHz,25MHzand20MHzclocksforsystemtiming.Aninternalcrystaloscillatorcircuitprovides
the reference clock for the synthesizer.
Scrambler/De-scrambler (100BaseTX only)
The purpose of the scrambler is to spread the power spectrum of the signal in order to reduce EMI and baseline wander.
10BaseT Transmit
When TXEN (transmit enable) goes high, data encoding and transmission will begin. The KS8721B/BT will continue to encode
and transmit data as long as TXEN remains high. The data transmission will end when TXEN goes low. The last transition
occurs at the boundary of the bit cell if the last bit is zero, or at the center of the bit cell if the last bit is one. The output driver
is incorporated into the 100Base driver to allow transmission with the same magnetics. They are internally wave-shaped and
pre-emphasized into outputs with a typical 2.5V amplitude. The harmonic contents are at least 27dB below the fundamental
when driven by an all-ones Manchester-encoded signal.
10BaseT Receive
On the receive side, input buffer and level detecting squelch circuits are employed. A differential input receiver circuit and a
PLL performs the decoding function. The Manchester-encoded data stream is separated into clock signal and NRZ data. A
squelch circuit rejects signals with levels less than 300mV or with short pulse widths in order to prevent noises at the RX+ or
RX- input from falsely trigger the decoder. When the input exceeds the squelch limit, the PLL locks onto the incoming signal
and the KS8721B/BT decodes a data frame. This activates the carrier sense (CRS) ad RXDV signals and makes the receive
data (RXD) available. The receive clock is maintained active during idle periods in between data reception.
SQE and Jabber Function (10BaseT only)
In 10BaseT operation, a short pulse will be put out on the COL pin after each packet is transmitted. This is required as a test
of the 10BaseT transmit/receive path and is called SQE test. The 10BaseT transmitter will be disabled and COL will go high
if TXEN is High for more than 20ms (Jabbering). If TXEN then goes low for more than 250ms, the 10BaseT transmitter will be
re-enabled and COL will go Low.
Auto-Negotiation
The KS8721B/BT performs auto-negotiation by hardware strapping option (pin 29) or software (Register 0.12). It will
automatically choose its mode of operation by advertising its abilities and comparing them with those received from its link
partner whenever auto-negotiation is enabled. It can also be configured to advertise 100BaseTX or 10BaseT in either full- or
half-duplex mode (please refer to “Auto-Negotiation” ). The auto-negotiation is disabled in the FX mode.
August 2003
11
KS8721B/BT
MICREL [ MICREL SEMICONDUCTOR ]
