Datasheet

Micrel, Inc.
KSZ8081MNX/KSZ8081RNB
August
19, 2015 19
Revision 1.4
Functional Description: 10Base-T/100Base-TX Transceiver
The KSZ8081 is an integrated single 3.3V supply Fast Ethernet transceiver. It is fully compliant with the IEEE 802.3
Specification, and reduces board cost and simplifies board layout by using on-chip termination resistors for the two
differential pairs and by integrating the regulator to supply the 1.2V core.
On the copper media side, the KSZ8081 supports 10Base-T and 100Base-TX for transmission and reception of data over
a standard CAT-5 unshielded twisted pair (UTP) cable, and HP Auto MDI/MDI-X for reliable detection of and correction for
straight-through and crossover cables.
On the MAC processor side, the KSZ8081MNX offers the Media Independent Interface (MII) and the KSZ8081RNB offers
the Reduced Media Independent Interface (RMII) for direct connection with MII and RMII compliant Ethernet MAC
processors and switches, respectively.
The MII management bus option gives the MAC processor complete access to the KSZ8081 control and status registers.
Additionally, an interrupt pin eliminates the need for the processor to poll for PHY status change.
The KSZ8081MNX/RNB is used to refer to both KSZ8081MNX and KSZ8081RNB versions in this datasheet.
100Base-TX Transmit
The 100Base-TX transmit function performs parallel-to-serial conversion, 4B/5B encoding, scrambling, NRZ-to-NRZI
conversion, and MLT3 encoding and transmission.
The circuitry starts with a parallel-to-serial conversion, which converts the MII data from the MAC into a 125MHz serial bit
stream. The data and control stream is then converted into 4B/5B coding and followed by a scrambler. 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 6.49kΩ 1% resistor for the 1:1 transformer ratio.
The output signal has a typical rise/fall time of 4ns and complies with the ANSI TP-PMD standard regarding amplitude
balance, overshoot, and timing jitter. The wave-shaped 10Base-T output is also incorporated into the 100Base-TX
transmitter.
100Base-TX Receive
The 100Base-TX receiver function performs adaptive equalization, DC restoration, MLT3-to-NRZI conversion, data and
clock recovery, NRZI-to-NRZ conversion, de-scrambling, 4B/5B decoding, and serial-to-parallel conversion.
The receiving side starts with the equalization filter to compensate for inter-symbol interference (ISI) over the twisted pair
cable. Because the amplitude loss and phase distortion is a function of the cable length, the equalizer must adjust its
characteristics to optimize performance. In this design, the variable equalizer makes 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 self-adjusts against environmental changes such as temperature variations.
Next, the equalized signal goes through a DC-restoration and data-conversion block. The DC-restoration circuit
compensates for the effect of baseline wander and improves the dynamic range. The differential data-conversion circuit
converts 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 convert the NRZI signal to NRZ format. This signal is sent through the de-scrambler, then the 4B/5B decoder. Finally,
the NRZ serial data is converted to MII format and provided as the input data to the MAC.
Scrambler/De-Scrambler (100Base-TX Only)
The scrambler spreads the power spectrum of the transmitted signal to reduce electromagnetic interference (EMI) and
baseline wander. The de-scrambler recovers the scrambled signal.
10Base-T Transmit
The 10Base-T drivers are incorporated with the 100Base-TX drivers to allow for transmission using the same magnetic.
The drivers perform internal wave-shaping and pre-emphasis, and output 10Base-T signals with a typical amplitude of
2.5V peak. The 10Base-T signals have harmonic contents that are at least 27dB below the fundamental frequency when
driven by an all-ones Manchester-encoded signal.