KS8993

Micrel

KS8993

16

April 2009

Functional Overview: Physical Layer Transceiver

100BaseTX Transmit

The 100BaseTX transmit function performs parallel to serial conversion, 4B/5B coding, scrambling, NRZ to NRZI conversion,

MLT3 encoding and transmission. The circuit starts with a parallel to serial conversion, which converts the data from the MAC

into a 125MHz serial bit stream. The data and control stream is then converted into 4B/5B coding followed by a scrambler. The

serialized data is further converted from NRZ to NRZI format, then transmitted in MLT3 current output. The output current is

set by an external 1% 3.01k

Ω resistor for the 1:1 transformer ratio. It has a typical rise/fall time of 4ns and complies to the ANSI

TP-PMD standard regarding amplitude balance, overshoot and timing jitters.

100BaseTX Receive

The 100BaseTX 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 inter-symbol interference (ISI) over the twisted pair cable. Since the amplitude loss

and phase distortion is a function of the length of the cable, the equalizer has to adjust its characteristics 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 for the 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

convert the NRZI signal into the NRZ format. The signal is then sent through the de-scrambler followed by the 4B/5B decoder.

Finally, the NRZ serial data is provided as the input data to the MAC.

PLL Clock Synthesizer

The KS8993 generates clocks for the external MII and SNI interface based on the interface type selected.

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. The

data is scrambled by the use of an 11-bit wide linear feedback shift register (LFSR). This can generate a 2047-bit non-repetitive

sequence. The receiver will then de-scramble the incoming data stream with the same sequence at the transmitter.

100BaseFX Operation

100BaseFX operation is very similar to 100BaseTX operation with the differences being that the scrambler/de-scrambler and

MLT3 encoder/decoder are bypassed on transmission and reception. In this mode the auto-negotiation feature is bypassed

since there is no standard that supports fiber auto-negotiation.

100BaseFX Signal Detection

The physical port runs in 100BaseFX mode if FXSDx >.6V. This signal is referenced to VREFx which is set at 1/2 Vdd but can

be overridden by an external level. VREFx can be connected to the “minus” signal of a differential pair coming from the fiber

module (“plus connects to FXSDx) used to convey signal detect. When FXSDx is below .6V then 100BaseFX mode is disabled.

100BaseFX Far End Fault

Far end fault occurs when the signal detection is logically false from the receive fiber module. When this occurs, the

transmission side signals the other end of the link by sending 84 1’s followed by a zero in the idle period between frames.

Far End Fault can be disabled by setting external hardware pin TEST[2]=0 and TEST[1] = float. See

“I/O Description” for pin

description.

10BaseT Transmit

The output 10BaseT driver is incorporated into the 100BaseT driver to allow transmission with the same magnetic. They are

internally wave-shaped and pre-emphasized into outputs with a typical 2.2V amplitude. The harmonic contents are at least

27dB below the fundamental when driven by an all-ones Manchester-encoded signal.

Special note for 10BaseT operation: With an operating voltage of 2.5V, the KS8993 does not always achieve the specified

transmit voltage swing greater than or equal to 2.2V as specified by IEEE 802.3. The important factor however is that the

KS8993 does adhere to the specified receive signal voltages using the IEEE twisted pair model with a 100

Ω load. The transmit

voltage swing can be increased to 2.2V or above by increasing the supply voltage to 2.65V if so desired.

10BaseT Receive

On the receive side, input buffer and level detecting squelch circuits are employed. A differential input receiver circuit and a

PLL perform the decoding function. The Manchester-encoded data stream is separated into clock signal and NRZ data. A