Philips Semiconductors

Product specification

NE83C92

Low-power coaxial Ethernet transceiver

1995 May 1

6

FUNCTIONAL DESCRIPTION

The NE83C92 is a low power BiCMOS coaxial Ethernet transceiver

which complies with the IEEE 802.3 specification and offers the

following features:

1. Low current consumption of typically 15mA when idling and

70mA while transmitting and no collision allows smaller DC-DC

converter to be used for the isolated power supply. (No external

pull-down resistors.)

2. Automatic selection of the AUI connector (for remote MAU)

instead of the direct local (Thin) coaxial connection is possible by

automatically placing the AUI drivers in high-impedance state

when the local coaxial cable is disconnected. This eliminates the

need for changing a jumper position on the Ethernet board when

selecting either one of the connections. (Automatic selection of

the local (Thin) connection is done by disconnecting the AUI

cable and reconnecting the local coaxial one, which allows the

NE83Q92 to automatically activate itself.)

3. High-efficiency AUI drivers for the RX

± and CD± ports

automatically power down when idling and are powered up when

a receive signal is detected. This is very important/useful for

power sensitive applications such as lap-top computers or

PCMCIA cards.

4. The NE83C92 advanced AUI driver (RX

± and CD±) design does

require external pull-down resistors (500

Ω) to drive a terminated

(78

Ω) AUI cable. However, these drivers will operate correctly

without the external resistors for integrated/local MAU

applications where no AUI cable is used. Hence, they can be

retro-fitted into existing 8392 designs with or without external

pull-down resistors depending on the application. An extra

current of 7mA/output (for 500

Ω resistors) would be generated,

by these resistors, regardless of whether the transceiver is idle

or responding to traffic.

Receiver Functions

The receiver consists of an input buffer, a cable equalizer, a 4-pole

Bessel low pass filter, a squelch circuit and a differential line driver.

The buffer provides high input resistance and low input capacitance

to minimize loading and reflections on the coaxial cable.

The equalizer is a high pass filter that compensates for the low pass

effect of the coaxial cable and results in a flatband response over all

signal frequencies to minimize signal distortion.

The 4-pole Bessel low pass filter extracts the average DC voltage

level on the coaxial cable for use by the receiver squelch and

collision detection circuits.

The receiver squelch circuit prevents noise on the coaxial cable

from falsely triggering the receiver in the absence of a true signal.

At the beginning of a packet, the receiver turns on when the DC

level from the low pass filter exceeds the DC squelch threshold and

the received packet has started with a 01 bit sequence with

acceptable timing parameters. For normal signal levels this will take

less than 500ns, or 5 bits. However, at the end of a packet, a fast

receiver turn off is needed to reject both dribble bits on the coaxial

cable and spurious responses due to settling of the on-chip

bandpass filter. This is accomplished by an AC timing circuit that

disables the receiver if the signal level on the coaxial cable remains

high for typically 250ns and only enables the receiver again after

approximately .5

µs. Figures 3 and 4 illustrate receiver timing.

The differential line driver provides typically +900mV signals to the

DTE with less than 7ns rise and fall times. When in idle state (no

received signal) its outputs provide <20mV differential voltage offset

to minimize DC standing current in the isolation transformer.

Transmitter Functions

The transmitter has differential inputs and an open collector current

driver output. The differential input common mode voltage is

established by the CTI and should not be altered by external

circuitry. Controlled rise and fall times of 25ns (+5ns) minimize

higher harmonic components in the transmitted spectrum, while

matching of these rise and fall times to typically 2ns minimizes

signal jitter. The drive current levels of the CTI are set by an on-chip

bandgap voltage reference and an external 1% resistor. An on-chip

isolation diode is provided to reduce the transmitter’s coaxial cable

load capacitance. For Thin Ethernet applications, no further external

isolation diode is required, since the NE83C92 meets the capacitive

loading specifications. For Ethernet applications a further external

diode should be added to reduce loading capacitance.

The transmitter squelch circuit ensures that the transmitter can only

be enabled if the transmitted packet begins with a 01 bit sequence

where the negative-going differential signals are typically greater

than 225mV in magnitude and 25ns in duration.

The transmitter will be disabled at the end of a packet if there are no

negative going signals of greater than 225mV for more than typically

150ns. Figure 5 illustrates transmitter timing.

Collision Functions

The collision detection scheme implemented in the NE83C92 is

receive mode detection, which detects a collision between any two

stations on the network with certainty at all times, irrespective of

whether or not the local DTE is producing one of the colliding

signals. This is the only detection scheme allowed by the IEEE

802.3 standard for both repeater and non-repeater nodes.

The collision circuitry consists of the 4-pole Bessel low pass filter, a

comparator, a precision voltage reference that sets up the collision

threshold, a heartbeat generator, a 10MHz oscillator, and a

differential line driver.

The collision comparator monitors the DC level at the output of the

low pass filter and enables the line driver if it is more negative than

the collision threshold. A collision condition is indicated to the DTE

by a 10MHz oscillation signal at the CD outputs and typically occurs

within 700ns of the onset of the collision. The collision signal begins

with a negative-going pulse and ends with a continuous high-to-idle

state longer than 170ns. Figure 6 illustrates collision timing.

At the end of every transmission, the heartbeat generator creates a

pseudo collision to ensure that the collision circuitry is properly

functioning. This pseudo collision consists of a 1

µs burst of 10MHz

oscillation at the line driver outputs approximately 1

µs after the end

of the transmission. The heartbeat function can be disabled

allows the CTI to be used in repeater applications. Figure 7

illustrates heartbeat timing.

Jabber Functions

The jabber timer monitors the transmitter and inhibits transmission if

it is active for longer than typically 30ms. The jabber circuit then

enables the collision outputs for the remainder of the data packet

and for typically 450ns (unjab time) after it has ended. At this point

the transmitter becomes uninhibited. Figure 8 illustrates jabber

timing.