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AM79C98

Twisted-Pair Ethernet Transceiver (TPEX)

Advanced Micro Devices 

standard is generated at a transmitter and passed

through 100 m of twisted-pair cable.

The polarity detection/correction algorithm will remain

“armed” until two consecutive packets with valid ETD of

identical polarity are detected. When “armed,” the re-

ceiver is capable of changing the initial or previous

polarity configuration based on the most recent ETD

polarity.

On receipt of the first packet with valid ETD following

reset or Link Fail, TPEX will utilize the inferred polarity

information to configure its RXD± input, regardless of

its previous state. On receipt of a second packet with a

valid ETD with correct polarity, the detection/correction

algorithm will “lock in” the initial polarity. If the second

(or subsequent) packet is not detected as confirming

the previous polarity decision, the most recently de-

tected ETD polarity will be used as the new default.

Note that packets with invalid ETD have no effect on

updating the previous polarity decision. Once two con-

secutive packets with valid ETD have been received,

TPEX will disable the detection/correction algorithm

until either a Link Fail condition occurs or PRDN/RST is

asserted.

During polarity reversal, the RXPOL pin is internally

pulled HIGH. During normal polarity conditions, the

RXPOL pin is driven LOW and is capable of directly

driving a “Polarity OK” LED using an integrated 16 mA

driver. If desired, the polarity reversal function can be

disabled by grounding the RXPOL pin.

Twisted-Pair Interface Status

Two outputs (XMT and RCV) indicate whether TPEX is

transmitting (AUI to twisted-pair) or receiving (twisted-

pair to AUI). Both signals are asserted during a colli-

sion. In Link Fail mode, RCV is disabled. In Jabber

Detect mode, XMT is disabled. Both signals are active

HIGH.

Collision Detect Function

Simultaneous carrier sense (presence of valid data sig-

nals) by both the AUI DO+/– pair and the RXD+/– pair

constitutes a collision, thereby causing a 10 MHz signal

to be asserted on the CI+/– output pair. The CI+/– output

meets the drive requirements for the AUI. This 10 MHz

signal will remain on the CI+/– pair until one of the two

colliding states changes from active to idle. The CI+/–

output pair stays HIGH for two bit times at the end of a

collision, decreasing to the idle level within eighty bit

times after the last LOW-to-HIGH transition. Both the

XMT and RCV pins are driven HIGH during collision.

Signal Quality Error (SQE) Test (Heartbeat)

Function

When the SQE TEST pin is driven LOW, TPEX will

routinely exercise the collision detection circuitry by

generating an SQE message at the end of every trans-

mission. This signal is a self-test indication to the DTE

that the MAU collision circuitry is functional. An SQE

message consists of a 10 MHz signal on the CI+/–

pair with a duration of 8 bit times (800 ns). When en-

abled, an SQE Test will occur at the end of every trans-

mission, starting eight bit times (800 ns) after the last

transition of the transmitted signal. For repeater appli-

cations, the SQE Test function can be disabled by tying

the SQE TEST pin HIGH or by leaving it disconnected.

Jabber Function

The Jabber function inhibits the twisted-pair transmit

function of TPEX if the DO+/– circuit is active longer

than the time permitted to transmit the maximum-

length 802.3/Ethernet data packet (50 ms nominal).

This prevents any one node from disrupting the net-

work due to a “stuck on” or faulty transmitter. If this

maximum transmit time is exceeded, TPEX transmitter

circuitry is disabled and a 10 MHz signal is driven onto

the CI+/– pair. Once the transmit data stream is re-

moved from the DO+/– pair of inputs, an “unjab” time of

250 ms to 750 ms will elapse before TPEX removes the

10 MHz signal from the CI+/– pair and re-enables the

transmit path.

Power Down

In addition to on-board power-on-reset circuitry, the

PRDN/RST pin is used as the master reset for TPEX.

PRDN/RST must be driven LOW for a minimum of two

microseconds for reset to occur. The PRDN/RST pin

can also be used to put the TPEX into an inactive state,

causing the device to consume less power. This feature

is useful in battery-powered or low-duty-cycle systems.

Driving PRDN/RST LOW resets the internal logic of

TPEX and places the device into idle mode. In this

mode, the twisted-pair driver pins (TXD+/–,TXP+/–)

are driven LOW, the AUI pins (CI+/–, DI+/–) are driven

HIGH, the LNKST and RXPOL pins are in the inactive

state, and XMT and RCV are LOW. TPEX will remain in

idle as long as PRDN/RST is asserted. Following the

rising edge of the signal on PRDN/RST, TPEX will re-

main in the reset state for 10 µs.

Test Modes

TPEX implements two types of loopback test modes

suitable for Station (DTE) or Repeater applications.

The Test mode is entered by driving the TEST pin

HIGH. The two types of test modes available are:

1. Station (DTE): SQE TEST pin LOW. Data on DO+/–

pair is transmitted onto the TXD+/– and TXP+/–

pairs and data on the RXD+/– input pair is transmit-

ted onto the DI+/– output pair. The jabber function

and collision detection functions are disabled.

2. Repeater: SQE TEST pin HIGH. Data on DO+/–

pair is looped back onto the DI+/– pair and data on

the RXD+/– pair is retransmitted on the twisted-pair

drivers (TXD+/– and TXP+/– pairs).

Am79C98

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