AM79C98 Advanced Micro Devices, AM79C98 Datasheet - Page 8

• Manufacturer Part Number: AM79C98
• Description: Twisted-Pair Ethernet Transceiver (TPEX)
• Manufacturer: Advanced Micro Devices

8
FUNCTIONAL DESCRIPTION
The Twisted-Pair Ethernet Transceiver (TPEX) com-
plies with the requirements specified by the IEEE 802.3
standard for the attachment unit interface (AUI) and the
standard for 10BASE-T medium attachment unit
(MAU). TPEX also implements a number of features in
addition to the IEEE 802.3 standard. An outline of func-
tions implemented by the Am79C98 is given below.
Attachment Unit Interface
(DO+/–, DI+/–, CI+/–)
The AUI electrical and functional characteristics com-
ply with those specified by the IEEE 802.3, Sections 7
and 14 (drafted). The AUI pins can be wired directly to
the isolation transformer, for a remote MAU application,
or to another device (e.g., Am7992 serial interface
adapter). The end-of-packet SQE Test function (heart-
beat) can be disabled to allow the device to be em-
ployed in a repeater application.
Twisted-Pair Transmit Function
Data transmission to the 10BASE-T medium occurs
when valid AUI signals appear on the DO+/–differential
pair. This data stream is routed to the differential driver
circuitry in the TXD+/– pins. The driver circuitry pro-
vides necessary electrical driving capability and pre-
distortion control for transmitting signals over
maximum-length twisted-pair cable, as specified by the
IEEE 802.3 10BASE-T standard. The transmit function
meets the propagation delays and jitter specified by the
standard. During transmission, the XMT pin is driven
HIGH and can be used for status information.
Twisted-Pair Receive Function
The receiver complies with the receiver specifications
of the IEEE 802.3 10BASE-T standard, including noise
immunity and received signal rejection criteria (“Smart
Squelch”). Signals meeting these criteria appearing at
the RXD+/– differential input pair are routed to the DI+/–
outputs. The receiver function meets the propagation
delays and jitter requirements specified by the stan-
dard. Receiver squelch level drops to approximately
half its threshold value after unsquelch to allow recep-
tion of minimum amplitude signals and to offset carrier
fade in the event of worst-case signal attenuation and
crosstalk noise conditions. During receive, the RCV pin
is driven HIGH and can be used for status information.
Link Test Function
The Link Test function is implemented as specified by
the IEEE 802.3 10BASE-T standard. During periods of
transmit pair inactivity, Link Test pulses will be periodi-
cally sent over the twisted-pair medium to allow con-
stant monitoring of medium integrity. When the Link
Test function is enabled, the absence of Link Test
pulses on the RXD+/– pair will cause the TPEX to go
into a Link Fail state. In Link Fail state, data transmission,
Am79C98
data reception, and the collision detection functions are
disabled, and remain disabled until valid data or >2
consecutive Link Test pulses appear on the RXD+/–
pair. During Link Fail, the LNKST pin is internally pulled
HIGH. When the link is identified as functional, the
LNKST pin is driven LOW and is capable of directly
driving a “link OK” LED. In order to interoperate with
systems that do not implement Link Test, this function
can be disabled by grounding the LNKST pin. When
disabled, the driver and receiver functions remain en-
abled irrespective of the presence or absence of data
or Link Test pulses on the RXD+/– pair. The transmitter
continues to generate Link Test pulses in the absence
of transmit data even if the Link Test function is
disabled.
Polarity Detection and Reversal
The TPEX receive function includes the ability to invert
the polarity of the signals appearing at the RXD pair if
the polarity of the received signal is reversed (such as
in the case of a wiring error). This feature allows data
packets received from a reverse-wired RXD input pair
to be corrected in the TPEX prior to transfer to the DTE
via the AUI interface (DI ). The polarity detection func-
tion is activated following reset or Link Fail, and will re-
verse the receive polarity based on both the polarity of
any previous Link Test pulses and the polarity of subse-
quent packets with a valid end transmit delimiter (ETD).
When in the Link Fail state, TPEX will recognize Link
Test pulses of either positive or negative polarity. Exit
from the Link Fail state is caused by the reception of
five to six consecutive Link Test pulses of identical po-
larity. On entry to the Link Pass state, the polarity of the
last five Link Test pulses is used to determine the initial
receive polarity configuration and the receiver is recon-
figured to subsequently recognize only Link Test pulses
of the previously established polarity. This link pulse
algorithm is employed only until ETD polarity determi-
nation is made, as described later in this section.
Positive Link Test pulses are defined as received sig-
nals with a positive amplitude greater than 520 mV and
a pulse width of 60 ns to 200 ns. This positive excursion
may be followed by a negative excursion. This definition
is consistent with the expected received signal at a cor-
rectly wired receiver when a Link Test pulse that fits the
template of Figure 14-12 in the 10BASE-T standard is
generated at a transmitter and passed through 100 m
of twisted-pair cable.
Negative Link Test pulses are defined as received sig-
nals with a negative amplitude greater than 520 mV
and a pulse width of 60 ns to 200 ns. This negative ex-
cursion may be followed by a positive excursion. This
definition is consistent with the expected received signal
at a reverse wired receiver when a Link Test pulse that
fits the template of Figure 14-12 in the 10BASE-T