AM79C98 Advanced Micro Devices, AM79C98 Datasheet - Page 9

• Manufacturer Part Number: AM79C98
• Description: Twisted-Pair Ethernet Transceiver (TPEX)
• Manufacturer: 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-
Am79C98
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 s.
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+/–
2. Repeater: SQE TEST pin HIGH. 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.
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).
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