AM79C940JCW Advanced Micro Devices, AM79C940JCW Datasheet - Page 43

• Manufacturer Part Number: AM79C940JCW
• Description: Media Access Controller for Ethernet (MACE)
• Manufacturer: Advanced Micro Devices

receiver is capable of changing the initial or previous po-
larity configuration based on the most recent ETD polar-
ity.
On receipt of the first packet with valid ETD following re-
set or Link Fail, the MACE device will utilize the inferred
polarity information to configure its RXD input, regard-
less of its previous state. On receipt of a second packet
with a valid ETD with correct polarity, the detection/cor-
rection algorithm will lock-in the received polarity. If the
second (or subsequent) packet is not detected as con-
firming the previous polarity decision, the most recently
detected ETD polarity will be used as the default. Note
that packets with invalid ETD have no effect on updating
the previous polarity decision. Once two consecutive
packets with valid ETD have been received, the MACE
device will disable the detection/correction algorithm
until either a Link Fail condition occurs or a hardware or
software reset occurs.
During polarity reversal, the RXPOL pin should be exter-
nally pulled HIGH and the Reversed Polarity bit
(REVPOL in the PHY Configuration Control register) will
be set. During normal polarity conditions, the RXPOL
pin is driven LOW (capable of directly driving a Polarity
OK LED using an integrated 12 mA driver) and the REV-
POL bit will be cleared.
If desired, the polarity correction function can be dis-
abled by setting the Disable Auto Polarity Correction bit
(DAPC bit in the PHY Configuration Control register).
However, the polarity detection portion of the algorithm
continues to operate independently, and the RXPOL pin
and the REVPOL bits will reflect the polarity state of the
receiver.
Twisted Pair Interface Status
Three outputs (TXEN, RXCRS and CLSN) indicate
whether the MACE device is transmitting (MENDEC
to Twisted Pair), receiving (Twisted Pair to MENDEC),
or in a collision state with both functions active
simultaneously.
The MACE device will power up in the Link Fail state.
The normal algorithm will apply to allow it to enter the
Link Pass state. On power up, the TXEN, RXCRS and
CLSN) pins will be in a high impedance state until they
are enabled by setting the Enable PLS I/O bit (ENPLSIO
in the PLS Configuration Control register) and the
10BASE-T port enters the Link Pass state.
In the Link Pass state, transmit or receive activity which
passes the pulse width/amplitude requirements of the
DO or RXD inputs, will be indicated by the TXEN or
RXCRS pin respectively going active. TXEN, RXCRS
and CLSN are all asserted during a collision.
In the Link Fail state, TXEN, RXCRS and CLSN are
inactive.
Am79C940
In jabber detect mode, the MACE device will activate the
CLSN pin, disable TXEN (regardless of Manchester
data output from the MENDEC), and allow the RXCRS
pin to indicate the current state of the RXD pair. If there
is no receive activity on RXD , only CLSN will be active
during jabber detect. If there is RXD activity, both
CLSN and RXCRS will be active.
If the SLEEP pin is asserted (regardless of the program-
ming of the AWAKE or RWAKE bits in the PHY Configu-
ration Control register), the TXEN, RXCRS and CLSN
outputs will be placed in a high impedance state.
Collision Detect Function
Simultaneous activity (presence of valid data signals)
from both the internal MENDEC transmit function (indi-
cated externally by TXEN active) and the twisted pair
RXD pins constitutes a collision, thereby causing an
external indication on the CLSN pin, and an internal indi-
cation which is returned to the MAC core. The TXEN,
RXCRS and CLSN pins are driven high during collision.
Signal Quality Error (SQE) Test
(Heartbeat) Function
The SQE Test message (a 10 MHz burst normally re-
turned on the AUI CI pair at the end of every transmis-
sion) is intended to be a self-test indication to the DTE
that the MAU collision circuitry is functional and the AUI
cable/connection is intact. This has minimal relevance
when the 10BASE-T MAU is embedded in the LAN con-
troller. A Collision Error (CERR bit in the Interrupt Regis-
ter) will be reported only when the 10BASE-T port is in
the link fail state, since the collision circuit of the MAU
will be disabled, causing the absence of the SQE Test
message. In GPSI mode the external encoder/decoder
is responsible for asserting the CLSN pin after each
transmission. In DAI mode SEQ Test has no relevance.
Jabber Function
The Jabber function inhibits the twisted pair transmit
function of the MACE device if the TXD /TXP circuits
are active for an excessive period (20–150 ms). This
prevents any one node from disrupting the network due
to a stuck-on or faulty transmitter. If this maximum trans-
mit time is exceeded, the data path through the
10BASE-T transmitter circuitry is disabled (although
Link Test pulses will continue to be sent), the CLSN pin
is asserted, the Jabber bit (JAB in the Interrupt Register)
is set and the INTR pin will be asserted providing the
JABM bit (Interrupt Mask Register) is cl eared. Once the
internal transmit data stream from the MENDEC stops
(TXEN deasserts), an unjab time of 250–750 ms will
elapse before the MACE device deasserts the CLSN in-
dication and re-enables the transmit circuitry.
When jabber is detected, the MACE device will assert
the CLSN pin, de-assert the TXEN pin (regardless of
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