AM79C940JC/W AMD [Advanced Micro Devices], AM79C940JC/W Datasheet - Page 42

• Manufacturer Part Number: AM79C940JC/W
• Manufacturer: AMD [Advanced Micro Devices]

mit Frame Status (bit 7) after the packet has been
transmitted.
Digital Attachment Interface (DAI)
The Digital Attachment Interface is a simplified electri-
cal attachment specification which allows MAUs which
do not require the DC isolation between the MAU and
DTE (e.g. devices compatible with the 10BASE-T Stan-
dard and 10BASE-FL Draft document) to be imple-
mented. All data transferred across the DAI port is
Manchester Encoded. Decoding and encoding is
performed by the MENDEC.
The DAI port will accept receive data on the basis that
the RXCRS input is active, and will take the data pre-
sented on the RXDAT input as valid Manchester data.
Transmit data is sent to the external transceiver by the
MACE device asserting TXEN and presenting compli-
mentary data on the TXDAT pair. During idle, the
MACE device will assert the TXDAT+ line high, and the
TXDAT line low, while TXEN is maintained inactive
(high). The MACE device implements logical collision
detection and will use the simultaneous assertion of
TXEN and RXCRS to internally detect a collision con-
dition, take appropriate internal action (such as abort
the current transmit or receive activity), and provide
external indication using the CLSN pin. Any external
transceiver utilized for the DAI interface must not loop
back the transmit data (presented by the MACE de-
vice) on the TXDAT pins to the RXDAT pin. Neither
should the transceiver assert the RXCRS pin when
transmitting data to the network. Duplication of these
functions by the external transceiver (unless the MACE
device is in the external loop back test configuration)
will cause false collision indications to be detected.
In order to provide an integrity test of the connectivity
between the MACE device and the external transceiver
similar to the SQE Test Message provided as a part of
the AUI functionality, the MACE device can be pro-
grammed to operate the DAI port in an external loop-
back test. In this case, the external transceiver is
assumed to loopback the TXDAT data stream to the
RXDAT pin, and assert RXCRS in response to the
TXEN request. When in the external loopback mode of
operation (programmed by LOOP [1-0] = 01), the
MACE device will not internally detect a collision condi-
tion. The external transceiver is assumed to take action
to ensure that this test will not disrupt the network. This
type of test is intended to be operated for a very limited
period (e.g. after power up), since the transceiver is as-
sumed to be located physically close to the MACE
device and with minimal risk of disconnection (e.g. con-
nected via printed circuit board traces).
Note that when the DAI port is selected, LCAR errors
will not occur, since the MACE device will internally loop
back the transmit data path to the receiver. This loop
back function must not be duplicated by a transceiver
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Am79C940
which is externally connected via the DAI port, since this
will result in a condition where a collision is generated
during any transmit activity.
The transmit function of the DAI port is protected by a
jabber mechanism which will be invoked if the TXDAT
and TXEN circuit is active for an excessive period (20 -
150 ms). This prevents a single node from disrupting
the network due to a stuck-on or faulty transmitter. If
this maximum transmit time is exceeded, the DAI port
transmitter circuitry is disabled, 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 cleared. Once the
internal transmit data stream from the MENDEC stops
(TXEN deasserts), an unjab time of 250 ms-750 ms will
elapse before the MACE device deasserts the CLSN
indication 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
internal MENDEC activity) and set the TXDAT+ and
TXDAT pins to their inactive state.
10BASE-T Interface
Twisted Pair Transmit Function
Data transmission over the 10BASE-T medium
requires use of the integrated 10BASE-T MAU, and
uses the differential driver circuitry in the TXD and
TXP pins. The driver circuitry provides the necessary
electrical driving capability and the pre-distortion con-
trol for transmitting signals over maximum length
Twisted Pair cable, as specified by the 10BASE-T
supplement to the IEEE 802.3 Standard. The transmit
function for data output meets the propagation delays
and jitter specified by the standard. During normal
transmission, and providing that the 10BASE-T MAU is
not in a Link Fail or jabber state, the TXEN pin will
be driven HIGH and can be used indirectly to drive a
status LED.
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 this criteria appearing at the
RXD differential input pair are routed to the internal
MENDEC. The receiver function meets the propaga-
tion delays and jitter requirements specified by the
10BASE-T Standard. The receiver squelch level drops
to half its threshold value after unsquelch to allow
reception of minimum amplitude signals and to mitigate
carrier fade in the event of worst case signal attenua-
tion and crosstalk noise conditions. During receive, the
RXCRS pin is driven HIGH and can be used indirectly
to drive a status LED.
Note that the 10BASE-T Standard defines the receive input
amplitude at the external Media Dependent Interface