AM79C940KCSLASHW datasheet(42/144 Pages) AMD

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Am79C940

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

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

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