AM79C940VCW datasheet(51/122 Pages) AMD

51 / 122 page

AMD

Am79C940

current XMTFW value in the FIFO Configuration Control

read pointer will be moved to the start of the next frame

or free location in the FIFO, and the write pointer will be

unaffected.

TDTREQ will not be re-asserted until the

Transmit Frame Status has been read.

After a RTRY error, all further packet transmission will

be suspended until the Transmit Frame Status is read,

regardless of whether additional packet data exists in

the FIFO to be transmitted. Receive FIFO read opera-

tions are not impaired.

Packets experiencing 16 unsuccessful attempt to trans-

mit will not be re-tried. Recovery from this condition

must be performed by upper layer software.

Abnormal network conditions include:

(a) Loss of carrier.

(b) Late collision.

These should not occur on a correctly configured 802.3

network, but will be reported if the network has been in-

correctly configured or a fault condition exists.

(a) A loss of carrier condition will be reported if the

MACE device cannot observe receive activity while it is

transmitting. After the MACE device initiates a transmis-

sion it will expect to see data

looped-back on the receive

input path. This will internally generate a carrier sense,

indicating that the integrity of the data path to and from

the external MAU is intact, and that the MAU is operating

correctly.

When the AUI port is selected, if carrier sense does not

become active in response to the data transmission, or

becomes inactive before the end of transmission, the

loss of carrier (LCAR) error bit will be set in the Transmit

Frame Status (bit 7) after the packet has been transmit-

ted. The packet will not be re-tried on the basis of an

LCAR error.

When the 10BASE-T port is selected, LCAR will be re-

ported for every packet transmitted during the Link fail

condition.

When the GPSI port is selected, LCAR will be reported if

the RXCRS input pin fails to become active during a

transmission, or once active, goes inactive before the

end of transmission.

When the DAI port is selected, LCAR errors will not oc-

cur, since the MACE device will internally loop back the

transmit data path to the receiver. The loop back feature

must not be performed by the external transceiver when

the DAI port is used.

During internal loopback, LCAR will not be set, since the

MACE device has direct control of the transmit and re-

ceive path integrity. When in external loopback, LCAR

will operate normally according to the specific port which

has been selected.

(b) A late collision will be reported if a collision condition

exists or commences 64 byte times (512 bit times) after

the transmit process was initiated (first bit of preamble

commenced). The MACE device will abandon the trans-

mit process for the particular frame, complete transmis-

sion of the jam sequence (32-bit all zeroes pattern),

de-assert the

TDTREQ pin, report the Late Collision

(LCOL) and Transmit Status Valid (XMTSV) in the

Transmit Frame Status, and set the XMTINT bit in the

Interrupt Register, causing activation of the external

INTR pin providing the interrupt is unmasked.

Once the XMTINT condition has been externally recog-

nized, the Transmit Frame Counter (XMTFC) can be

read to determine whether the tail end of the frame that

suffers the LCOL error is still in the host memory (i.e.,

when XMTFC = 0). This XMTFC read should be re-

quested before the Transmit Frame Status read since

reading the XMTFS would cause the XMTFC to decre-

ment. If the tail end of the frame is indeed still in the host

memory, the host is responsible for ensuring that the tail

end of the frame does not get written into the FIFO and

does not get transmitted as a whole frame. It is recom-

mended that the host clear the tail end of the frame from

the host memory before requesting the XMTFS read so

that after the XMTFS read,when the MACE device re-

asserts

TDTREQ, the tail end of the frame does not get

written into the FIFO. The Transmit Frame Status read

will indicate that the LCOL error occurred. The read op-

eration on the Transmit Frame Status will update the

FIFO read and write pointers. If no

End-of-Frame write

(

EOF pin assertion) had occurred during the FIFO write

sequence, the entire transmit path will be reset (which

will update the Transmit FIFO watermark with the cur-

rent XMTFW value in the FIFO Configuration Control

pointer will be moved to the start of the next frame or free

location in the FIFO, and the write pointer will be unaf-

fected.

TDTREQ will not be re-asserted until the Trans-

mit Frame Status has been read.

After an LCOL error, all further packet transmission will

be suspended until the Transmit Frame Status is read,

regardless of whether additional packet data exists in

the FIFO to be transmitted. Receive FIFO operations

are unaffected.

Packets experiencing a late collision will not be re-tried.

Recovery from this condition must be performed by up-

per layer software.

pletion of a transmitted message, the AUI CI

± pair is

asserted by some transceivers as a self-test. When the

AUI port has been selected, the integral Manchester En-

coder/Decoder will expect the SQE Test Message

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