LAN91C111 datasheet(39/125 Pages) MICROCHIP

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DS00002276A-page 39

LAN91C111

The transmit byte count least significant bit will be assumed 0 by the controller regardless of the value written in memory.

DATA AREA - The data area starts at offset 4 of the packet structure and can extend up to 2043 bytes.

The data area contains six bytes of DESTINATION ADDRESS followed by six bytes of SOURCE ADDRESS, followed

by a variable-length number of bytes. On transmit, all bytes are provided by the CPU, including the source address. The

LAN91C111 does not insert its own source address. On receive, all bytes are provided by the CSMA side.

The 802.3 Frame Length word (Frame Type in Ethernet) is not interpreted by the LAN91C111. It is treated transparently

as data both for transmit and receive operations.

CONTROL BYTE - For transmit packets the CONTROL BYTE is written by the CPU as:

ODD - If set, indicates an odd number of bytes, with the last byte being right before the CONTROL BYTE. If clear, the

number of data bytes is even and the byte before the CONTROL BYTE is not transmitted.

CRC - When set, CRC will be appended to the frame. This bit has meaning only if the NOCRC bit in the TCR is set.

For receive packets the CONTROL BYTE is written by the controller as:

ODD - If set, indicates an odd number of bytes, with the last byte being right before the CONTROL BYTE. If clear, the

number of data bytes is even and the byte before the CONTROL BYTE should be ignored.

8.2

Receive Frame Status

This word is written at the beginning of each receive frame in memory. It is not available as a register.

ALGNERR - Frame had alignment error.

BROADCAST - Receive frame was broadcast. When a broadcast packet is received, the MULTCAST bit may be also

set on the status word in addition to the BRODCAST bit. The software implement may just ignore the MULTCAST bit if

for BRODCAST packet.

BADCRC - Frame had CRC error, or RX_ER was asserted during reception.

ODDFRM - This bit when set indicates that the received frame had an odd number of bytes.

TOOLNG - Frame length was longer than 802.3 maximum size (1518 bytes on the cable).

TOOSHORT - Frame length was shorter than 802.3 minimum size (64 bytes on the cable).

ODD

CRC

0000

ODD

00000

HIGH

BYTE

ALGN

ERR

BROD

CAST

BAD

CRC

ODD

FRM

TOOLNG

TOO

SHORT

LOW

BYTE

HASH VALUE

MULT

CAST

Reserved

321

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