LAN91C110 datasheet(13/55 Pages) SMSC

13 / 55 page

FEAST Fast Ethernet Controller for PCMCIA and Generic 16-Bit Applications

SMSC DS – LAN91C110 REV. B

Page 13

Rev. 09/05/02

DATA STRUCTURES AND REGISTERS

PACKET FORMAT IN BUFFER MEMORY

The packet format in memory is similar for the Transmit and Receive areas. The first word is reserved for the status word.

The next word is used to specify the total number of bytes, and it is followed by the data area. The data area holds the

packet itself.

FIGURE 5 - DATA PACKET FORMAT

TRANSMIT PACKET

RECEIVE PACKET

STATUS WORD

Written by CSMA upon transmit

completion (see Status Register)

Written by CSMA upon receive

completion (see RX Frame

Status Word)

BYTE COUNT

Written by CPU

Written by CSMA

DATA AREA

Written/modified by CPU

Written by CSMA

CONTROL BYTE

Written by CPU to control

odd/even data bytes

Written by CSMA; also has

odd/even bit

BYTE COUNT - Divided by two, it defines the total number of words including the STATUS WORD, the BYTE

COUNT WORD, the DATA AREA and the CONTROL BYTE.

The receive byte count always appears as even; the ODDFRM bit of the receive status word indicates if the low byte of

the last word is relevant.

RESERVED

BYTE COUNT (always even)

STATUS WORD

DATA AREA

LAST DATA BYTE (if odd)

bit0

bit15

RAM

OFFSET

(DECIMAL)

2046 Max

CONTROL BYTE

Last Byte

1st Byte

2nd Byte

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