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DM8108

8 port 10/100M Fast Ethernet Switching Controller

Unspecified 

DM8108

8 port 10/100M Fast Ethernet Switching Controller

Media Access Control

The MAC Engine incorporates the essential protocol

requirement for an Ethernet IEEE-802.3 compliant

node, and provides the interface between the FIFO

subsystem and the MII. The MAC has two primary

attributes:

Transmit and receive message data

encapsulation

The MAC will discard the illegally short (less than 64

bytes of frame data) or oversized (greater than 1536

bytes) messages to be transmitted or received.

T Framing (frame boundary delimitation, frame

synchronization)

The MAC engine will automatically handle the

construction of the transmit frame. Once the

transmit FIFO has been filled to the predetermined

threshold and access of the channel is permitted, the

MAC will commence the following for transmission:

The receiving section of the MAC will detect an

incoming preamble sequence when the RXDV signal

is activated by the external PHY. The MAC will

discard the preamble and begin searching for the SFD.

Once the SFD is detected, all the subsequent nibbles

are treated as part of the frame. The MAC will discard

the message if it is shorter than 64-bytes or longer

than 1518 (1536) bytes. The received frame will be

sent to Receiving Buffer for switching.

If the frame terminates or suffers a collision before

64-bytes (after SFD) have been received, the MAC

will automatically delete the frame from FIFO.

T Addressing (source and destination address

handling)

The MAC intercepts the source and destination

address from the incoming frame and send them

to switching engine for the following purposes:

. To update the address table

. To learn the switching target

. To detect the DM8108 predefined address for

the device control functions.

T Error detection (physical medium transmission

errors)

During transmission, if the switching engine

failed to keep the transmit FIFO filled sufficiently,

cause an underflow, the MAC engine will

guarantee the message is either sent as runt

packet (which will be detected by the receiving

station) or as an invalid FCS (which will cause

the receiver to reject the packet).

During reception, the FCS is generated on every

nibble (including the dribbling bits) coming from

the cable, although the internally saved FCS

value is only updated on the eighth bit (on each

byte boundary). The MAC engine will ignore up

to 7 additional bits at the end of a message

(dribbling bits), that can occur under normal

network operating conditions.

Preamble

1010…1010

7

Bytes

SFD

10101011

1

Bytes

Destination

Address

6

Bytes

Source

Address

6

Bytes

Length

Data FCS

2

40 – 1500

4

Bytes

Bytes Bytes

14

Preliminary

Version: DM8108-DS-P02

November 25, 1999

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