8101 datasheet(33/172 Pages) LSI

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Transmit MAC

2-15

2.7 Transmit MAC

To generate an Ethernet MAC frame from the transmit FIFO, the transmit

MAC section:

• Generates preamble and SFD

• Pads undersize packets with zeros to meet minimum packet size

requirements

• Calculates and appends a CRC value to the packet

• Maintains a minimum interpacket gap

Each of the above four operations can be individually disabled and

altered if desired. The transmit MAC then sends the fully formed Ethernet

packet to the 8B10B PCS block for encoding. The transmit MAC section

also generates MAC control frames.

2.7.1 Preamble and SFD Generation

The transmit MAC normally appends the preamble and SFD to the

packet. To program the controller to not append the preamble and SFD

to the transmit packet, clear the TXPRMBL bit in “Register 7–

Conﬁguration 1‚" Section 4.3.8.

2.7.2 AutoPad

The transmit MAC normally AutoPads packets. AutoPadding is the

process of automatically adding enough zeroes in packets with data

ﬁelds less than 46 bytes to make the data ﬁeld exactly 46 bytes in length

which meets the 46-byte minimum data ﬁeld requirement of IEEE 802.3.

To program the controller to not AutoPad, clear the APAD bit in

“Register 7–Conﬁguration 1‚" Section 4.3.8.

2.7.3 CRC Generation

The transmit MAC normally appends the CRC value to the packet. To

program the controller to not append the CRC value to the end of the

packet from the transmit FIFO, assert the TXCRCn pin or clear the

TXCRC bit in Register 7 “Conﬁguration 1”, as described in Table 2.3 and

as described in “Register 7–Conﬁguration 1‚" Section 4.3.8.

8101 Datasheet(PDF) 33 Page - LSI Computer Systems