
Functional Description
Copyright 1997-2002 by LSI Logic Corporation. All rights reserved.
35 of 128
April, 2002
The reception of MAC Control frames is unaffected by these bits and is
controlled by other bits described in
Section 3.16, “MAC Control Frames,”
page 52
.
3.5.8 Frame Validity Checks
The receive MAC determines the validity of each receive packet by
checking for (1) valid FCS, (2) oversize packet, and (3) undersize packet.
Valid FCS is determined by computing the CRC value on the incoming
receive packet per IEEE 802.3 specifications and comparing it against
the actual CRC value present in the FCS field of the received packet. If
the values are not the same, (1) the frame is determined to be invalid
and the packet is discarded, if the discard feature is enabled (2) the CRC
error detect bit is set in the RX Status register, (3) the CRC error bit is
set in the receive status word for the packet and (4) the interrupt pin for
the port is asserted provided the interrupt function is enabled. Refer to
Section 3.9, “Packet Discard,” page 41
, for more information about
discards. The device can be programmed to not discard and accept all
receive packets with bad FCS by setting the CRC error accept bit in the
RX Command register.
Oversize packets are packets whose length is greater than the maximum
packet size. If a received packet is an oversize packet, then (1) the
packet is determined to be invalid and it is discarded if the discard
feature is enabled, (2) the oversize packet detect bit is set in the RX
Status register, and (3) the interrupt pin for the port is asserted provided
the interrupt function is enabled. Refer to
Section 3.9, “Packet Discard,”
page 41
for more information about discards. The maximum packet size
can be programmed to be any one of sixteen values between 1518 bytes
and 1533 bytes, exclusive of preamble & SFD, by appropriately setting
the maximum packet size select bits in the Configuration 5 register. The
device also can be programmed to not discard and accept all receive
oversize packets, regardless of size, by setting the oversize packet
accept bit in the RX Command register.
Undersize packets are packets whose length is less than the minimum
packet size. Minimum packet size is defined to be 64 bytes, exclusive of
preamble & SFD. If a received packet is an undersize packet, then (1)
the frame is determined to be invalid and it is discarded if the discard
feature is enabled, (2) the undersize packet detect bit is set in the RX
Status register, and (3) the interrupt pin for the port is asserted provided