Datasheet

 2010 Microchip Technology Inc. DS39935C-page 99
ENC424J600/624J600
10.1 CRC Error Collection Filter
The CRC Error Collection filter allows applications to
accept frames with an invalid Frame Check Sequence
(FCS). This filter is primarily intended for performing
network, cable and layout noise immunity diagnostics.
The filter computes the CRC over incoming frame data
and compares the result with the FCS appended at the
end of each frame. If the computed CRC does not
match the FCS, the filter will accept the frame. If the
CRC is correct or the CRC Error Collection filter is
disabled, the frame passes through to the next lower
priority filter.
This filter is disabled at power-up. To enable the filter,
set CRCEEN (ERXFCON<7>). Enabling this filter will
cause frames with bit transmission errors and/or invalid
data to be accepted into the packet buffer. To comply
with IEEE 802.3 specifications, this filter should be
disabled.
10.2 Runt Error Collection Filter
The Runt Error Collection filter allows applications to
accept frames shorter than 64 bytes (counting from the
start of the Ethernet source address to the end of the
Frame Check Sequence, inclusive). Runt packets are
ordinarily generated by early half-duplex collisions and
should not be treated as valid packets. This filter is
primarily intended for detecting duplex mismatches or
performing network utilization diagnostics.
The filter checks the length of each frame and accepts
any frame with a length of 63 or fewer bytes. Frames
that are 64 bytes or longer are passed on to the next
lower priority filter.
This filter is disabled at power-up. To enable the filter,
set RUNTEEN (ERXFCON<5>). If the filter is disabled,
all frames will be passed on to the next lower priority
filter, regardless of length.
To comply with IEEE 802.3 specifications and properly
discard collision fragments, this filter should be
disabled.
10.3 CRC Error Rejection Filter
The CRC Error Rejection filter verifies the Frame
Check Sequence of incoming frames. If the CRC is
invalid, the frame is discarded. Frames with a valid
CRC will be passed on to the next filter.
The MAC truncates received frames if they exceed the
length specified by the MAC maximum frame length
register, MAMXFL. Because the Frame Check
Sequence is always transmitted as the last four bytes
of any Ethernet frame, reception of an oversize frame
that becomes truncated will almost always result in an
invalid CRC. When enabled, the CRC Error Rejection
filter will discard these truncated frames, as well as
collision fragments, and other frames that become
corrupted during transmission.
This filter is enabled at power-up. To disable this filter,
clear CRCEN (ERXFCON<6>). If the filter is disabled,
all frames will be passed on to the next lower priority
filter, regardless of CRC validity.
10.4 Runt Error Rejection Filter
The Runt Error Rejection filter checks the length of
each incoming frame. If the length of the frame is less
than the Ethernet minimum of 64 bytes, the frame will
be discarded. Frames of 64 bytes or larger will be
passed on to the next filter.
This filter is enabled at power-up. To disable this filter,
clear RUNTEN (ERXFCON<4>). If the filter is disabled,
all frames will be passed on to the next lower priority
filter, regardless of length.
10.5 Unicast Collection Filter
The Unicast Collection filter checks the destination
address of each incoming frame. If the destination
address exactly matches the local MAC address,
defined by the MAADR registers, the frame will be
accepted. If there is a mismatch, the frame will be
passed on to the next filter.
This filter is enabled at power-up. To disable this filter,
clear UCEN (ERXFCON<3>). If the filter is disabled, all
frames will be passed on to the next lower priority filter,
regardless of destination address.
10.6 Not-Me Unicast Collection Filter
The Not-Me Unicast Collection filter checks the desti-
nation address of incoming frames. If the destination
address is a Unicast address, but does not exactly
match the contents of the MAADR registers, the frame
will be accepted. This will include any frame specifically
addressed to another station, but will not include Multi-
cast or Broadcast frames. If the packet is a Multicast,
Broadcast or Unicast frame for the local device, the
frame will be passed on to the next filter.
This filter is disabled at power-up. To enable this filter,
set NOTMEEN (ERXFCON<2>). If the filter is disabled,
all frames will be passed on to the next lower priority
filter, regardless of destination address.
Note: Enable this filter for normal IEEE 802.3
compliant operation.
Note: Enable this filter for normal IEEE 802.3
compliant operation.