Datasheet

ENC424J600/624J600
DS39935C-page 100  2010 Microchip Technology Inc.
10.7 Multicast Collection Filter
The Multicast Collection filter checks the destination
address of incoming frames. If the Least Significant bit
(LSb) of the first byte of the destination address is set,
the frame will be accepted. This represents all Multicast
frames. If the frame has a Unicast destination, it will be
passed on to the next filter.
This filter is disabled at power-up. To enable this filter,
set MCEN (ERXFCON<1>). If the filter is disabled, all
frames will be passed on to the next lower priority filter,
regardless of destination address.
10.8 Broadcast Collection Filter
The Broadcast Collection filter checks the destination
address of incoming frames. If the destination address
is FF-FF-FF-FF-FF-FF, the frame will be accepted.
Frames matching this filter are designated as being
broadcast to all nodes that receive them. All frames
with other address values will be passed on to the next
filter.
This filter is enabled at power-up. To disable this filter,
clear BCEN (ERXFCON<0>). If the filter is disabled, all
frames will be passed on to the next lower priority filter,
regardless of destination address.
10.9 Hash Table Collection Filter
The Hash Table filter accepts frames based on their des-
tination address, and is configurable for up to 64 different
hash values. This filter allows the device to accept
frames for multiple destination addresses (without
accepting all Not-Me traffic as described in
Section 10.6 “Not-Me Unicast Collection Filter”). It
can also be used to accept traffic for one or more
specific Multicast groups (without accepting all Multi-
cast traffic as described in
Section 10.7 “Multicast
Collection Filter”
). Note that Hash Table collisions are
possible, so applications should still verify the MAC
address of accepted frames. This filter simply reduces
the amount of incoming traffic for these applications.
The filter performs a 32-bit CRC over the six destina-
tion address bytes in the packet, using the polynomial,
4C11DB7h. From the resulting 32-bit binary number, a
6-bit value is taken from bits<28:23>. This value, in
turn, points to a location in a table formed by the Ether-
net Hash Table registers, ETH1 through ETH4. If the bit
in that location is set, the packet meets the Hash Table
filter criteria and is accepted. The specific pointer
values for each bit location in the table are shown in
Table 10-1.
An example of the Hash Table operation is shown in
Example 10-1. In this case, the destination address,
01-00-00-00-01-2C, produces a Hash Table Pointer
value of 34h, which points to bit 4 of ETH4. If this bit is
‘
1’, the packet will be accepted. If this Hash Table bit is
‘
0’, the packet will be passed to the next lower priority
filter.
By extension, if every bit in the Hash Table is set, the
filter criteria will always be met, so all packets will be
accepted if no higher priority filter has rejected the
packet. Similarly, clearing every bit in the Hash Table
registers means that the filter criteria will never be met,
so all packets will be passed on to the next lower
priority filter.
This filter is disabled at power-up. To enable this filter,
set HTEN (ERXFCON<15>). If the filter is disabled, all
frames will be passed on to the next lower priority filter,
regardless of destination address or Hash Table
register values.
EXAMPLE 10-1: DERIVING A HASH TABLE
LOCATION
TABLE 10-1: BIT ASSIGNMENTS IN HASH TABLE REGISTERS
Packet Destination Address:
01-00-00-00-01-2C (hex)
Result of CRC-32 with 4C11DB7h:
1101 1010 0000 1011 0100 0101 0111 0101
(binary)
Pointer Derived from bits<28:23> of CRC Result:
110100
(binary) or 34 (hex)
Corresponding Hash Table Location:
EHT4<4>
Register
Bit Numbers in Hash Table
1514131211109876543210
EHT1 0F 0E 0D 0C 0B 0A 09 08 07 06 05 04 03 02 01 00
EHT2 1F 1E 1D 1C 1B 1A 19 18 17 16 15 14 13 12 11 10
EHT3 2F 2E 2D 2C 2B 2A 29 28 27 26 25 24 23 22 21 20
EHT4 3F 3E 3D 3C 3B 3A 39 38 37 36 35 34 33 32 31 30