PIC18F97J60_11 Datasheet, PDF (259/492 Pages) Microchip Technology – 64/80/100-Pin, High-Performance, 1-Mbit Flash Microcontrollers with Ethernet

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19.8 Receive Filters
To minimize microcontroller processing overhead, the
Ethernet module incorporates a range of different
receive filters which can automatically reject packets
which are not needed. Six different types of packet
filters are implemented:
• Unicast
• Multicast
• Broadcast
• Pattern Match
• Magic Packet™
• Hash Table
The individual filters are all configured by the ERXFCON
register (Register 19-20). More than one filter can be
active at any given time. Additionally, the filters can be
configured by the ANDOR bit to either logically AND or
logically OR the tests of several filters. In other words,
the filters may be set so that only packets accepted by
all active filters are accepted, or a packet accepted by
any one filter is accepted. The flowcharts in Figure 19-13
and Figure 19-14 show the effect that each of the filters
will have, depending on the setting of ANDOR.
The device can enter Promiscuous mode and receive
all legal packets by setting the ERXFCON register to
20h (enabling only the CRC filter for valid packets). The
proper setting of the register will depend on the
application requirements.
19.8.1 UNICAST FILTER
The Unicast receive filter checks the destination
address of all incoming packets. If the destination
address exactly matches the contents of the MAADR
registers, the packet meets the Unicast filter criteria.
19.8.2 MULTICAST FILTER
The Multicast receive filter checks the destination
address of all incoming packets. If the Least Significant
bit of the first byte of the destination address is set, the
packet meets the Multicast filter criteria.
19.8.3 BROADCAST FILTER
The Broadcast receive filter checks the destination
address of all incoming packets. If the destination
address is FF-FF-FF-FF-FF-FF, the packet meets the
Broadcast filter criteria.
19.8.4 HASH TABLE FILTER
The Hash Table receive filter is typically used to receive
traffic sent to a specific Multicast group address.
Because it checks the specific destination address of
packets, it is capable of filtering out more unwanted
packets than the Multicast filter.
The filter performs a 32-bit CRC over the six destination
address bytes in the packet, using the polynomial,
4C11DB7h. From the resulting 32-bit binary number, a
6-bit value is derived from bits<28:23>. This value, in
turn, points to a location in a table formed by the Ether-
net Hash Table registers, ETH0 through ETH7. 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 19-9.
An example of the Hash Table operation is shown in
Example 19-1. In this case, the destination address,
01-00-00-00-01-2C, produces a Table Pointer value of
34h, which points to bit 4 of ETH6. If this bit is ‘1’, the
packet will be accepted.
By extension, clearing every bit in the Hash Table
registers means that the filter criteria will never be met.
Similarly, if every bit in the Hash Table is set, the filter
criteria will always be met.
TABLE 19-9:
Register
7
EHT0
07
EHT1
0F
EHT2
17
EHT3
1F
EHT4
27
EHT5
2F
EHT6
37
EHT7
3F
BIT ASSIGNMENTS IN HASH
TABLE REGISTERS
Bit Number in Hash Table
6543210
06 05 04 03 02 01 00
0E 0D 0C 0B 0A 09 08
16 15 14 13 12 11 10
1E 1D 1C 1B 1A 19 18
26 25 24 23 22 21 20
2E 2D 2C 2B 2A 29 28
36 35 34 33 32 31 30
3E 3D 3C 3B 3A 39 38
EXAMPLE 19-1: DERIVING A HASH TABLE
LOCATION
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:
ETH6<4>
 2011 Microchip Technology Inc.
DS39762F-page 259