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CHAPTER 3 FUNCTIONAL DESCRIPTION
User’s Manual S14054EJ4V0UM
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3.5.3 CRC check
Each port of the
µ
PD98431 automatically calculates a 4-byte frame check sequence (FCS) from the receive
packet data and compares it with the CRC data suffixed to the receive packet. The result of comparison is reported
to the host system as status information.
3.5.4 Packet filtering
The
µ
PD98431 can filter receive packets under the following conditions. Filtering can be set for each port. Two or
more filtering conditions can be used in combination.
•
Destination address
•
Short packet: Packet with packet length of less than 64 bytes
•
CRC error packet
•
Control frame
For details of the procedure to filter packets by using the destination address as a condition, refer to
3.5.5
Address filtering
. Filtering for CRC error packets, control frames, and short packets can be specified by using the
RFIC2 register. Depending on the setting of this register, a packet in which a CRC error has occurred, control
frames, and short packets can be eliminated from the receive FIFO. All the filtering conditions may be canceled. If
all of them are canceled, all the received packets are transferred to the host system.
3.5.5 Address filtering
The
µ
PD98431 can execute filtering by using the destination address of a receive packet and eliminate a packet
that does not satisfy a given condition. The address filtering condition is set using the AFR, HT1, and HT2 registers.
A filtering condition can be also set for each address type of unicast address, multicast address, and broadcast
address. In addition, two or more of the filtering conditions described below can be used in combination.
(1) Filtering of unicast address
The station address set by the LSA1 and LSA2 registers is used as a unicast address and is compared with the
destination address of the receive packet. If the two addresses coincide, the receive packet is accumulated to
the receive FIFO. Coincidence of the unicast address is detected for each receive packet, unless the PRO bit of
the AFR register is set to 1.
(2) Filtering of multicast address
A multicast address is filtered in two ways. When the PRM bit of the AFR register is set to 1, all multicast
packets are stored in the receive FIFO.
When the AMC bit of the AFR register is set to 1, only multicast packets that coincide with the hash table
prepared by the HT1 and HT2 registers are stored in the receive FIFO. Coincidence is detected by using the
hash table as shown below:
The CRC of the received multicast address is calculated. As a result, a 32-bit CRC is obtained. By using the
bits 28 to 23 of this 32-bit CRC, the hash table is referenced. The following polynomial expression is used for
CRC calculation.
X
32
+ X
26
+ X
23
+ X
22
+ X
16
+ X
12
+ X
11
+ X
10
+ X
8
+ X
7
+ X
5
+ X
4
+ X
2
+ X + 1
If the bit of the HT1 or HT2 register indicated by these high-order 6 bits is set to 1, the multicast packet is stored
in the receive FIFO. To set the hash table, the CRC of a multicast address must be calculated and the
corresponding bit must be set to 1 in advance.