AN2692 Freescale Semiconductor / Motorola, AN2692 Datasheet - Page 20

• Manufacturer Part Number: AN2692
• Description: MC9S12NE64 Integrated Ethernet Controller
• Manufacturer: Freescale Semiconductor / Motorola

MC9S12NE64 Ethernet Media Access Controller (EMAC)
Unicast Filter Mode
In unicast filter mode, the EMAC uses the MAC unicast address (MACAD) registers, which contain the
unique 6-byte address. The destination address of an incoming packet is compared to the MACAD value.
If the incoming frame’s destination address field does not match the MACAD value, the frame is rejected.
To set the device to unicast filter mode, the PROM bit must be cleared.
Broadcast Filter Mode
A broadcast Ethernet frame is identified by an Ethernet frame with a destination MAC address of FF-FF-
FF-FF-FF-FF (6-byte address of all 1s). A network uses the broadcast message format to address a
particular message to every device on the network. The address resolution protocol (ARP) uses this
mechanism when executing an address request process.
Setting the BCREJ bit of the RXCTS register will block broadcast messages from being accepted by the
MC9S12NE64. If the PROM bit is set, all address filtering is overridden and all Ethernet data frames are
accepted by the MC9S12NE64, regardless of the destination MAC address.
Multicast Filter Mode and Hash Table
In general, multicast Ethernet packets are used by network devices to address a particular message to
devices on the network that belong to a particular group. The developer or network administrator should
assign mutlicast groups.
If the value of the left-most bit of the MAC hardware address is 1, the Ethernet packet is considered to be
multicast. If the value of the left-most bit is 0, the Ethernet packet is considered to be unicast.
Using the CONMC bit, the MC9S12NE64 can be configured to either accept all incoming Ethernet packets
that are defined as multicast, or to accept only those multicast Ethernet packets that match the EMAC
multicast hash table.
The multicast hash table is implemented by the multicast hash table (MCHASH) registers,
MCHASH[63:0]. To initialize MCHASH, the 6-byte destination address of a multicast group must be
mapped into 1 of 64 bits of the MCHASH register. The mapping into MCHASH is accomplished by
calculating a 32-bit CRC value of the 6-byte destination address and then selecting the six most significant
bits of the CRC-encoded result. Next, the MCHASH register bit position that corresponds to the 6-bit value
must be set. Then, in multicast filter mode, the EMAC calculates the hash value of an incoming Ethernet
packet; if that value matches one of the initialized values, the packet would pass the hash table test and
be accepted.
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CONMC = 0 — All multicast Ethernet messages will be accepted, regardless of the contents of
multicast hash table
CONMC = 1 and PROM = 0 — Only multicast Ethernet packets that pass the multicast hash table
test are accepted by the MC9S12NE64
The broadcast message is a special case of a multicast message, but the
CONMC bit has no effect on broadcast messages.
MC9S12NE64 Integrated Ethernet Controller, Rev. 0.2
NOTE
Freescale Semiconductor