RM0319 Datasheet, PDF (171/368 Pages) STMicroelectronics – SPEAr320S architecture and functionality

RM0319
Fast Ethernet ports (RMII0/RMII1/MII1)
Note:
To receive all multicast frames, the hash register should be set with all ones and the
multicast hash enable bit should be set in the network configuration register.
External address matching
As an alternative to using the built in frame filtering functionality, a user may implement their
own external frame filtering block.The external address input signal (eam) is provided for
this purpose and enabled by bit 9 in the network configuration register.
If the eam signal is asserted before 16 bytes have been received the frame will be copied to
memory.
Copy all frames (or Promiscuous Mode)
If the copy all frames bit is set in the network configuration register then all non-errored
frames will be copied to memory. For example, frames that are too long, too short, have FCS
errors or have rx_er asserted during reception will be discarded and all others will be
received. Frames with FCS errors are copied to memory if bit 19 in the network
configuration register is set.
Type ID checking
The contents of the type_id register are compared against the length/type ID of received
frames (that is. bytes 13 and 14). Bit 22 in the receive buffer descriptor status is set if there
is a match. The reset state of this register is zero which is unlikely to match the length/type
ID of any valid Ethernet frame.
A type ID match does not affect whether a frame is copied to memory.
VLAN support
An Ethernet encoded 802.1Q VLAN tag looks like this.
:
TPID (Tag Protocol Identifier) 16 bits
0x8100
TCI (Tag Control Information) 16 bits
First 3 bits priority, then CFI bit, last 12 bits VID
The VLAN tag is inserted at the 13th byte of the frame adding an extra four bytes to the
frame.
If the VID (VLAN identifier) is null (0x000) this indicates a priority-tagged frame.
The MAC can support frame lengths up to 1536 bytes, 18 bytes more than the original.
Ethernet has a maximum frame length of 1518 bytes. This is achieved by setting bit 8 in the
network configuration register.
The following bits in the receive buffer descriptor status word give information about VLAN
tagged frames:-
● Bit 21 set if receive frame is VLAN tagged (that is. type id of 0x8100)
● Bit 20 set if receive frame is priority tagged (that is. type id of 0x8100 and null VID). (If
bit 20 is set bit 21 will be set also.)
● Bit 19, 18 and 17 set to priority if bit 21 is set
● Bit 16 set to CFI if bit 21 is set
Doc ID 022640 Rev 3
171/368