Altera Triple Speed Ethernet MegaCore Function - Manual (Page 43)

The backoff time is a multiple of slot times. One slot is equal to a 512 bit times period. The number of the
delay slot times, before the Nth retransmission attempt, is chosen as a uniformly distributed random integer
in the following range:
0
≤
r < 2
k
k = min(n, N), where n is the number of retransmissions and N = 10.
For example, after the first collision, the backoff period, in slot time, is 0 or 1. If a collision occurs during
the first retransmission, the backoff period, in slot time, is 0, 1, 2, or 3.
The maximum backoff time, in 512 bit times slots, is limited by N set to 10 as specified in the IEEE Standard
802.3.
If collision occurs after 16 consecutive retransmissions, the MAC function reports an excessive collision
condition by setting the
EXCESS_COL
bit in the
command_config
register to 1, and discards the current frame
from the transmit FIFO buffer.
In networks that violate standard requirements, collision may occur after the transmission of the first 64
bytes. If this happens, the MAC function stops transmitting the current frame, discards the rest of the frame
from the transmit FIFO buffer, and resumes transmitting the next available frame.
MAC Receive Datapath
The MAC function receives Ethernet frames from the network via a PHY and forwards the payload with
relevant frame fields to the user application after performing checks, filtering invalid frames, and removing
the preamble and SFD.
Preamble Processing
The MAC function uses the SFD (
0xD5
) to identify the last byte of the preamble. If an SFD is not found after
the seventh byte, the MAC function rejects the frame and discards it.
The IEEE standard specifies that frames must be separated by an interpacket gap (IPG) of at least 96 bit
times. The MAC function, however, can accept frames with an IPG of less than 96 bit times; at least 8-bytes
and 6-bytes in RGMII/GMII (1000 Mbps operation) and RGMII/MII (10/100 Mbps operation) respectively.
The MAC function removes the preamble and SFD fields from valid frames.
Collision Detection in Half-Duplex Mode
In half-duplex mode, the MAC function checks for collisions during frame reception. When collision is
detected during the reception of the first 64 bytes, the MAC function discards the frame if the
RX_ERR_DISC
bit is set to 1. Otherwise, the MAC function forwards the frame to the user application with error.
Address Checking
The MAC function can accept frames with the following address types:
• Unicast address—bit 0 of the destination address is 0.
• Multicast address—bit 0 of the destination address is 1.
• Broadcast address—all 48 bits of the destination address are 1.
The MAC function always accepts broadcast frames. If promiscuous mode is enabled ( PROMIS_EN bit in
the
command_config
register = 1), the MAC function omits address filtering and accepts all frames.
Altera Corporation Functional Description
Send Feedback
4-7 MAC Receive Datapath
UG-01008
2014.06.30