K60P100M100SF2RM Datasheet, PDF (1195/1809 Pages) Freescale Semiconductor, Inc – K60 Sub-Family Reference Manual

Chapter 44 10/100-Mbps Ethernet MAC (ENET)
The back-off time is represented by an integer multiple of slot times (one slot is equal to
a 512-bit time period). The number of the delay slot times, before the n^th re-transmission
attempt, is chosen as a uniformly-distributed random integer in the 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 is 0 or 1 slot time. If a collision
occurs on the first retransmission, the backoff period is 0, 1, 2, or 3 and so on.
The maximum backoff time (in 512-bit time slots) is limited by N = 10 as specified in the
IEEE 802.3 standard.
If a collision occurs after 16 consecutive retransmissions, the core reports an excessive
collision condition (ENETn_EIR[RL] interrupt bit and TxBD[EE]) and discards the
current packet from the FIFO.
In networks violating the standard requirements, a collision may occur after transmission
of the first 64 bytes. In this case, the core stops the current packet transmission and
discards the rest of the packet from the transmit FIFO. The core resumes transmission
with the next packet available in the core transmit FIFO.
44.4.6 Full Duplex Flow Control Operation
Three conditions are handled by the core's flow control engine:
• Remote device congestion — The remote device connected to the same Ethernet
segment as the core reports a congestion requesting the core to stop sending data
• Core FIFO congestion — When the core's receive FIFO reaches a user-
programmable threshold (RX section empty), the core sends a pause frame back to
the remote device requesting the data transfer to stop
• Local device congestion — Any device connected to the core can request (typically,
via the host processor) the remote device to stop transmitting data
44.4.6.1 Remote Device Congestion
When the MAC transmit control gets a valid pause quanta from the receive path and if
ENETn_RCR[FCE] is set, the MAC transmit logic:
• Completes the transfer of the current frame
K60 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.
1195