Cirrus Logic CS8900A - Manual (Page 34)

34 DS271F5
CS8900A
Crystal LAN™ Ethernet Controller
CIRRUS LOGIC PRODUCT DATASHEET
3.9.5.4 Collision Resolution
If a collision is detected while the CS8900A is
transmitting, the MAC responds in one of three
ways depending on whether it is a normal col-
lision (within the first 512 bits of transmission)
or a late collision (after the first 512 bits of
transmission):
3.9.5.5 Normal Collisions
If a collision is detected before the end of the
preamble and SFD, the MAC finishes the pre-
amble and SFD, transmits the jam sequence
(32-bit pattern of all 0’s), and then initiates
Backoff. If a collision is detected after the
transmission of the preamble and SFD but be-
fore 512 bit times, the MAC immediately termi-
nates transmission, transmits the jam
sequence, and then initiates Backoff. In either
case, if the Onecoll bit (Register 9, TxCMD, Bit
9) is clear, the MAC will attempt to transmit a
packet a total of 16 times (the initial attempt
plus 15 retransmissions) due to normal colli-
sions. On the 16th collision, it sets the 16coll
bit (Register 8, TxEvent, Bit F) and discards
the packet. If the Onecoll bit is set, the MAC
discards the packet without attempting any re-
transmission.
3.9.5.6 Late Collisions
If a collision is detected after the first 512 bits
have been transmitted, the MAC immediately
terminates transmission, transmits the jam se-
quence, discards the packet, and sets the Out-
of-window bit (Register 8, TxEvent, Bit 9). The
CS8900A does not initiate backoff or attempt
to retransmit the frame. For additional informa-
tion about Late Collisions, see Out-of-Window
Error in this section.
3.9.5.7 Backoff
After the MAC has completed transmitting the
jam sequence, it must wait, or “Back off”, be-
fore attempting to transmit again. The amount
of time it must wait is determined by one of two
Backoff algorithms: the Standard Backoff algo-
rithm (ISO/IEC 4.2.3.2.5) or the Modified
Backoff algorithm. The host selects which al-
gorithm through the ModBackoffE bit (Register
13, LineCTL, Bit B).
3.9.5.8 Standard Backoff
The Standard Backoff algorithm, also called
the “Truncated Binary Exponential Backoff”, is
described by the equation:
0
≤ r ≤ 2 k
where r (a random integer) is the number of
slot times the MAC must wait (1 slot time = 512
Tx
Frame
Ready and Not
in Backoff?
Transmit
Frame
Start Monitoring
Network Activity
Network
Active?
Network
Active?
Yes
No
Yes
No
No
Yes
Wait
9.6 μs
Figure 11. Simple Deferral