SMSC LAN9420 - Manual (Page 121)

Single-Chip Ethernet Controller with HP Auto-MDIX Support and PCI Interface
Datasheet
SMSC LAN9420/LAN9420i 121 Revision 1.22 (09-25-08)
DATASHEET
7-6 BackOff Limit (BOLMT)
The BOLMT bits allow the user to set its back-off limit in a relaxed or
aggressive mode. According to IEEE 802.3, the MAC has to wait for a
random number [r] of slot-times (
Note 4.4 ) after it detects a collision, where:
(eq.1)0 < r <
2
K
The exponent K is dependent on how many times the current frame to be
transmitted has been retried, as follows:
(eq.2)K = min (n, 10) where n is the current number of retries.
If a frame has been retried three times, then K = 3 and r= 8 slot-times
maximum. If it has been retried 12 times, then K = 10, and r = 1024 slot-
times maximum.
An LFSR (linear feedback shift register) 20-bit counter emulates a 20bit
random number generator, from which r is obtained. Once a collision is
detected, the number of the current retry of the current frame is used to
obtain K (eq.2). This value of K translates into the number of bits to use
from the LFSR counter. If the value of K is 3, the MAC takes the value in
the first three bits of the LFSR counter and uses it to count down to zero
on every slot-time. This effectively causes the MAC to wait eight slot-times.
To give the user more flexibility, the BOLMT value forces the number of bits
to be used from the LFSR counter to a predetermined value as in the table
below.
Thus, if the value of K = 10, the MAC will look at the BOLMT if it is 00, then use
the lower ten bits of the LFSR counter for the wait countdown. If the BOLMT is 10,
then it will only use the value in the first four bits for the wait countdown, etc.
Note 4.4 Slot-time = 512 bit times. (See IEEE 802.3 Spec., Secs. 4.2.3.25
and 4.4.2.1)
R/W 00b
5 Deferral Check (DFCHK)
When set, enables the deferral check in the MAC. The MAC will abort the
transmission attempt if it has deferred for more than 24,288 bit times.
Deferral starts when the transmitter is ready to transmit, but is prevented
from doing so because the CRS is active. Defer time is not cumulative. If
the transmitter defers for 10,000 bit times, then transmits, collides, backs
off, and then has to defer again after completion of back-off, the deferral
timer resets to 0 and restarts. When reset, the deferral check is disabled in
the MAC and the MAC defers indefinitely. R/W 0b
4 RESERVED RO -
3 Transmitter enable (TXEN)
When set, the MAC’s transmitter is enabled and it will transmit frames from
the buffer onto the cable. When reset, the MAC’s transmitter is disabled and
will not transmit any frames. R/W 0b
BITS DESCRIPTION TYPE DEFAULT
BOLMT Value # Bits Used from LFSR Counter
2’b00 10
2’b01 8
2’b10 4
2’b11 1