LAN89730 datasheet(163/218 Pages) MICROCHIP

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DS60001348A-page 163

LAN89730

7:6

BackOff Limit (BOLMT)

The BOLMT bits allow the user to set its back-off limit in a relaxed or aggres-

sive mode. According to IEEE 802.3, the MAC has to wait for a random num-

ber [r] of slot-times (Note 6-13) after it detects a collision, where:

(eq.1)0 < r < 2K

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 maxi-

mum. If it has been retried 12 times, then K = 10 and r = 1024 slot-times max-

imum.

An LFSR (linear feedback shift register) 20-bit counter emulates a 20-bit ran-

dom 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 flex-

ibility, 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 6-13

Slot-time = 512 bit times. (See IEEE 802.3 Spec., Sections

4.2.3.25 and 4.4.2.1)

R/W

00b

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

RESERVED

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

Receiver Enable (RXEN)

When set (‘1’), the MAC’s receiver is enabled and will receive frames from the

internal PHY. When reset, the MAC’s receiver is disabled and will not receive

any frames from the internal PHY.

R/W

Bits

Description

Type

Default

BOLMT Value

# Bits Used from LFSR Counter

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