參數(shù)資料
型號(hào)： MB86965A
元件分類： 微控制器/微處理器
英文描述： 2 CHANNEL(S), 10M bps, LOCAL AREA NETWORK CONTROLLER, PQFP16
封裝： PLASTIC, QFP-160
文件頁(yè)數(shù)： 42/76頁(yè)
文件大小： 394K
代理商： MB86965A
MB86965
47
TRANSMITTER CIRCUITS
Circuits within the Transmitter include a transmitter state
machine, a small FIFO for pipe-lining the packet data,
preamble generator, CRC generator, end-of-packet
symbol generator, parallel-to-serial converter, Manches-
ter encoder, waveform generator, transmit filter, twisted-
pair line driver, backoff generator, inter-packet gap-timer,
and time domain reflectometer (TDR) counter.
The transmitter state machine, which sequences Trans-
mitter events (idle, preamble, data, CRC, inter-packet
gap, jam and backoff), detects various transmit error
conditions and sets appropriate bits within the DLCR
registers.
The pipe-line FIFO provides elastic buffering that the
Buffer Controller loads with data to be transmitted. The
EtherCoupler’s CRC generator calculates the Ethernet
32-bit CRC on the destination and source address, the
length field, and the (ISO/ANSI/IEEE 8802-3 Ethernet
specification) data field that appends to the packet.
TRANSMIT ERROR PROCESSING
Transmit error status bits in Transmit Status register
DLCR0 report possible transmit errors, the last two of
which can be enabled separately to generate interrupts: 1)
CR Lost, DLCR0<4>, loss of carrier during transmission,
usually a medium fault or collision; 2) Col, DLCR0<2>,
collision; and 3) 16 Col, DLCR0<1>, 16 consecutive
collisions.
If it detects a collision during transmission, EtherCoupler
automatically tries to retransmit the packet until sixteen
attempts
have
been
made.
Collision
counter
DLCR4<7:4> automatically increments after each
Collision up to the sixteenth, at which time it rolls over to
zero. (Bit 7 is the most-significant of the four bits.)
Appropriate status bits in the Transmit Status register are
set when a collision terminates transmission. The
occurrence of 16 collisions may indicate a network
problem, such as a disconnected cable or terminator, that
produces false collisions. While rare, 16 collisions may
normally occur.
A pseudo-random number generator, which provides
collision backoff clocked at the 10-megahertz bit rate so
that distances between stations become part of the
randomizing function, is sampled at the time of collision,
so as to mask all but the appropriate number of bits
specified by the 8802-3 backoff algorithm. This value is
then counted down at the slot-time rate (512 bits per
second) to generate the backoff interval. Only one bit is
used for a first collision, giving a backoff of either 0 or
51.2 microseconds. A second consecutive collision uses
two bits, and so forth, up to ten bits. The tenth through
16th collisions use 10 bits to provide a pseudo-random
backoff interval from 0 to 52.38 milliseconds that, per
8802-3, is the so-called binary exponential backoff for
collisions.
TIME DOMAIN REFLECTOMETRY
The TDR function counts the actual number of bits
transmitted for each packet before a collision indication,
carrier loss indication or completion of transmission. A
complete transmission with no error indications clears the
TDR counter. [Refer to register descriptions for Time
Domain
Reflectometry
registers,
DLCR14
and
DLCR15.]
MEDIA ACCESS CONTROL
The EtherCoupler transmitter state machine implements
the 8802-3 networks media-access protocol, CSMA/CD
(Carrier Sense, Multiple Access with Collision Detec-
tion). Carrier sense means that EtherCoupler monitors the
network for any other node carrier, and defers
transmission (collision avoidance) while other nodes are
transmitting. Collision detection handles collisions that
may still occur when two nodes separated on the network
by several microseconds begin transmitting at nearly the
same time. All nodes monitor the network for collisions
and, when involved in one, transmit a 32-bit Jam to
reinforce the collision and then terminate transmission.
After waiting a pseudo-random backoff interval, the node
automatically retries transmission of the packet.
Packets must be separated by at least 9.6 microseconds, a
timing gap during which trunk cabling is idle. The
EtherCoupler’s Transmitter state machine that measures
this interval, starting from the end of a packet on the
network, does not transmit until the end of this interval. If
carrier reappears on the network during the first
two-thirds of the interval, EtherCoupler resets the interval
timer to re-time the interval from the end of the new
transmission.
Superimposition of two packets corrupts data and carrier
indications, such as during a collision. During the last
one-third of the interval, EtherCoupler ignores the
occurrence of a carrier indication, in accordance with
8802-3, and ensures fairness and equality in access to the
network. If one station begins transmission slightly ahead
of another, there is no advantage to the earlier start. Both
nodes transmit, a collision occurs, and backoff interval
differentials resolve the media-access contention.
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