83C795 Datasheet, PDF (89/136 Pages) List of Unclassifed Manufacturers – Ethernet System Controller

83C795
LAN CONTROLLER OVERVIEW
7.6 TRANSMITTER SECTION
7.6.1 Basic Function
T he trans mitter section generates s erial stream of
NR Z data. It produces preamble and the S F D field
at thebeginningof aframe, then datais s hiftedfrom
the F IF O serially followed by the CR C field. T he
trans mitter checks for collisions andretrans mits the
frames if neces s ary, it counts interframe gap and
implements random backoff algorithm. It maintains
the trans mit s tatis tics and generates s tatus
information on each attempted trans mis s ion.
S election of optional operating modes of the
T r ans mi tter s ecti on i s done pr i mar i l y by
programming the T rans mit Configuration regis ter.
7.6.2 Preamble Generator
At the beginning of each frame, the trans mitter
generates 56 bits of preamble(an alternating’1010’
pattern). Immediately after this , it generates aS tart
F rame Delimiter s equence which is ’10101011.’
7.6.3 Transmit Serializer
T he T rans mit S erializer converts 8 bits of parallel
data from the F IF O into s erial transmit NR Z data.
Data is s hifted out least-s ignificant-bit (LS B) firs t.
S erial data is clocked onto an internal s ignal (T XD)
by the ris ing edge of the transmit clock. T his signal
pass es to the Manches ter E ncoder which encodes
it anddrives the s electeds erial interface. When the
encoder is being bypass ed, the serial data drives
the XT XD pin directly.
7.6.4 CRC Generator
T he trans mitter calculates the CR C s erially and
appends it to each frame. CR C is clocked out
mos t-significant-bit (MS B) firs t. T he trans mitter can
be configured to exclude attachment of the
computed CR C by s etting the CR CN option bit in
the T r ans mi t Confi gur ati on R egi s ter
(T CON.CR CN). T his is us eful for some bridging
applications in which the original checks um mus t
remain attached to the packet until the final
des tination.
7.6.5 Transmit Protocol FSM
T ransmit Protocol F S M controls trans miss ion of
frames, defers to active carriers and collisions ,
monitors collis ion conditions , and initiates both
backoff and re-transmis sion when needed.
7.6.5.1 Interframe Gap and Deference
Deference is initiated when both XCR S and XCOL
have terminated at the end of a frame.
T he trans mitter deference logic initiates a 2-part
timer at the end of network activity. While this timer
is running, no frame trans miss ion will be initiated.
T hefirs tpart of thetimer (interF rameS pacingPart1)
is us ed to obs erve the network for transmis sion
activity by other s tations . I f this s tation is
trans mitting, carrier is s ens ed, or collis ion is
detected during this part of the timer, the timer will
be reset to zero andheldthere until the termination
of line activity. When the firs t part of the timer
elapses , line activity is no longer obs erved and the
timer runs to completion.
If any frame is queued up for trans mis s ion at the
moment of timer expiration, trans mis s ion will be
initiated regardless of line activity.
T he combination of interF rame S pacingPart1 and
i nter F r ame S paci ngP ar t2 makes up the
Inter-F rame Gap (IF G) as defined by the 802.3
s pecification. T he interF rame S pacingPart1 is 6.0
µs ec and interFrame S pacingPart2 is 3.6 µsec.
7.6.5.2 Collision Handling Logic
When collis ion is detected by the trans mitter
s ection during the firs t s lot time of an active
trans miss ion, the transmis sion does not terminate
immediately. Ins tead, the preamble is allowed to
finish andthejams equenceis transmitted. T hejam
s equence consists of 32 bits of logic’1’s . If collision
is detectedafter thes lot timeis pas s ed, the83C795
will abort the trans mis s ion with jam and without
retry and ’Out of Window Collis ion’ bit is s et in the
trans miss ion s tatus regis ter.
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