intd.
83C152 HARDWARE DESCRIPTION
5.1 GSC Transmitter Error Conditions
The GSC Transmitter seetion reports three kinds of
error conditions:
TCDT —Transmitter CollisionDetector
UR
—Underrun
in Transmit FIFO
NOACK— No Acknowledge
Thesebits reaidein the TSTATregister.User software
ean readthem,but onlythe GSChardwarecan writeto
them. The GSChardwarewill set them in responseto
the variouserror conditionsthat they represent.When
user softwaresets the TEN biLthe GSChardwarewill
at that timeclearthesetlags.Thisis the onlywaythese
flagscan be cleared.
The logicalORofthesethree bits flagsthe GSCTrans-
mit Error interrupt (GSCTE)and clears the TEN bit,
as shownin Figure5.2.Thusany detectederror condi-
tion abortsthe transmission.No CRCbitsare transmit-
ted. In SDLC mode, no EOF tlag is generated. In
CSMA/CD mode, an EOF is generated by default,
since the GTXD pin is pulled to a logic 1 and held
there.
TheTCDTbit can getset onlyif theGSCis eonfigured
to CSMA/CD mode. In that case, the GSC hardware
setsTCDTwhena collisionis detectedduring a trana-
rnission,and the collisionwasdetectedafter TFIFO has
baa accesed. Alao, the GSC hardware sets TCDT
whena detectedecdlisioncausesthe TCDCNTregister
to overflow.
TheUR bit can getset onlyif the DMAbit in TSTAT
is set. The DMA bit beingset informsthe GSC hard-
ware that TFIFO is beingseMeed by DMA. In that
caaGifthe GSCgoeato fetchanotherbytefromTFIFO
and findsit empty, and the byte count register of the
DMA channelservicingTFfFO is not zero, it sets the
UR bit.
If the DMA hardware is not being used to aerviee
TFIFO,the UR bit cannotgetset. If the DMA bit is O,
then when the GSC finds TFIFO empty, it assumes
that the transmissionofdata is completeand the trans-
missionof CRCbits can begin.
The
NOACKbit is fictional only in CSMA/CD
mode andonlywhenthe HABENbit in RSTATis set.
The HABEN bit turns on the Hardware Baaed Ac-
knowledgefeature, as deacribedin Seetion3.2.6.If this
featureis not invoked,the NOACKbit willstay at O.
:E=ii
270427-49
Figure 5.2. Transmit Error Ffsgs (Logic for Clearing TEN, Setting TDN)
7-62
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