int#
83C152 HARDWAREDESCRIPTION
1
PRBS
I
I
AND
II
LOAD
BKOFF
8
SLOT
CLOCK
6
COMP
+ BKOFF=MYSLOT
6
—
270427-38
Figure 3.5. BackOffTimer Logic
In either of these cases, the transmitter is disabled
most protocols, the slot period must be equal to or
(TEN = O)and the Transmit Error flag TCDT is set.
The automatic restatl is eaneeled.
Where the Normal and Alternate Random backoffal-
gorithmsdifferis that in Normal Random baekoffthe
BKOFF timer starts countingdown as soon as a line
idle condition is detected, whereas in Alternate Ran-
dom backoffthe BKOFF timer doesn’tstart counting
downtill the IFS expirea.
The
Alternate Random mode was deaigned for net-
worksin whichthe slot time is leasthan the IFS. If the
randomlyaasignedbackofftime for a giventranstm
“tter
happens
to be O,thenit isfreeto transmit as soonasthe
IFS ends. If the slot time is shorter than the IFS, Nor-
mal hndom mode would nearly
guarantee that if
there’sa first collisionthere will be a secondcollision.
The situation is avoidedin Alternate Random mode,
sincethe BKOFF countdowndoean’tstart till the IFS
is over.
The unit of countto the BKOFF timer is the slottime.
The slot time is measured in bit-times, and is deter-
mined by a CPU write to the register SLOTTM.The
slot time clockis a l-bytedowncmnter whichstarts its
countdownfrom the value written to SLOTTM.It is
decrementedeach bit time when a backoffis in prog-
ress, and whenit gets to 1it generatesone tick in the
slottime
clock.Thenextstate after 1is the reloadvalue
whichwas writtento SLOTTM.If Ois the valuewnt-
terrto SLOTTM,the slot time clock will equal 256bit
times.
A CPU write to SLOTTMamesses
the reload register.
A CPUread ofSLOITM acassea
the downcounter.In
7-26
greater than the longestround trip propagationtime
plus thejam time.
Deterministic Backoff
In the Determinestic backoffmod%the GSCisassigned
(insoftware)a slotnumber.Theslotassignmentk writ-
ten to the low 6 bits of the register MYSLOT.This
same registeralso Contain$in the 2 highbit positions,
the controlbits DCJ and DCR.
Slotassignmentsthereforecan run fromOto 63.It will
turn out that the higherthe slotassignment,the sooner
the GSCwillget to restart its transmissionin the event
of a collision.
The higheatslot assignmentin the networkis written
by each station’ssoftwareinto its TCDCNT register.
Normallythe higheatslot assignmentis just the total
numberof stationsthat are goingto participate in the
backoffalgorithm.
In deterministicbackoffmodea collisionwillnot cause
a 1to be sltitledintoTCDCNT.TCDCNTwillstill be
ANDedwithPRBSandthe resultloadedintoBKOFF.
In order to insurethat all stationshavethe samevalue
loaded into BKOFF, which determinesthe first slot
number to cccur, the PRBS should be leaded with
OFFH;the PRBSwill maintain this value until either
the 8XC152isreset or the user writessomeother value
into PRBS.After BKOFF is loadedit beginscounting
downslot timesas soonas the IFS ends.Slottimes are
definedbythe user,the samewayas before,by loading
SLOTTMwith the numberofbit timesper slot.
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