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Ineithercase,receiveflowcontrolpreventsframereceptionbyissuingtheflowcontrolappropriateforthe
currentmodeofoperation.ReceiveflowcontrolpreventsreceptionofframesontheEMACuntilallofthe
triggeringconditionsclear,atwhichtimeframesmayagainbereceivedbytheEMAC.
ReceiveflowcontrolisenabledbytheRXBUFFERFLOWENbitintheMACcontrolregister
(MACCONTROL).TheEMACisconfiguredforcollisionorIEEE802.3Xflowcontrolusingthe
FULLDUPLEXbitinMACCONTROL.Receiveflowcontrolistriggeredwhenthenumberoffreebuffersin
anyenabledreceivechannelfreebuffercountregister(RXnFREEBUFFER)islessthanorequaltothe
receivechannelflowcontrolthresholdregister(RXnFLOWTHRESH)value.Receiveflowcontrolis
independentofreceiveQOS,exceptthatbothusethefreebuffervalues.
2.9.1.3.1Collision-BasedReceiveBufferFlowControl
Collision-basedreceivebufferflowcontrolprovidesameansofpreventingframereceptionwhenthe
EMACisoperatinginhalf-duplexmode(theFULLDUPLEXbitisclearedinMACCONTROL).When
receiveflowcontrolisenabledandtriggered,theEMACgeneratescollisionsforreceivedframes.Thejam
sequencetransmittedisthe12-bytesequenceC3.C3.C3.C3.C3.C3.C3.C3.C3.C3.C3.C3h.Thejam
sequencebeginsnolaterthanapproximatelyasthesourceaddressstartstobereceived.Notethatthese
forcedcollisionsarenotlimitedtoamaximumof16consecutivecollisions,andareindependentofthe
normalback-offalgorithm.
Receiveflowcontroldoesnotdependonthevalueoftheincomingframedestinationaddress.Acollision
isgeneratedforanyincomingpacket,regardlessofthedestinationaddress,ifanyEMACenabled
channel’sfreebufferregistervalueislessthanorequaltothechannel’sflowthresholdvalue.
2.9.1.3.2IEEE802.3x-BasedReceiveBufferFlowControl
IEEE802.3x-basedreceivebufferflowcontrolprovidesameansofpreventingframereceptionwhenthe
EMACisoperatinginfull-duplexmode(theFULLDUPLEXbitissetinMACCONTROL).Whenreceive
flowcontrolisenabledandtriggered,theEMACtransmitsapauseframetorequestthatthesending
stationstoptransmittingfortheperiodindicatedwithinthetransmittedpauseframe.
TheEMACtransmitsapauseframetothereservedmulticastaddressatthefirstavailableopportunity
(immediatelyifcurrentlyidleorfollowingthecompletionoftheframecurrentlybeingtransmitted).The
pauseframecontainsthemaximumpossiblevalueforthepausetime(FFFFh).TheEMACcountsthe
receivepauseframetime(decrementsFF00hto0)andretransmitsanoutgoingpauseframe,ifthecount
reaches0.Whentheflowcontrolrequestisremoved,theEMACtransmitsapauseframewithazero
pausetimetocancelthepauserequest.
Notethattransmittedpauseframesareonlyarequesttotheotherendstationtostoptransmitting.
Framesthatarereceivedduringthepauseintervalarereceivednormally(providedthereceiveFIFOisnot
full).
Pauseframesaretransmittedifenabledandtriggered,regardlessofwhetherornottheEMACis
observingthepausetimeperiodfromanincomingpauseframe.
TheEMACtransmitspauseframesasdescribedbelow:
•The48-bitreservedmulticastdestinationaddress01.80.C2.00.00.01h.
•The48-bitsourceaddress(setusingtheMACSRCADDRLOandMACSRCADDRHIregisters).
•The16-bitlength/typefieldcontainingthevalue88.08h.
•The16-bitpauseopcodeequalto00.01h.
•The16-bitpausetimevalueofFF.FFh.Apause-quantumis512bit-times.Pauseframessentto
cancelapauserequesthaveapausetimevalueof00.00h.
•Zeropaddingto64-bytedatalength(EMACtransmitsonly64-bytepauseframes).
•The32-bitframe-checksequence(CRCword).
Allquantitiesarehexadecimalandaretransmittedmost-significantbytefirst.Theleast-significantbit(LSB)
istransferredfirstineachbyte.
IftheRXBUFFERFLOWENbitinMACCONTROLisclearedto0whilethepausetimeisnonzero,thenthe
pausetimeisclearedto0andazerocountpauseframeissent.
36EthernetMediaAccessController(EMAC)/SPRU941A–April2007
ManagementDataInput/Output(MDIO)
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