輸入/輸出接口（2）1四.可編程通信接口16550/82501.RS-232接口標準

DB-25型連接器外形

2

用于異步通信RS-232C的引腳定義*RS-232還有DB-9的連接器3下面介紹RS-232用于異步通信各信號的作用及對應的引腳功能。1．保護地線（引腳1）2．信號地線（引腳7）3．請求發(fā)送（引腳4）、清除發(fā)送（引腳5）

4．數(shù)據(jù)設備就緒（引腳6）5．數(shù)據(jù)終端就緒（引腳20）6．載波檢測（引腳8）7．振鈴指示（引腳22）8．數(shù)據(jù)信號速率選擇（引腳23）9．發(fā)送數(shù)據(jù)引腳（引腳2）10.接收數(shù)據(jù)（引腳3）返回本節(jié)

RS-232主要功能信號4串行通信接口的典型連接5串行接口適配器由地址譯碼器、時鐘電路、16550/8250UART、數(shù)據(jù)緩沖器、發(fā)送接收驅動器和RS-232插座等組成。由地址譯碼器接收系統(tǒng)發(fā)出的尋址信息，并使能有關的端口寄存器。串行端口COM1和COM2的地址如表所示。其作用為：2.串行接口適配器(接口卡)

串行通信靠發(fā)送器、接收器和線纜三部分來實現(xiàn)的。在程序控制下它主要完成以下任務：（1）數(shù)據(jù)的串——并及并——串轉換。（2）線路和MODEM的控制及狀態(tài)檢測。（3）信號電平TTL與EIA電平的轉換。（4）發(fā)送和接收數(shù)據(jù)。返回本節(jié)6串行通信適配器框圖7Auniversalasynchronousreceiver/transmitter(UART).Operationspeed:0-1.5MBaud(Baudis#ofbitstransmitted/sec,includingstart,stop,dataandparity).Includes:?AprogrammableBaudrategenerator.?SeparateFIFObuffersforinputandandoutputdata(16byteseach).Asychronousserialdata:Transmittedandreceivedwithoutaclockortimingsignal.Two10-bitframesofasynchronousdata.7-or8-bitASCII,e.g.worw/oparity,ispossible.3.可編程通信接口芯片16550/8250功能與引腳(DIP40/PLCC40）*FIFO---FirstInFirstOut*PLCC---PlasticLead-lessChipCarrier塑料無引線芯片載體8

Twoseparatesectionsareresponsiblefordatacommunications:ReceiverTransmitterCanfunctionin:simplex:transmitonlyhalf-duplex:transmitandreceivebutnotsimultaneouslyfull-duplex:transmitandreceivesimultaneouslyThe16550cancontrolamodemthroughDSR,DTR,CTS,RTS,RIandDCD.Inthiscontext,themodemiscalledthedatasetwhilethe16550iscalledthedataterminal.可編程通信接口芯片16550引腳信號916550引腳功能A0,A1andA2(input):Selectaninternalregisterforprogramminganddatatransfer.?ADS(input):Addressstrobeusedtolatchaddressandchipselect.NotneededonIntelsystems--connectedtoground.?BAUDOUT(output):ClocksignalfromBaudrategeneratorintransmitter.?CS0,CS1,CS2(input):Chipselects?CTS(input):Cleartosend--indicatesthatthemodemordatasetisreadytoexchangeinformation.(Usedinhalf-duplextoturnthelinearound).10D7-D0(input/output):Thedatabuspinsareconnectedtothemicroprocessordatabus.DCD(input):Thedatacarrierdetect--usedbythemodemtosignalthe16550thatacarrierispresent.DDIS(output):Disabledriveroutput--setto0toindicatethatthemicroprocessorisreadingdatafromtheUART.Usedtochangedirectionofdataflowthroughabuffer.DSR(input):Datasetreadyisaninputto16550--indicatesthatthemodem(dataset)isreadytooperate.DTR(output):Dataterminalreadyisanoutput--indicatesthatthedataterminal(16550)isreadytofunction.INTR(output):Interruptrequestisanoutputtothemicro--usedtorequestaninterrupt.ReceivererrorDatareceivedTransmitbufferempty11MR(input):Masterreset--connecttosystemRESETOUT1,OUT2(output):Userdefinedoutputpinsformodemorotherdevice.RCLK(input):Receiverclock--clockinputtothereceiversectionoftheUART.Always16XthedesiredreceiverBaudrate.RD,RD(input):Readinputs(eithercanbeused)--causedatatobereadfromtheregistergivenbytheaddressinputs.RI(input):Ringindicatorinput--setto0bymodemtoindicatetelephoneisringing.RTS(output):Request-to-send--signaltomodem,indicatingUARTwishestosenddata.SIN(input),SOUT(output):Serialdatapins,inandout.RXRDY(input):Receiverready--usedtotransferreceiveddataviaDMAtechniques.TXRDY(output):Transmitterready--usedtotransfertransmitterdataviaDMA.WR,WR(input):Write(eithercanbeused)--connectstomicrowritesignaltotransfercommandsanddatato16550.XIN,XOUT:Mainclockconnections--acrystaloscillatorcanbeused.12

16550電路應用連接與8088微處理器連接，端口地址為00F0H~00F7H的165501316550/8250UART內部寄存器框圖4.16550/8250UART內部寄存器14中斷允許寄存器(A2A1A0=001)?中斷標識寄存器(A2A1A0=010)15FIFO控制寄存器(A2A1A0=010)初始化FIFO的狀態(tài)16線路控制寄存器(A2A1A0=011)設定串行數(shù)據(jù)格式和波特率17線路狀態(tài)寄存器(A2A1A0=101)發(fā)送或接收時查詢是否準備就緒，有否錯誤185.編程與控制5.1初始化---編程線路控制寄存器、波特率發(fā)生器和FIFOTwophases:Initialization,operation.Initialization:AfterRESET,thelinecontrolregisterandbaudrategeneratorneedtobeprogrammed.Linecontrolregistersetsthe#ofdatabits,#ofstopbitsandtheparity.Addressedatlocation011.Stopbits:S=1,1.5stopbitsusedfor5databits,2usedfor6,7or8.19Initialization(cont.)ST,PandPEusedtosendevenoroddparity,tosendnoparityortosenda1ora0intheparitybitpositionforalldata.Noparity,both0--usedforinternetconnections.SB=1causesabreaktobetransmittedonSOUT.Abreakisatleasttwoframeof0data.DL=1enablesprogrammingofthebaudratedivisor.20Initialization(cont.)Baudrategeneratorisprogrammedwithadivisorthatsetsbaudrateoftransmitter.Baudrategeneratorisprogrammedat000and001.Port000usedtoholdleastsignificantbyte,001mostsignificant.Valueuseddependsonexternalclock/crystalfrequency.For18.432MHzcrystal,10,473gives110bandrate,30gives38,400baud.Note,numberprogrammedgeneratesaclock16XthedesiredBaudrate.Last,theFIFOcontrolregistermustbeprogrammedat010.21

波特率與除數(shù)寄存器值的關系*波特率=晶體頻率/(N*除數(shù)值)-----N取1，16，32等*本例若選取晶體頻率為18.432MHZ，N=16，則除數(shù)值應乘以1022PC系統(tǒng)中串行端口寄存器地址23例：一個異步系統(tǒng)需要7位數(shù)據(jù)位、奇校驗、9600的波特率及1位停止位，端口地址為3F8H~3FFH（COM1），其初始化程序為

LINEEQU3FBH；線路控制寄存器

LSBEQU3F8H；波特率除數(shù)寄存器低位

MSBEQU3F9H；波特率除數(shù)寄存器高位

FIFOEQU3FAH；FIFO控制寄存器

STARTPROCNEARMOVAL，10001010B；允許除數(shù)鎖存

MOVDX，LINE

指向除數(shù)寄存器

OUTDX，AL24

MOVAL，120；除數(shù)值120，波特率9600

MOVDX，LSBOUTDX，ALINCDX；指向高位

MOVAL，0OUTDX，AL

INCDX；指向線路控制寄存器

INCDXMOVAL，00001010B；7位數(shù)據(jù)位

OUTDX，AL

1位停止位，奇校驗

無間隔發(fā)送

25

MOVAL，00000111B；初始化FIFO，F(xiàn)IFO使能

DECDX清除接收器/發(fā)送器

OUTDX，AL

RETSTARTENDP5.2發(fā)送串行數(shù)據(jù)---線路狀態(tài)寄存器、發(fā)送器鎖存26Operating:Statuslineregistergivesinformationabouterrorconditionsandstateofthetransmitterandreceiver.Thisregisterneedstobetestedinsoftwareroutinesdesignedtousethe16550totransmit/receivedata.SupposeaprogramwantstosenddataoutSOUT.ItneedstopooltheTHbittodetermineiftransmitterisreadytoreceivedata.Toreceiveinformation,theDRbitistested.27例：將AH的內容發(fā)送給16550并通過其串行數(shù)據(jù)引腳SOUT輸出，其程序為

LSTATEQU3FDH；線路狀態(tài)寄存器

DATAEQU3F8H；發(fā)送器鎖存

SENDPROCNEARPUSHAXPUSHDXSEND1：MOVDX，LSTATINAL，DX；讀線路狀態(tài)

TESTAL，20H；測試TH位

JZSEND1；等待發(fā)送器準備就緒28

MOVAL，AH；取數(shù)據(jù)

SUBDX，5；指向發(fā)送器鎖存

OUTDX，AL；發(fā)送數(shù)據(jù)

POPDXPOPAXRETSENDENDP5.3接收串行數(shù)據(jù)---線路狀態(tài)寄存器、接收緩沖器29Operating:Itisalsoagoodideatocheckforerrors.Parityerror（1）:Receiveddatahaswrongerror--transmissionbitflipduetonoise.Framingerror（1）:Startandstopbitsnotintheirproperplaces.Thisusuallyresultsifthereceiverisreceivingdataattheincorrectbaudrate.Overrunerror（1）:DatahasoverruntheinternalreceiverFIFObuffer.SoftwareisfailingtoreadthedatafromtheFIFO.Breakindicatorbit:Softwareshouldcheckforthisaswell,i.e.twoconsecutiveframesof0s..30例：從16550中讀取接收到的信息，若檢測到一個錯誤，則在AL中返回一個ASCII碼‘？’，若無錯誤，則在AL中返回接收到的字符。其程序為

LSTATEQU3FDH