GPS授時(shí)系統(tǒng)的硬件組成
GPS衛(wèi)星時(shí)鐘接收機(jī)由GPS接收天線(5MHz,3V)、GPS OEM接收板(GARMIN公司的GPSl5 L)、ADM202E、單片機(jī)(AT89S51)、鎖存器(74LS574)、雙口RAM(CY7C13l的容量為1K)組成。硬件結(jié)構(gòu)如圖3所示。
由于AT89S5l型單片機(jī)的輸出為0V~5V的_TTL電平,而OEM板配置的是RS-232標(biāo)準(zhǔn)串行接口,二者的性能規(guī)范不一致,不能直接進(jìn)行通信。為使1TTL電平與RS-232標(biāo)準(zhǔn)協(xié)調(diào),采用一種雙路發(fā)送/接收集成電路(ADM202E)。該電路的功耗低,只需單電源(+5V)供電,片內(nèi)具有兩套電壓提升器構(gòu)成的4倍電壓變換器,能產(chǎn)生+10V和-10V電壓,這樣就使單片機(jī)和0EM板之間的電壓完全匹配。
在AT89S51型單片機(jī)系統(tǒng)中。當(dāng)對(duì)外部數(shù)據(jù)存儲(chǔ)器進(jìn)行讀寫(xiě)操作即執(zhí)行[email=MOVX@DPTR,A]MOVX@DPTR,A[/email],A(累加器A中數(shù)據(jù)送片外數(shù)據(jù)存儲(chǔ)器),MOVX A,@DPTR(片外數(shù)據(jù)存儲(chǔ)器中數(shù)據(jù)送累加器A1指令時(shí),DPTR的低8位地址在鎖存信號(hào)ALE的下降沿被鎖存在地址鎖存器中。高8位地址直接送到目標(biāo).然后8位數(shù)據(jù)直接送往目標(biāo)器件。
雙端口。RAM最重要的特點(diǎn)是每個(gè)器件有2組數(shù)據(jù)總線、地址總線及控制總線。只要不是同時(shí)訪問(wèn)同一個(gè)存儲(chǔ)單元,就允許2個(gè)端口同時(shí)對(duì)片內(nèi)任何存儲(chǔ)單元進(jìn)行獨(dú)立的讀,寫(xiě)操作而互不干擾;如果2個(gè)端口同時(shí)訪問(wèn)同一個(gè)存儲(chǔ)單元.則由片內(nèi)的仲裁邏輯決定訪問(wèn)哪個(gè)端口。
在硬件設(shè)計(jì)中,經(jīng)常需要2個(gè)設(shè)備有權(quán)訪問(wèn)同一塊存儲(chǔ)區(qū),如果使用常規(guī)的單端口存儲(chǔ)器,那么當(dāng)某一個(gè)設(shè)備正在訪問(wèn)時(shí).別的設(shè)備則無(wú)法訪問(wèn).嚴(yán)重降低了工作效率.而且不論是采用HOLD信號(hào)使某一設(shè)備“掛起”的方法.還是采用二套總線二選一的方法,都給邏輯電路的設(shè)計(jì)增添了負(fù)擔(dān)。相比之下,使用雙端口RAM的好處至少有二點(diǎn):首先,可將2個(gè)設(shè)備的總線與雙端口RAM的2個(gè)端口分別相連,邏輯設(shè)計(jì)簡(jiǎn)潔明了;其次.只要安排得當(dāng),雙口RAM無(wú)沖突地為2個(gè)端口服務(wù),則可使訪問(wèn)效率比常規(guī)單端口RAM提高l倍。
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