摘? 要: 介紹了數(shù)據(jù)收發(fā)器CY7B923/933的性能特點、結(jié)構(gòu)原理、工作模式及應(yīng)用電路。在一VME總線系統(tǒng)中,采用該收發(fā)器及UTP雙絞線實現(xiàn)了400Mbps的串行數(shù)據(jù)傳輸。
????關(guān)鍵詞: 點對點串行通信? 串行數(shù)據(jù)傳輸? 8B/10B編碼模式? 同步
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串行數(shù)據(jù)傳輸可實現(xiàn)長距離高速通信,且電纜線少、成本低、安裝方便,在計算機網(wǎng)絡(luò)中得到了廣泛應(yīng)用,并日趨成為長距離數(shù)據(jù)通信的主要方式。CYPRESS公司的數(shù)據(jù)收發(fā)器CY7B923/933使用方便、可靠性好,可廣泛用于長距離高速點對點串行通信中,如光纖通信設(shè)備、工作站與海量存儲器的互連、視頻點播、語音圖像多媒體傳輸設(shè)備及其它長距離高速串行通信場合。CY7B923/933性能特點如下：
·適用于Fibre Channel、IBM ESCON、ATM;
·8B/10B編碼傳輸或不編碼直傳,傳輸速為160、330、400 Mbps三檔;
·最大傳輸距離(330Mbps的參考值)為:光纖幾千米,同軸電纜150米,屏蔽雙絞線80米,非屏蔽雙絞線40米;
·具有較強的自檢功能;
·單一+5V電源;
????·TTL同步I/O。
1 結(jié)構(gòu)原理及功能
圖1是CY7B923的邏輯方塊圖,其功能是將8bit并行TTL輸入數(shù)據(jù)(D0～D7)轉(zhuǎn)換為PECL串行差分位流輸出。輸入寄存器用于輸入數(shù)據(jù),時序與標(biāo)準(zhǔn)FIFO一致， 使能端
可選擇時鐘上升沿寫入數(shù)據(jù)。輸入數(shù)據(jù)由編碼器編碼為便于串行鏈路傳輸?shù)?B/10B 傳輸碼,由SC/
(專用字/數(shù)據(jù))端電平控制,將D0～D7輸入編成數(shù)據(jù)碼或?qū)Ｓ米执a。時鐘發(fā)生器含有一個鎖相環(huán)PLL,將發(fā)送器時鐘CKW(數(shù)據(jù)按字節(jié)輸入的時鐘)十倍頻形成位時鐘,驅(qū)動移位寄存器。 移位寄存器將數(shù)據(jù)變成串行位流,分三路同時輸出,FOTO可關(guān)閉OUTA和OUTB輸出。自檢邏輯產(chǎn)生通信系統(tǒng)自檢用發(fā)送信號。
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圖2是CY7B933的邏輯方塊圖,其功能是將INA或INB 的PECL串行差分輸入位流轉(zhuǎn)換為8bit并行TTL數(shù)據(jù)輸出Q7～Q0。時鐘同步器內(nèi)嵌鎖相環(huán)PLL跟蹤輸入數(shù)據(jù)位流的頻率,產(chǎn)生與發(fā)送時鐘同步的時鐘。移位寄存器按位接收串行數(shù)據(jù),在同步時鐘作用下將數(shù)據(jù)按位送往成幀器" title="成幀器" target="_blank">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" 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title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器" title="成幀器">成幀器。成幀器使位流數(shù)據(jù)正確地按字節(jié)恢復(fù)。解碼器將成幀后的 8B/10B代碼解碼為原始數(shù)據(jù)或?qū)Ｓ米?用RVS=H作數(shù)據(jù)違規(guī)或RD(Running Disparity)錯的報錯信號。串/并轉(zhuǎn)換后的數(shù)據(jù)從輸出寄存器Q7～Q0并行輸出。
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2 工作模式及8B/10B編碼
CY7B923/933有編碼、直傳兩種工作模式及自檢模式,由MODE、
端信號控制。
2.1 編碼模式
MODE=L,收發(fā)送器工作于8B/10B編碼模式,即8位數(shù)據(jù)或?qū)Ｓ米钟?B923編碼為10位的傳輸碼再逐位傳送。10位傳輸碼包括了所有256個8位組合,這些代碼符合ANSI X3.230(Fibre Channel)及IBM ESCON Channel規(guī)范。使用傳輸碼的優(yōu)點是：(1)使傳輸過程1與0的數(shù)目均衡,每個8B的數(shù)據(jù)或?qū)Ｓ米謱?yīng)兩個10B的傳輸碼,用參數(shù)RD表征傳輸1及0的狀況。若1比0多則RD取正值,下一個字節(jié)發(fā)送0比1多的傳輸碼。反之,則RD取負(fù)值,下次發(fā)送1比0多的傳輸碼，這樣可保證傳輸?shù)闹绷鞒煞纸咏?,基線漂移減至最小;(2)附加位能保證串行位流中有足夠的躍變,使接收器能從這些躍變中恢復(fù)發(fā)送時鐘;(3)這種編碼大大增加了探測位錯或多位錯的可能性,一些專用字使用獨特的位組合,容易識別,可幫助接收器數(shù)據(jù)成幀;(4)可用同一種編碼傳輸數(shù)據(jù)或?qū)Ｓ米帧Ｈ魹閿?shù)據(jù),SC/
端應(yīng)加低電平,編碼使用數(shù)據(jù)字符集;若為專用字。編碼采用專用字代碼集,專用字符包含F(xiàn)ibre Channel、ESCON和其它專用系統(tǒng)以及用于診斷的所有協(xié)議字符。診斷字符可用于測試Fibre Channel等的連接、診斷系統(tǒng)對故障的響應(yīng)、系統(tǒng)的時序等。8B數(shù)據(jù)或?qū)Ｓ米志幋a為10B傳輸碼通過查編碼表實現(xiàn)。對應(yīng)于RD正負(fù)兩種取值,每個數(shù)據(jù)或?qū)Ｓ米钟袃煞N編碼,發(fā)送器先取正或負(fù)值為RD初值,根據(jù)要發(fā)送的數(shù)據(jù)或?qū)Ｓ米旨爱?dāng)前的RD值,查8B對應(yīng)的10B代碼即實際發(fā)送的傳輸碼,并計算該傳輸碼的RD值作為當(dāng)前值,與下一個要傳送的數(shù)據(jù)共同查表確定下一個傳輸碼。詳見參考文獻[1]。
2.2 直傳模式
MODE=H， 收發(fā)送器工作于直傳模式,即收發(fā)送器直接傳輸數(shù)據(jù)不編碼,編碼和解碼由外部協(xié)議控制器完成。輸入分成10位,數(shù)據(jù)需預(yù)先編碼,編碼方式任意,但需保證有足夠的躍變密度(每10位至少一次),使接收器鎖相環(huán)能正確鎖定相位,且編碼需與傳輸介質(zhì)兼容。
無論是編碼模式還是直傳模式,若處于空閑狀態(tài)(時鐘CKW上升沿時
),編碼器將自動插入同步字K28.5(即C5.0)以保證接收器與發(fā)送器處在同步狀態(tài)。
2.3 自檢模式
,收發(fā)送器工作于自檢模式,7B923可分別發(fā)送兩種信號來檢測整個通信鏈路是否正常:一種是1與0相間的連續(xù)信號;另一種是循環(huán)發(fā)送的偽隨機序列信號。若為后者,7B923輸入寄存器轉(zhuǎn)換成線性反饋移位寄存器LFSR,它可產(chǎn)生一組511個字節(jié)的偽隨機代碼序列,包括了所有的數(shù)據(jù)字和專用字。7B933將輸出寄存器也變成LFSR,產(chǎn)生與發(fā)送器相同的偽隨機代碼序列,并與接收的序列相比較。若相同,則整個通信系統(tǒng)工作正常,否則為異常,使RVS=H給出錯誤信號。每次循環(huán),7B923的
及7B933的
都將輸出一脈沖,此脈沖可用于記錄循環(huán)次數(shù)以控制自檢時間。7B923還可發(fā)送違規(guī)信號,以檢測報錯系統(tǒng)是否正常。這些信號都大大方便了故障診斷。
3 CY7B923/933應(yīng)用電路
圖3為采用CY7B923/933實現(xiàn)的單向數(shù)據(jù)傳輸?shù)膽?yīng)用電路。主機發(fā)送控制命令,通過控制邏輯控制CY7B923/933工作在傳輸或自檢模式。這里控制邏輯用FPGA實現(xiàn),也可采用其它數(shù)字邏輯器件實現(xiàn)?？刂齐娐窇?yīng)包含對、RVS端的檢測,以判斷系統(tǒng)工作正常與否,及時進行錯誤處理。主機數(shù)據(jù)傳輸速度一般與CY7B923/933不一致,發(fā)送端及接收端都需設(shè)置數(shù)據(jù)緩存,緩存一般用FIFO,我們采用CYPRESS公司的CY7C45X系列,它可與CY7B923/933組成無縫接口。若用FIFO的標(biāo)志位作控制信號,則應(yīng)注意標(biāo)志延遲時間。CY7B933的輸出信號CKR,可作為其輸出數(shù)據(jù)寫入FIFO的時鐘,可作寫FIFO使能信號,使用的優(yōu)點是同步字串到來它無效,自動濾除同步字串,這就保證了只將有效數(shù)據(jù)寫入FIFO， 避免同步字將FIFO填滿。電路板設(shè)計應(yīng)采用四層板,CY7B923/933的位置應(yīng)靠近雙絞線接口使連線最短,連線長度應(yīng)盡量保持對稱。輸出輸入端都應(yīng)接匹配電阻,如圖3所示。R1、R2阻值由下式給出:R1=(VccRt)/Vtt；R2=(VccRt)/(Vcc-Vtt)。式中,Rt為傳輸線阻抗；Vtt=Vcc-2V。如Vcc=5V,UTP雙絞線阻抗為110Ω,則R1=183Ω，R2=275Ω。
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發(fā)送數(shù)據(jù)前,先進行系統(tǒng)自檢,然后進入傳輸狀態(tài)。數(shù)據(jù)在發(fā)送時鐘CKW上升沿進入發(fā)送器,通過發(fā)送器的延遲為21tB-10ns(tB為移位時鐘周期，tB=1/10CKW),經(jīng)傳輸線、變壓器進入接收器,數(shù)據(jù)通過接收器的延時為24tB+10ns。變壓器隔離傳輸線上可能的高電位,并有帶通濾波的作用。當(dāng)7B933的RF=H,成幀器使能,成幀器開始搜尋數(shù)據(jù)流中的同步字K28.5,并以K28.5邊界為基準(zhǔn),確定后續(xù)數(shù)據(jù)的字節(jié)邊界。一些隨機錯誤可導(dǎo)致串行數(shù)據(jù)變成同步字K28.5,造成錯誤成幀。為避免出錯,可在數(shù)據(jù)同步過程中關(guān)閉成幀器,即令成幀器使能端RF=L,或令RF=H超過2048字節(jié),此時需在連續(xù)5字節(jié)中有兩個K28.5才能啟動成幀功能,可大大減少偶然錯誤導(dǎo)致誤成幀。
作者在一VME總線長距離高速點對點串行通信系統(tǒng)中，采用最大數(shù)據(jù)傳輸率為330Mbps的標(biāo)準(zhǔn)CY7B923/933及UTP雙絞線,實現(xiàn)了400Mbps的串行數(shù)據(jù)傳輸,工作穩(wěn)定可靠,誤碼率小于10-10。該工作是在中科院高能物理研究所過雅南研究員、趙棣新研究員的指導(dǎo)下完成的,在此表示衷心地感謝!
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參考文獻
1 HOTLink Transmitter/Receiver Data Sheet.Cypress Semiconductor? Corporation， 1997