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通(tong)用(yong)串行(xing)(xing)總(zong)線(xian)Universal Serial Bus,縮(suo)寫為USB,是一(yi)個標(biao)準的(de)(de)(de)外部(bu)總(zong)線(xian)標(biao)準。在(zai)計算機領域，它是已(yi)經獲得廣泛使(shi)用(yong)的(de)(de)(de)技術(shu)，常用(yong)的(de)(de)(de)電(dian)(dian)子(zi)設(she)(she)備(bei)上幾乎都有(you)USB接口。USB最(zui)初用(yong)于(yu)規范個人電(dian)(dian)腦PC與(yu)外部(bu)設(she)(she)備(bei)之(zhi)間(jian)的(de)(de)(de)連接與(yu)通(tong)訊(xun)，一(yi)般由PC來(lai)充當(dang)host端,各(ge)種(zhong)外部(bu)設(she)(she)備(bei)作為device端，所有(you)的(de)(de)(de)通(tong)信(xin)都必(bi)須(xu)在(zai)host與(yu)device之(zhi)間(jian)進行(xing)(xing)，各(ge)device之(zhi)間(jian)無(wu)法進行(xing)(xing)直接通(tong)信(xin)。隨著(zhu)USB技術(shu)的(de)(de)(de)不(bu)(bu)斷廣泛應用(yong)，各(ge)種(zhong)帶USB接口的(de)(de)(de)電(dian)(dian)子(zi)設(she)(she)備(bei)不(bu)(bu)斷涌現,形成了現成遍地開(kai)花的(de)(de)(de)局面。這樣一(yi)來(lai)沒有(you)PC就無(wu)法使(shi)用(yong)USB進行(xing)(xing)通(tong)信(xin)，顯得十分(fen)不(bu)(bu)方(fang)便(bian)。于(yu)是USB OTG技術(shu)開(kai)始出現。USB OTG是USB On-The-Go的(de)(de)(de)縮(suo)寫，是近年發(fa)展起來(lai)的(de)(de)(de)技術(shu)，主(zhu)要(yao)應用(yong)于(yu)各(ge)種(zhong)不(bu)(bu)同(tong)的(de)(de)(de)設(she)(she)備(bei)或(huo)移(yi)動(dong)設(she)(she)備(bei)間(jian)的(de)(de)(de)聯接，進行(xing)(xing)數據(ju)交換(huan)，特(te)別是PAD、移(yi)動(dong)電(dian)(dian)話(hua)、消費類設(she)(she)備(bei)。

在(zai)以(yi)(yi)前(qian)(qian)傳(chuan)統的(de)(de)(de)通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)方式中(zhong)(zhong)，例如以(yi)(yi)前(qian)(qian)個人(ren)計(ji)算機的(de)(de)(de)串(chuan)(chuan)口和并(bing)(bing)口。以(yi)(yi)前(qian)(qian)的(de)(de)(de)RS232串(chuan)(chuan)行通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin),因(yin)為每次(ci)僅(jin)能傳(chuan)送一(yi)個bit的(de)(de)(de)數(shu)(shu)據(ju)，波特(te)率(lv)(lv)也無法獲得更進(jin)一(yi)步(bu)的(de)(de)(de)提(ti)(ti)高(gao)(gao)，數(shu)(shu)據(ju)傳(chuan)輸速(su)(su)(su)率(lv)(lv)很低。相比串(chuan)(chuan)行通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)，并(bing)(bing)行通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)能同時傳(chuan)送多個比特(te)位，可(ke)以(yi)(yi)顯著加快(kuai)(kuai)數(shu)(shu)據(ju)通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)速(su)(su)(su)率(lv)(lv)。然而并(bing)(bing)行通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)，需(xu)要(yao)(yao)多根數(shu)(shu)據(ju)線(xian)(xian)來(lai)同時傳(chuan)送數(shu)(shu)據(ju)的(de)(de)(de)各bit位，如果傳(chuan)輸距(ju)(ju)(ju)離(li)(li)(li)稍長，通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)線(xian)(xian)路的(de)(de)(de)成(cheng)本(ben)明(ming)顯增(zeng)加，不(bu)太適(shi)合距(ju)(ju)(ju)離(li)(li)(li)稍遠的(de)(de)(de)數(shu)(shu)據(ju)傳(chuan)輸。然而不(bu)管(guan)是(shi)(shi)串(chuan)(chuan)行還(huan)是(shi)(shi)并(bing)(bing)行通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin),由于它們(men)直接(jie)使用(yong)電(dian)平(ping)的(de)(de)(de)高(gao)(gao)低來(lai)表示(shi)二進(jin)制的(de)(de)(de)01信(xin)(xin)(xin)(xin)(xin)(xin)(xin)號,使得電(dian)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)號在(zai)傳(chuan)輸過程中(zhong)(zhong)抗(kang)干(gan)擾(rao)能力差(cha)(cha)，隨著通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)距(ju)(ju)(ju)離(li)(li)(li)的(de)(de)(de)增(zeng)加，信(xin)(xin)(xin)(xin)(xin)(xin)(xin)號衰減快(kuai)(kuai)。要(yao)(yao)想提(ti)(ti)高(gao)(gao)通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)距(ju)(ju)(ju)離(li)(li)(li)，同時提(ti)(ti)高(gao)(gao)通(tong)(tong)(tong)(tong)(tong)訊(xun)速(su)(su)(su)率(lv)(lv)，就(jiu)需(xu)要(yao)(yao)采(cai)用(yong)差(cha)(cha)分(fen)線(xian)(xian)來(lai)傳(chuan)輸數(shu)(shu)據(ju)。USB就(jiu)是(shi)(shi)基于這一(yi)原理(li)，通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)過程中(zhong)(zhong)僅(jin)需(xu)要(yao)(yao)D+，D-這一(yi)對差(cha)(cha)分(fen)數(shu)(shu)據(ju)線(xian)(xian)，使得通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)線(xian)(xian)路簡單,通(tong)(tong)(tong)(tong)(tong)信(xin)(xin)(xin)(xin)(xin)(xin)(xin)速(su)(su)(su)率(lv)(lv)有了(le)巨大(da)的(de)(de)(de)提(ti)(ti)高(gao)(gao),同時抗(kang)干(gan)擾(rao)能力強(qiang)。USB接(jie)口還(huan)能供電(dian)，支持熱插(cha)拔(ba)。自從(cong)該技術在(zai)PC中(zhong)(zhong)應用(yong)以(yi)(yi)來(lai)，就(jiu)受到了(le)廣泛歡(huan)迎。

USB接口(kou)可(ke)以(yi)進行供(gong)電(dian)，這(zhe)樣小型(xing)的(de)設(she)備(bei)(bei)就不需要單(dan)獨供(gong)電(dian)，簡化(hua)了設(she)備(bei)(bei)的(de)供(gong)電(dian)設(she)計(ji)。于是各種小型(xing)的(de)電(dian)子設(she)備(bei)(bei)都特別(bie)喜歡使用USB接口(kou)。嵌(qian)入(ru)式設(she)備(bei)(bei)中，經常都需要該設(she)備(bei)(bei)充當device的(de)角(jiao)色。下面以(yi)device的(de)設(she)計(ji)為例(li)，討論一下USB接口(kou)的(de)軟硬件設(she)計(ji)。比如一個簡單(dan)的(de)基于單(dan)片機的(de)USB數據采集器，可(ke)以(yi)如下設(she)計(ji):

這是(shi)一個基(ji)于AVR單片(pian)機的通用USB數(shu)據(ju)采集器，使用了一款經典(dian)的USB接(jie)(jie)口(kou)芯片(pian)PDIUSBD12,單片(pian)機與(yu)(yu)PDIUSBD12之間(jian)直接(jie)(jie)利(li)(li)用傳統的數(shu)據(ju)、地(di)址和控制三(san)總線方(fang)式連接(jie)(jie)，有利(li)(li)于最大化數(shu)據(ju)傳輸。PDIUSBD12芯片(pian)引出差分(fen)數(shu)據(ju)線D+,D-與(yu)(yu)USB插(cha)頭相連。

軟(ruan)件開發上，需要實現USB協議棧，按傳(chuan)統的軟(ruan)件分(fen)層(ceng)設(she)計(ji)方(fang)法來進行設(she)計(ji)即可，比如該數據采集器的軟(ruan)件層(ceng)次結構如下(xia):

當USB檢測到device插入(ru)后，就會自動和(he)(he)設(she)備進(jin)行前期的多次對話(hua)，完成必(bi)要的初始化和(he)(he)加載(zai)任(ren)務，我們設(she)計device時，就需要不斷地(di)向host返回相應的信息，例(li)如:

利用(yong)傳統(tong)的(de)單片機前后臺(tai)程序(xu)設(she)計(ji)方法就可以完成。該數據(ju)采(cai)集器是個通用(yong)的(de)USB device設(she)計(ji)模塊，其它USB設(she)備只(zhi)需(xu)在其基礎上做相應的(de)軟硬件修改即可實現。