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小(xiao)(xiao)系(xi)統(tong)，指的(de)(de)(de)(de)是的(de)(de)(de)(de)用少的(de)(de)(de)(de)元件(jian)組成的(de)(de)(de)(de)單片機(ji)可以工作的(de)(de)(de)(de)系(xi)統(tong)。一(yi)般在設(she)計小(xiao)(xiao)系(xi)統(tong)的(de)(de)(de)(de)時(shi)(shi)候分為這四種必(bi)要的(de)(de)(de)(de)電路(lu)，分別是1、電源供(gong)電電路(lu);2、單片機(ji)復位電路(lu);3、時(shi)(shi)鐘振蕩(dang)電路(lu);4、程(cheng)序(xu)的(de)(de)(de)(de)下載接口電路(lu)。這次基于我們研發(fa)的(de)(de)(de)(de)WiFi小(xiao)(xiao)車51核心控制板的(de)(de)(de)(de)小(xiao)(xiao)系(xi)統(tong)原理圖(tu)來(lai)講一(yi)下51單片機(ji)的(de)(de)(de)(de)小(xiao)(xiao)系(xi)統(tong)。

WiFi小車51核心控制板使用(yong)的(de)(de)(de)單片(pian)機(ji)是STC15W4K32S4系(xi)列(lie)單片(pian)機(ji)，這個(ge)系(xi)列(lie)單片(pian)機(ji)為寬(kuan)電壓單片(pian)機(ji)，指的(de)(de)(de)是工作(zuo)電壓寬(kuan)它的(de)(de)(de)工作(zuo)電壓可為2.5V-5.5V，為了方便大家使用(yong)我們選擇了與(yu)多核心實驗(yan)箱同款的(de)(de)(de)單片(pian)機(ji)IAP15W4K61S4，具有4K的(de)(de)(de)SRAM和61KB的(de)(de)(de)Flish程(cheng)序存儲空間，帶有8路PWM其中包含6路15位的(de)(de)(de)PWM和2路CCP，4個(ge)串行通信(xin)接口，5個(ge)內部(bu)定時器和8路10位的(de)(de)(de)ADC通道(dao)。這款單片(pian)機(ji)是可以(yi)直(zhi)接在線對程(cheng)序進(jin)行仿真調試(shi)的(de)(de)(de)，方便開發(fa)和學習。

電(dian)源(yuan)供電(dian)電(dian)路對于一個完整的(de)電(dian)子產品來講(jiang)，首先面臨的(de)問(wen)題(ti)就是系(xi)統的(de)供電(dian)，所以電(dian)源(yuan)是整個系(xi)統平穩(wen)運行的(de)基礎(chu)。在設(she)計的(de)時(shi)候會在單(dan)片機每個電(dian)源(yuan)引腳(jiao)旁邊放上一個濾波電(dian)容(rong)來保障輸入到單(dan)片機電(dian)源(yuan)引腳(jiao)的(de)電(dian)壓是穩(wen)定(ding)的(de)，若(ruo)單(dan)片機的(de)時(shi)鐘頻率(lv)較(jiao)高(gao)，建議(yi)(yi)這(zhe)處的(de)電(dian)容(rong)設(she)置(zhi)成0.01uF;若(ruo)單(dan)片機的(de)時(shi)鐘頻率(lv)較(jiao)低，建議(yi)(yi)設(she)置(zhi)為0.1uF。

復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)電(dian)路的(de)(de)作用(yong)(yong)是(shi)給單(dan)片(pian)(pian)(pian)(pian)機(ji)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)，使單(dan)片(pian)(pian)(pian)(pian)機(ji)恢復(fu)(fu)(fu)到起始(shi)(shi)狀(zhuang)態程序(xu)從頭開始(shi)(shi)執行，好比電(dian)腦在(zai)(zai)使用(yong)(yong)過(guo)程中(zhong)出現了死機(ji)的(de)(de)情況，按(an)下重啟按(an)鍵電(dian)腦內部程序(xu)從頭開始(shi)(shi)運行。單(dan)片(pian)(pian)(pian)(pian)機(ji)也(ye)(ye)一(yi)樣(yang)，當單(dan)片(pian)(pian)(pian)(pian)機(ji)在(zai)(zai)工作過(guo)程中(zhong)也(ye)(ye)會受到環(huan)境等因素出現程序(xu)跑(pao)飛的(de)(de)現象。從單(dan)片(pian)(pian)(pian)(pian)機(ji)的(de)(de)數據手(shou)冊上(shang)得知(zhi)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)電(dian)路的(de)(de)工作原理是(shi)在(zai)(zai)需要復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)的(de)(de)時候給單(dan)片(pian)(pian)(pian)(pian)機(ji)的(de)(de)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)引腳輸入兩個(ge)機(ji)器(qi)周期以上(shang)的(de)(de)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)信(xin)號(hao)(hao)。這個(ge)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)信(xin)號(hao)(hao)分兩種(zhong)(zhong)，一(yi)種(zhong)(zhong)是(shi)高電(dian)平復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)，一(yi)種(zhong)(zhong)是(shi)低電(dian)平復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)。如(ru)下圖這兩種(zhong)(zhong)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)電(dian)路左側(ce)(ce)為低電(dian)平復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)電(dian)路，右側(ce)(ce)是(shi)高電(dian)平復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)。具體選用(yong)(yong)那種(zhong)(zhong)就需要看所使用(yong)(yong)的(de)(de)單(dan)片(pian)(pian)(pian)(pian)機(ji)是(shi)那種(zhong)(zhong)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)電(dian)路，原理都(dou)是(shi)一(yi)樣(yang)的(de)(de)只是(shi)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)信(xin)號(hao)(hao)不同，而(er)51單(dan)片(pian)(pian)(pian)(pian)機(ji)常(chang)用(yong)(yong)的(de)(de)是(shi)低電(dian)平復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)信(xin)號(hao)(hao)。

由(you)圖(tu)中可以看(kan)出來這(zhe)(zhe)兩(liang)種復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)是(shi)(shi)(shi)由(you)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)和電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻串聯構成(cheng)的(de)(de)(de)(de)，不同的(de)(de)(de)(de)是(shi)(shi)(shi)我們考慮(lv)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)在直流中為短路(lu)(lu)的(de)(de)(de)(de)情(qing)況下單片機(ji)的(de)(de)(de)(de)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)引腳(jiao)所(suo)(suo)接入(ru)的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)信號不一(yi)(yi)樣。高(gao)(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)中復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)引腳(jiao)接的(de)(de)(de)(de)是(shi)(shi)(shi)低電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)信號，反(fan)之低電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)引腳(jiao)接高(gao)(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)。復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)通電(dian)(dian)(dian)(dian)(dian)(dian)(dian)之后由(you)于(yu)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)要(yao)進行充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)所(suo)(suo)以RST端的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓并不是(shi)(shi)(shi)突變的(de)(de)(de)(de)，如在高(gao)(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)中，系統一(yi)(yi)上電(dian)(dian)(dian)(dian)(dian)(dian)(dian)RST腳(jiao)將會出現(xian)一(yi)(yi)段時(shi)間(jian)(jian)的(de)(de)(de)(de)高(gao)(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)，而且(qie)這(zhe)(zhe)個(ge)高(gao)(gao)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)平(ping)持續的(de)(de)(de)(de)時(shi)間(jian)(jian)是(shi)(shi)(shi)由(you)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)的(de)(de)(de)(de)RC值(zhi)決定的(de)(de)(de)(de)，所(suo)(suo)以選用合適(shi)的(de)(de)(de)(de)組(zu)合的(de)(de)(de)(de)RC值(zhi)就可保(bao)證單片機(ji)能(neng)夠可靠(kao)的(de)(de)(de)(de)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)。通常會選用0.1uF的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)和10K歐姆的(de)(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)阻值(zhi)，在工作電(dian)(dian)(dian)(dian)(dian)(dian)(dian)壓是(shi)(shi)(shi)5V的(de)(de)(de)(de)情(qing)況下電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)充(chong)滿(man)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)(de)(de)(de)時(shi)間(jian)(jian)大概為1ms，這(zhe)(zhe)1ms的(de)(de)(de)(de)充(chong)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)時(shi)間(jian)(jian)遠大于(yu)了單片機(ji)的(de)(de)(de)(de)兩(liang)個(ge)機(ji)器周期。在這(zhe)(zhe)兩(liang)種復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)中還(huan)包含了自動(dong)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)和手動(dong)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)，復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)在系統上電(dian)(dian)(dian)(dian)(dian)(dian)(dian)的(de)(de)(de)(de)時(shi)候(hou)會完(wan)成(cheng)一(yi)(yi)次(ci)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)，除此(ci)之外可以通過復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)電(dian)(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)上的(de)(de)(de)(de)按鍵(jian)來實現(xian)手動(dong)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)，手動(dong)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)的(de)(de)(de)(de)原理是(shi)(shi)(shi)按下按鍵(jian)后將電(dian)(dian)(dian)(dian)(dian)(dian)(dian)容(rong)(rong)器兩(liang)端短路(lu)(lu)直接將復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)信號輸入(ru)到RST引腳(jiao)來實現(xian)復(fu)(fu)(fu)位(wei)(wei)(wei)(wei)(wei)。

時(shi)(shi)鐘(zhong)震蕩(dang)電(dian)(dian)路(lu)：前面的電(dian)(dian)源電(dian)(dian)路(lu)比較(jiao)好理解，但是(shi)什么事時(shi)(shi)鐘(zhong)電(dian)(dian)路(lu)呢(ni)?它的作用(yong)是(shi)什么呢(ni)?其實時(shi)(shi)鐘(zhong)震蕩(dang)電(dian)(dian)路(lu)在單(dan)片(pian)機中的作用(yong)就(jiu)像(xiang)是(shi)人(ren)的心臟一(yi)樣，每時(shi)(shi)每刻(ke)都不(bu)斷的跳動著，所以時(shi)(shi)鐘(zhong)電(dian)(dian)路(lu)是(shi)是(shi)為單(dan)片(pian)機內部各部分正常的工作提供驅(qu)動力(li)。WiFi小車上的時(shi)(shi)鐘(zhong)震蕩(dang)電(dian)(dian)路(lu)如下圖(tu)所示。

經常的(de)我們的(de)震(zhen)蕩(dang)(dang)電(dian)路(lu)使用(yong)石英晶(jing)體(ti)振(zhen)(zhen)蕩(dang)(dang)器(qi)作(zuo)為單片(pian)機的(de)時(shi)鐘源，如果有時(shi)鐘信號的(de)時(shi)候(hou)也可(ke)以(yi)直(zhi)接輸(shu)入到TXAL1引腳(jiao)。如果使用(yong)無源晶(jing)振(zhen)(zhen)的(de)話在(zai)(zai)單片(pian)機的(de)XTAL1和(he)XTAL2腳(jiao)接上一(yi)個石英晶(jing)體(ti)震(zhen)蕩(dang)(dang)器(qi)，它(ta)在(zai)(zai)通(tong)電(dian)的(de)時(shi)候(hou)會產(chan)生(sheng)特定的(de)頻(pin)率(lv)的(de)震(zhen)蕩(dang)(dang)，后通(tong)過電(dian)路(lu)可(ke)以(yi)輸(shu)出(chu)一(yi)個頻(pin)率(lv)穩定的(de)時(shi)鐘信號來驅動單片(pian)機工作(zuo)。

時鐘(zhong)電路還要用到(dao)兩個電容C1，C2。這(zhe)兩個電容和內部電路共同組成一(yi)定頻率的(de)震蕩，電容起固定頻率的(de)作用。一(yi)般在設(she)計的(de)時候要注意是晶(jing)振、外部電容器(qi)與(yu)單片機之間的(de)走線(xian)要盡(jin)量短，如(ru)果線(xian)太長，會使它對干擾變(bian)得非常敏感(gan),而且還會給振蕩器(qi)增加寄(ji)生電容。

在WiFi小車(che)控制板(ban)上用(yong)(yong)的(de)單片機其實內部自帶了一個RC振(zhen)蕩(dang)器作(zuo)為(wei)時(shi)鐘源(yuan)來 ，但(dan)是(shi)它也可以(yi)使(shi)用(yong)(yong)外(wai)部晶(jing)振(zhen)。一般RC振(zhen)蕩(dang)器容易發生(sheng)溫漂，使(shi)時(shi)鐘精度受到影響(xiang)，所(suo)以(yi)我們在這里依(yi)然(ran)使(shi)用(yong)(yong)了外(wai)部的(de)石(shi)英晶(jing)體(ti)振(zhen)蕩(dang)器作(zuo)為(wei)時(shi)鐘振(zhen)蕩(dang)電路(lu)了。

還(huan)有一(yi)個(ge)部分(fen)就是單片機(ji)下(xia)(xia)載(zai)(zai)程序的(de)(de)(de)接口(kou)(kou)(kou)(kou)(kou)，這個(ge)就根據使用(yong)不同(tong)型(xing)號的(de)(de)(de)單片機(ji)和(he)(he)不同(tong)的(de)(de)(de)下(xia)(xia)載(zai)(zai)方式(shi)來(lai)決定的(de)(de)(de)。WiFi小車51核(he)心控制(zhi)板(ban)中(zhong)使用(yong)的(de)(de)(de)單片機(ji)是可(ke)以(yi)(yi)(yi)(yi)使用(yong)串(chuan)口(kou)(kou)(kou)(kou)(kou)進行下(xia)(xia)載(zai)(zai)和(he)(he)仿真的(de)(de)(de)，所以(yi)(yi)(yi)(yi)需(xu)要留出(chu)一(yi)個(ge)串(chuan)口(kou)(kou)(kou)(kou)(kou)作為下(xia)(xia)載(zai)(zai)程序的(de)(de)(de)接口(kou)(kou)(kou)(kou)(kou)，這個(ge)引(yin)出(chu)的(de)(de)(de)下(xia)(xia)載(zai)(zai)口(kou)(kou)(kou)(kou)(kou)除了可(ke)以(yi)(yi)(yi)(yi)下(xia)(xia)載(zai)(zai)程序外，還(huan)是普(pu)(pu)通(tong)的(de)(de)(de)串(chuan)口(kou)(kou)(kou)(kou)(kou)，可(ke)以(yi)(yi)(yi)(yi)向(xiang)普(pu)(pu)通(tong)的(de)(de)(de)串(chuan)口(kou)(kou)(kou)(kou)(kou)一(yi)樣做串(chuan)口(kou)(kou)(kou)(kou)(kou)通(tong)信使用(yong)。

有了(le)這四(si)種(zhong)電(dian)路單(dan)片(pian)機就可(ke)以穩定的工(gong)作啦(la)，我(wo)們在WiFi小車上接上我(wo)們的RGBLED燈、超聲波測距、電(dian)機驅動電(dian)路、按(an)鍵等外圍(wei)設備，就可(ke)以用單(dan)片(pian)機控制和通信啦(la)!