描述
1768-L45
技巧3——在啟動(dòng)時(shí)執(zhí)行RAM檢查
為了建立一個(gè)更加可靠和扎實(shí)的系統(tǒng),確保系統(tǒng)硬件正常工作非常重要。畢竟硬件會(huì)發(fā)生故障。(幸運(yùn)的是軟件永遠(yuǎn)不會(huì)發(fā)生故障,軟件只會(huì)做代碼要它做的事,不管是正確的還是錯(cuò)誤的)。在啟動(dòng)時(shí)驗(yàn)證RAM的內(nèi)部或外部沒有問(wèn)題,是確保硬件可以如預(yù)期般運(yùn)作的一個(gè)好方法。
有許多不同的方法可用于執(zhí)行RAM檢查,但常用的方法是寫入一個(gè)已知的模式,然后等上一小段時(shí)間再回讀。結(jié)果應(yīng)該是所讀就是所寫。真相是,在大多數(shù)情況下RAM檢查是通過(guò)的,這也是我們想要的結(jié)果。但也有極小的可能性檢查不通過(guò),這時(shí)就為系統(tǒng)標(biāo)示出硬件問(wèn)題提供了極好的機(jī)會(huì)。
技巧4——使用堆棧監(jiān)視器
對(duì)許多的嵌入式開發(fā)者而言,堆棧似乎是一股相當(dāng)神秘的力量。當(dāng)奇怪的事情開始發(fā)生,工程師終于被難倒了,他們開始思考,也許堆棧中發(fā)生了什么事。結(jié)果是盲目地調(diào)整堆棧的大小和位置等等。但該錯(cuò)誤往往是與堆棧無(wú)關(guān)的,但怎能如此確定?畢竟,有多少工程師真的實(shí)際執(zhí)行過(guò)最壞情況下的堆棧大小分析?
堆棧大小是在編譯時(shí)就靜態(tài)分配好的,但堆棧是以動(dòng)態(tài)的方式使用的。隨著代碼的執(zhí)行,應(yīng)用程序需要的變量、返回的地址和其它信息被不斷存儲(chǔ)在堆棧中。這種機(jī)制導(dǎo)致堆棧在其分配的內(nèi)存中不斷增長(zhǎng)。然而,這種增長(zhǎng)有時(shí)會(huì)超出編譯時(shí)確定的容量極限,導(dǎo)致堆棧破壞相鄰內(nèi)存區(qū)域的數(shù)據(jù)。
絕對(duì)確保堆棧正常工作的一種方法是實(shí)現(xiàn)堆棧監(jiān)視器,將它作為系統(tǒng)“保健”代碼的一部分(有多少工程師會(huì)這樣做?)。堆棧監(jiān)視器會(huì)在堆棧和“其它”內(nèi)存區(qū)域之間創(chuàng)建一個(gè)緩沖區(qū)域,并填充已知的位模式。然后監(jiān)視器會(huì)不斷的監(jiān)視圖案是否有任何變化。如果該位模式發(fā)生了改變,那就意味著堆棧增長(zhǎng)得太大了,即將要把系統(tǒng)推向黑暗地獄!此時(shí)監(jiān)視器可以記錄事件的發(fā)生、系統(tǒng)狀態(tài)以及任何其它有用的數(shù)據(jù),供日后用于問(wèn)題的診斷。
大多數(shù)實(shí)時(shí)操作系統(tǒng)(RTOS)或?qū)崿F(xiàn)了內(nèi)存保護(hù)單元(MPU)的微控制器系統(tǒng)中都提供有堆棧監(jiān)視器。可怕的是,這些功能默認(rèn)都是關(guān)閉狀態(tài),或者經(jīng)常被開發(fā)人員有意關(guān)閉。在網(wǎng)絡(luò)上快速搜尋一下可以發(fā)現(xiàn),很多人建議關(guān)閉實(shí)時(shí)操作系統(tǒng)中的堆棧監(jiān)視器以節(jié)省56字節(jié)的閃存空間。等等,這可是得不償失的做法!
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