而且市面很多帶單片機的產品中，時間都做不準，這絕對是設計問題。例如：筆者車子上的時鐘日差有10秒多。
一般大多數人所設計的定時器時常，是根據晶振標出的數據（如12M）計算而成的，固定不變，設計呆板。但普通晶振的實際振蕩頻率是不可能與標出的數據完全相同的，例如：12.00043M，11.99985……，這個誤差必然積累，所以時鐘就不準了。另外絕大多數設計也沒有考慮微調方案，對于日差幾秒無法控制。
日差1秒要求的精準是：1秒/（24小時*60分*60秒），對應12M晶振；當頻率是12.000014M或11.999986M，日差就有1.2秒。可見用普通晶振做的時間鐘，如果不采取修正措施，會因精度不夠，時鐘日差10多秒是很普遍的。
在筆者設計的有單片機時間產品中，隨機取10個，同時上電，3天后再看時鐘，它們之間的時間誤差，一般都不會超出1秒（普通晶振，不聯網）。許多產品月差在1秒之內。
1、實現方法：
1）將時間的定時中斷時常數做成可修正的，且加入微調常數，定時時常數與微調時常數在FLASH中取得。微調時常數用于修正時常數的小數點之后的部分。
2）程序可以對某端口輸入的秒脈沖再進行計算，并據之修正定時器的時常數，以及微調常數，并存入FLASH中。達成利用外部精準脈沖源對時鐘進行校準的目的，說白了，就是用外校消除普通晶振的個體差異。同時程序也可以輸出自己的秒脈沖，這樣就可以達成產品之間的互校。
3）選擇一個產品，對其時鐘進行精確校準（這需要外部精準的時鐘源。如果手中沒有，可以多花點時間，用手機、電視、廣播上的報時進行校準），這樣就可以將它做成一個自己的【標準秒脈沖時鐘源】。
4）產品出貨前，用自己的【標準秒脈沖時鐘源】校一下（將輸出的標準秒脈沖送入其它產品的校準端口，讓其它產品自動完成定時器時常數及微調常數的修正。）
如此處理，時間鐘的精度就取決于晶振的穩定度，而不是精度。而晶振的穩定度普遍可以達到PPM級，當環境溫度變化不大時，極易保證《11.57PPM，這就達成了日差小于1秒的目的。
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