6ES7972-0CB35-0XA0 對于時序的計算和分析在一些相關文獻里有詳細的介紹，下面列出需要設置和分析的8個方面：
1）寫建立分析：DQ vs. DQS
2）寫保持分析：DQ vs. DQS
3）讀建立分析：DQ vs. DQS
4）讀保持分析：DQ vs. DQS
5）寫建立分析：DQS vs. CLK
6）寫保持分析：DQS vs. CLK
7）寫建立分析：ADDR/CMD/CNTRL vs. CLK
8）寫保持分析：ADDR/CMD/CNTRL vs. CLK
表2舉了一個針對寫建立（Write Setup）分析的例子。表中的一些數據需要從控制器和存儲器廠家獲取，段”Interconnect”的數據是取之于SI仿真工具。
對于DDR2上面所有的8 項都是需要分析的，而對于DDR3，5項和6項不需要考慮。在PCB設計時，長度方面的容差必須要保證total margin是正的。
8
PCB Layout
在實際的PCB設計時，考慮到SI的要求，往往有很多的折中方案。通常，需要優先考慮對于那些對信號的完整性要求比較高的。畫PCB時，當考慮以下的一些相關因素，那么對于設計PCB來說可靠性就會更高。
1）首先，要在相關的EDA工具里設置好拓撲結構和相關約束。
2）將BGA引腳突圍，將ADDR/CMD/CNTRL引腳布置在DQ/DQS/DM字節組的中間，由于所有這些分組操作，為了盡可能少的信號交叉，一些獨立的管腳也許會被交換到其它區域布線。
3）由串擾仿真的結果可知，盡量減少短線（stubs）長度。通常，短線（stubs）是可以被削減的，但不是所有的管腳都做得到的。
在BGA焊盤和存儲器焊盤之間也許只需要兩段的走線就可以實現了，但是此走線必須要很細，那么就提高了PCB的制作成本，而且，不是所有的走線都只需要兩段的，除非使用微小的過孔和盤中孔的技術。最終，考慮到信號完整性的容差和成本，可能選擇折中的方案。
4）將Vref的去耦電容靠近Vref管腳擺放；Vtt的去耦電容擺放在最遠的一個SDRAM外端；VDD的去耦電容需要靠近器件擺放。小電容值的去耦電容需要更靠近器件擺放。
正確的去耦設計中，并不是所有的去耦電容都是靠近器件擺放的。所有的去耦電容的管腳都需要扇出后走線，這樣可以減少阻抗，通常，兩端段的扇出走線會垂直于電容布線。
5）當切換平面層時，盡量做到長度匹配和加入一些地過孔，這些事先應該在EDA工具里進行很好的仿真。
通常，在時域分析來看，差分線的正負兩根線要做到延時匹配，保證其誤差在+/- 2ps，而其它的信號要做到+/- 10 ps。
6ES7972-0CB35-0XA0
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