1794-IH16 如何證明一條內存條的PCB設計是好的呢？大家想的沒錯！可以拿一塊現成通用的CPU主板來搭配驗證。那如果主板上有的槽位驗證通過，有的槽位不滿足要求呢，這到底算誰的問題？
的確是這樣，隨著近年來國產化的需求越來越大，從芯片級的自研芯片到應用級的各種PCIE加速卡再到內存條，國產化的進程慢慢覆蓋到更大的硬件范圍。本文講述的是一款自研內存條的測試debug過程，看看通過這個案例能給大家帶來什么樣的一些思考。
這是一款DDR4內存條的測試debug案例，這家客戶找到我們幫忙進行該自研DIMM條的的PCB設計及后面的加工，樣板回來之后他們進行測試，向我們提出了問題。客戶是把我們設計加工好的內存條搭配一款現成的電腦主板平臺上進行測試得到的結論。可能大多數人看到測試的描述后的反應是會不會這款DIMM條的設計不太好導致的，因為主板是經過驗證的平臺，因為認為自研的DIMM條有問題的可能性更大。高速先生一開始也往這個方向來走，因此在重新審閱我們這款內存條的PCB設計的同時也check了一下我們的仿真報告。
而回看我們的仿真報告，由于在一開始設計和仿真的時候客戶也沒向高速先生說明是用哪個平臺的主板搭配進行驗證，因此高速先生采用的是模擬主板的仿真方式，按照經驗來模擬主板的走線進行仿真，認為主板是只帶一條內存條的情況進行仿真，從我們的仿真結果看，是能夠支持3200Mbps的速率的。
仿真結果明顯和實際測試的情況對不上，于是我們確認仿真方法沒什么大問題之后，開始去關注是不是對主板的仿真模型模擬得不準確導致的。于是我們按照客戶郵件給的主板型號去研究該主板的PCB設計，發現果然和我們主板的仿真模型不一樣，客戶驗證的平臺主板都是一個通道帶2條內存條的設計，如下所示：
憑借著對多負載的設計經驗和度，高速先生對客戶的測試場景也有了以下的疑問。
客戶也很快給了他們更具體的測試結果。
也就是說在1拖2的主板上，1和2槽位是一個通道，3和4槽位是一個通道，如果內存條插靠近后面的2或4槽位是OK的，插前面的1槽位是有問題的。
高速先生立馬就明白了個中的原因，于是按照該平臺主板的模型進行了更精確的仿真，發現仿真測試的確就能和測試的結果對上了。
由于1槽位是在拓撲的前端，因此會存在后面2槽位所帶來的走線stub，因此信號質量會明顯變差，而4槽位由于是在拓撲的末端，在3槽位的后面，理論上不存在stub，所以信號質量可以接受。
另外針對1槽位降頻到2133Mbps可以運行，我們也做了對于降頻的仿真，發現也基本能和測試結果對應上。
最后再說一點就是，如果我們能提前預知到客戶所要搭配的主板，并且能拿到仿真模型的話，我們的仿真結果確是可以和測試比較好的對應上的。另外也從這個案例清晰的說明，DDR系統能支持3200Mbps并不是一件容易做到的事情哈。
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