What's wrong in my design?

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I think the generation of sof does not require the setting of AS or PS during compilation. Only when you need pof, you need set either AS or PS in Configuration scheme under Device-> Device and Pin Options -> Configuration tab. (But make sure on hardware, the MSEL pins of master and slave devices are set to AS and PS respectively.)

So in your case, compile your designs of master device and slave device and generate their sof files separately.

When combining them with Convert Programming Files, add two sof files below Page_0 with master device sof on the top. Do not use Page_1. (First click Page_0 to highlight it, then click Add File.. button to add sof files, do not click Add Sof Data) And generate the pof file.

This is how I do with my project, which is the same case as yours.

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Thank wdshen a lot! your guidance makes the problem solved. Thanks again!