the right way of using CSAAT in SPI when PS == 1

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Have anyone used SPI in Variable Peripheral Select mode before?

I now have problem with the way to use CSAAT in my device driver code.

I want to use Variable Peripheral Select mode in my driver and some driver functions need the CSn pin to be kept low for several SPI transfers and some don't, so I have set the CSAAT bit in the begining of those functions which need this, and I set the LASTXFER in their last transfer. But I found out that if I don't clear CSAAT in the end of these functions, then the following executing function which don't needs CSn to be low between transfers will get low CSn. But if I clear the CSAAT in the end of these functions, then their CSn will not be low between transfers.

So what should I do then?