Using FDM and VDM when convienient?

Question

So I am curious about a potential use case for FDM. I don’t have physcial chip yet but i am looking to write a small single purpose driver to maximize performance for my usecase. I see that FDM could have a benifit in use with the DMA as i don’t have to have any extra interrupts or handling for toggling the CS between register configurations. but i want to use it in conjunction with VDM. And I couldn’t get a clear “yea thats fine” or “no that definatly will not work” out of the datasheet. It does say that in the FDM configuration the CS will be constantly selected but my use case isn’t to remove a cs pin but to generate a buffer for configuration of multiple registers and letting the DMA handle the SPI configuration. since the “Op Mode” definition makes it sound like i can send groups of fixed size data across the spi. as long as the SCSn is Low. Ideally i would treat FDM mostly like VDM pulling the CSn High and low after transfers but allowing me to set mutiple registers via a dma register without getting the microcontroller involved with the SCSn managment. If this works it would be nice to know.

Eugeny · Answer

VDM mode transaction can only be finished with CS high. FDM is a feature giving you some flexibility not to toggle CS between transactions because it involves predefined message format/length.