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STM32F7xx SDMMC_STA bit 9 reserved, acts like STBITERR?

Question asked by bittner.bruno on Apr 7, 2017

I have some SDMMC driver code carried over from STM32F4xx that uses bit 9 STBITERR of SDMMC_STA register.  In the STM32F7xx reference manual RM0410 DM00224583 DocID028270 Rev 2 dated April 2016, this bit in SDMMC_STA register is described as  "Reserved, must be kept at reset value." whereas the STM32F4xx RM0090 describes this bit as "STBITERR: Start bit not detected on all data signals in wide bus mode" in register SDIO_STA.  Similarly for the SDMMC_ICR and SDMMC_MASK registers (reserved, keep at reset value).

This bit seems to be doing something real, normally I return an error code on this condition, but if I don't test for it(since this bit doesn't exist in the F7) in my driver hangs. If I test for it but give normal return code, everything SEEMS to work fine.

What does this bit do in the F7?