Is the SPI flash module's CS pin exposed on ESP-12E module?

[liudr]

It's actually traditional to use gpio 15 for cs, as it's what used for cs when it's handled in hardware (of course, in our case we handle it ourselves, so you can use any pin, but since gpio 15 is already marked as cs...).

Do you mean that the SPI bus for sd card is software instead of hardware? Thanks.

[deshipu]

No, both SPI and HSPI are hardware, but the MicroPython implementation of SPI doesn't do CS toggling in SPI hardware – instead the drivers need to toggle that pin separately as a normal GPIO pin.