https://github.com/espressif/arduino-esp32/blob/9d547a8a441f70755c5b9d385e1092c1728da098/cores/esp32/esp32-hal-spi.h#L57 https://github.com/espressif/arduino-esp32/blob/9d547a8a441f70755c5b9d385e1092c1728da098/cores/esp32/esp32-hal-spi.c#L413