Master:       send command      send dummy byte
                V                 V                  V                 V
       MOSI:  ___X  command    X___X   dummy     X_...X   dummy     X___X   dummy     X__
       MISO:  ___X    IDLE     X___X    BUSY     X_...X   READY     X___X   answer    X__
       SCK :  ___|_|_|_|_|_|_|_|___|_|_|_|_|_|_|_|_...|_|_|_|_|_|_|_|___|_|_|_|_|_|_|_|__
                                V                    ^
Slave :                      interrupt               |send answer
                                |<------------------>|
                                  command handling on main loop