Master:       send command    delay       send dummy byte
                V |<--------------------->| V
       MOSI:  ___X  command    X_____________X   dummy     X____
       MISO:                      ___________X   answer    X__
       SCK :  ___|_|_|_|_|_|_|_|_____________|_|_|_|_|_|_|_|_____
                                V         ^
Slave :                      interrupt    |send answer
                                |<------->|
                                  command handling