I analyzed the signals and it is working correctly (CS, SCK and MOSI, see pic.). The measured clock frequency is about 8MHz. I tried to change it by varying the last parameter, which I thought is clock_div, because it is too fast for my SPI slave. But it does not change anything. The clock stays at 8Mhz, what ever I tried.
To send 3 bytes I used
What am I doing wrong?