Lab Equipment and What It Answers
The debugger on your Nucleo can single-step your code, read every register, and show you the contents of memory — and it is blind to everything that happens outside the package. It will tell you, truthfully, that you wrote 0xA5 to the SPI data register. It cannot tell you whether 0xA5 left the pin, whether the clock that carried it was clean, whether the device on the other end was even powered.