Ludovic,
As you know this specific line can go floating per design, if care is not taken. There are a number of of ways of ensuring it does not cause a problem. Here is a short list.
- Add a pull-up resistor (easy, secure, but cost extra and can add current consumption during run time)
- Configure the host MCU to enable its internal pull up (no added cost)
- Configure the host MCU to force the IO to ground during times where the MISO line can go floating.
In most case people implement option 2 or 3.
Regards,
Thomas