[sdiy] Beware Magic Numbers!`

[Byron G. Jacquot]

>The proper way of doing it is use defines (C) or EQUs (asm) in a header file
>and not use numbers directly in the code.

You can take that one step further, too.

If you're on a platform with C++ support (don't laugh, even the AVR has C++ these days), use an enum to define named values.  It also defines a type, which you can use to declare and pass items of that type.  It'll also give you some compile-time type checking -- it likely would have caught the mixup between 0 and SPI_MODE_0.

-Byron Jacquot