I remember reading this somewhere but cannot find the ratio of the rise time vs. the propagation time (i.e. the trace length?) of a signal when transmission line effects come into effect?
For example – if I have a SPI bus running at 12.5MHz. If it runs a few inches through the PCB trace, it is not a transmission line – but at what length goes it become a transmission line (at least in theory). How to calculate that?