Posted on May 02, 2017 at 11:00What is STPM3x's max SPI clock?
The datasheet (always a recommended read) specifies 50ns clock period, meaning 20MHz clock frequency.
These boards are connected by flying wires(5cm). The max SPI clock could be influenced by the flying wire?
Are you familiar with Laplace/Fourier transformations ?
Rectangular signals like the SPI signal can be expressed as a series of sinusoidal waves of increasing frequency.
Your cabling needs to support a bandwidth of about 100MHz for you signals, else the distortions make it increasingly unrecognizable.
or Do I need a impedance resistor?
Would probably help. And twisting each signal line with a ground wire, to get a consistent impedance.
You can check the signal quality on the (signal) receiver side - if you are familiar with RF measurements.
Probe impedance should match signal/cable impedance as well, else you measure rubbish ...
