I am busy designing a carrier board for the iMX6ULL Colibri module. I noticed that on the Iris Carrier Board, 22 Ohm resistors were added to pretty much all I/O lines.
Is there an advantage of using the 22R resistors over setting the pin driver strength in the Linux Device Tree? Even with the 22R resistors, one would still need to set the pin driver strength to ~30 Ohms to match the trace characteristic impedance of 50 Ohms. Is it not simpler to just set the pin driver strength to ~50 Ohms and not use the resistors?
Generally the serial resistors in Hardware are used to reduce the slew rate of the signals which reduces also the radiation problems and EMC issues. Alternatively the serial resistor prevents also in case of the shortcuts damage to the SOC Pins.
You are right, if you need the impedance matching, then you will need to do the correct combination of hardware and software settings. For the high speed signals as USB, ETHERNET no serial resistors should be used, however for LCD signals serial resistors are recommended.
the serial resistors’ main function is to form a low-pass filter with any downstream capacitance to slew-rate limit the signals
if impedance matching is required this should be done through software settings in conjunction with any serial resistance which may already be on the PCBA
