Many guides instruct that digital signals should have a series resistor placed by the driver, whose resistance is equal to the line impedance (Z0) minus the driver impedance (Zd). Here is one such guide.
However, I can't find output impedance specs for any microcontrollers. I've looked in the datasheets for the Renesas and PIC MCUs I use, and it's not even mentioned.
The situation I'm currently facing is a Renesas MCU talking to an SD card via 21cm-long traces, at 33MHz I think. (Some of the signals are bidirectional, but that's another story.) In order to avoid ringing and EMI emissions, which seem like they could be a problem with such long traces and high frequency, I want to terminate correctly.
Possible solutions - would it be likely that the driver impedance is really low, like only 1 ohm or so? I can calculate my line impedance, and if the driver impedance is far smaller, perhaps I could just neglect it?
Or, could I possibly attempt to detect the EMI caused by the ringing by (don't laugh) holding an AM radio next to the transmission lines, and try different termination resistor values until the ringing is minimized? Is there any better way to do this at home?