Doh! It turns out that all I had to do to make the twitching servo problem go away was to wire a ground pin on the micro-controller board to the common ground for everything else. I thought that I had done the equivalent of this by having the micro-controller board’s power line’s ground going to a common ground, but I guess there is a difference between logical ground and power ground, and without getting them all to the same place the servo did not have a good reference voltage for its signal line (or something).

Of course, I didn’t figure this out without first tearing apart everything, trying to rule out shorts or burned out components, testing things in isolation, even wiring up the signal line to my oscilloscope, etc…

Live and learn… Now I just need to rig up a ribbon cable so that all three servos can be wired up at the same time, then extend the proof-of-concept code to operate all three legs in concert, and we’ll have a walking robot…