So I seem to have figured out my issue. Long story short, it was the brushed motor putting noise on the power lines going to the nrf24 radio.
I was able to isolate the root cause to be a conducted susceptibility of the radio (ie. The motor was inducing noise onto the power lines of the radio) which caused communication issues between the radio and the arduino. Due to not having an O'scope at home to characterize the noise I was isolating the issue through process of elimination. I was able to get the same message length errors by unplugging the power lines to he radio after communications had been established. I then tried running my sketch, with the motor unplugged, and I never got the errors. I also powered the motor through a separate external power supply and never got the errors. This last step showed that there was a conducted issue instead of a radiated issue from the sparking of the brushes inside the motor.
I tried to clean up the power lines by doing the following, none of which caused the error to go away:
- added 2 additional .1uF caps to the motor (already had 1 from motor terminal to terminal, added 1 from each terminal to motor case)
- added a 100uF cap on he 3.3v bus of he arduino
- added additional 100nF and 10nF ceramic caps to the radio terminals to filter out higher frequencies that he 100uF and 47uF caps can't filter.
After doing all this I was surprised that the errors persisted, so I focused on cleaning up the regulation. After I did the following, most of my issues went away:
- moved ceramic caps (10nF and 10nF) directly on the nrf24 board.
- upgraded my 3.3v regulator to an LD33 TO-220 package
- added a series inductor (33uH) after the regulator and before the 147uF worth of of bypass caps
After I did this, most of the msg length errors went away with the exception of 1 or 2 right when i turn the motor on. I added a second 33uH inductor bringing my total series inductance to 66uH. With this last change all my message length errors went away.
