Kevin: Thanks; I'll check all of this information as you pointed out. FYI, the reason for my alternative testing is due to the fact that I find an extreme interaction between roll/pitch and mag heading that makes this system totally unusable for my application. Even modest changes in roll/pitch cause significant changes in mag heading, which cannot be tolerated in my situation. This seems to go beyond calibration, which itself is apparently widely discussed in this community, with no apparent "common" resolution as far as I can tell, though I have no doubt it's possible to do this accurately with some effort. This is not the problem, however, as I said. I don't know if this interaction is due to software or the sensors themselves. The software can probably be fixed, but not the sensors. Without a clear understanding of the problem source, I'll be unable to continue with this level of product, and will have to seek a "higher grade" system for continued development. Unfortunately, this project is cost sensitive, which may force me to terminate it. FYI, it's an aviation-grade project which has no need for mag heading, other than the fact that this is used to assist in the accurate determination of yaw. Be advised that your definition of yaw is different from that defined by the aviation community. Your definition includes any rotation about the vertical axis, whereas a pilot's definition only defines yaw in the case where the aircraft's turn is uncoordinated. That is, where the amount of rudder used in a turn isn't proportional to the angle of bank. This causes the "ball" in the inclinometer to move away from the center, indicating a slip or skid condition. Just like going around a curve in a car where the speed/curve radius doesn't match the "bank angle" of the road. To summarize, a coordinated turn in an aircraft produces no yaw. Pitch/roll and yaw must be totally independent to "line up" with the needs of my project.
I hope this explains my position more clearly. I had planned to use products such as yours to manufacture a unit for a specialized purpose in the aviation field, but am not hopeful at this point in time. If you have any further light to shed on this subject, I'll certainly consider it. The LSM6DS33 works OK, so the traces are good. You could send me the list you mentioned if you don't mind.
Addendum: Since writing the above, I've decided to change my approach. The only requirements are the detection of a predetermined maximum bank angle limit and a maximum yaw limit ("skid" condition). The latter is easily achieved through the use of a completely independent inclinometer, either commercially available or designed in-house, a very straightforward situation where yaw is detected by means of a hardware-only solution. This avoids all the complications of the current approach. I'll retain the gyro/accelerometer solely for determination of the bank angle.
Thanks for your help,