As a follow-up, my Hyperkin mouse exhibits the following sensitivity behaviours:
The reported sensitivity is always 0, and this report
doesn't change if I push the sensitivity button on the bottom of the mouse.
There are actually only two sensitivity settings for this device. High sensitivity produces values from -63 to 63 on each axis. Low sensitivity produces -31 to 31. It seems like the low setting just divides the magnitude in half. Hard to measure with any accuracy, but the response seems very linear vs. the mouse's physical speed. On both sensitivities it's easy to get reports of 1 displacement by moving slowly.
(Edit: this was updated twice with better measurements after producing test ROM below, and revising it.)
I don't see any jitter when the mouse is at rest. It's a very stable 0,0 report. The sign bit for a 0 seems to hold whatever it held last before reaching 0.
Checked the fifth byte read after the end of the report, and it seems to always be $80, which is curious.
Also, when moving the mouse while connected to my Famicom, I hear a faint sound, maybe at 850 or 1700 Hz. Assuming that's indicates some sort of periodic power draw from the mouse? (Another user who tested a Hyperkin with this ROM noticed a similar sound on their AVS.)
I visualized this with a sprite at 128,128 and a second sprite at 128+x,128+y, and I could easily see the hard square edge of the report range by moving the mouse vigorously. Unless someone else gets around to it before me, I'll follow up with a better test ROM later, but I'm not ready to release the thing I'm using at the moment, sorry.
(Edit: made the test ROM, see below.)
I'm curious about how the SNES' mouse differs in this respect. The information about sensitivity that I've seen is a bit vague. I'm also surprised that I can find many, many review videos for the Hyperkin mouse but none seem to notice this difference in sensitivity (did find a couple
amazon reviews that seem to address it though). Maybe it's a bit of a difficult thing to notice if you're not doing something scientific with it, but I'd be interested in hearing confirmation that the Hyperkin's sensitivity does work this way on an SNES as well, e.g. going into Mario Paint with it and trying the speed settings (does it make a difference, is it stuck on turtle, etc.).
Edit: corrected typo, the report doesn't change when you press the button on the bottom.