SolvedMarlin [BUG] TMC2209 Sensorless homing sensitivity is off

Description

I have just configured a TMC2209 for my X axis, with UART. It seems to work alright, but setting up the sensorless homing is being a pain (I had it working with an TMC2130 earlier).

If I set the sensitivity to anything above 105 (M914 X105) and try to home the axis, it moves for about 1mm and then immediately stops. I'm assuming this is the stallguard triggering, since I haven't been able to get a read with M119. As soon as I change the sensitivity to anything bellow that (M914 X104) it just tries to home forever until I kill the machine. I have tried setting up floats for the value but it won't accept them, and I also tried switching it with another TMC2209 with exactly the same result.

Steps to Reproduce

Config.zip

Using TMC2209 and these stepper motors, on the bugfix-2.0.x branch:

1. M914 X105

2. G1 X1

3. M914 X104

4. G1 X1

Expected behavior: Stallguard triggers when it hits something.

Actual behavior: Stallguard triggers too early (moving 1mm and hitting nothing) with the 105 sensitivity, and too late (dangerously so) with 104.

I'd appreciate any help debugging this, as I suspect it's something specific to my stepper motor/driver combination rather than a general issue.

@boelle Thank you for reopening it. I'm about to close it again, but I will update for the benefit of others. I spent the last couple of hours trying multiple things, and it looks like I managed to find a fix for the issue.

Apparently, lowering the homing speed to fairly low values fixes the issue. I set my HOMING_FEEDRATE_XY to (30*60) and with that, I can set my stallguard sensitivity values at 100 and it seems to work perfectly (I haven't had a single false positive or false negative so far). Setting it to (45*60) goes back to the behaviour I reported here, where it's either too sensitive or too insensitive without anything reasonable in-between.

So basically, @boelle was correct in saying this was a setup issue, but to my knowledge, the homing speed affecting the sensorless homing isn't documented anywhere. I'll try to add something to the docs about it.