docs: Warn against using the TMC2208 in standalone mode

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/FAQ.md

@@ -265,17 +265,46 @@ configured in Marlin.
 
 ### My TMC motor driver turns off in the middle of a print
 
-There have been reports of some TMC drivers being disabled in the
-middle of a print. (In particular, with the TMC2208 driver.) When this
-issue occurs, the stepper associated with the driver moves freely,
-while the print continues.
+Short answer: Do not use the TMC2208 driver in "standalone mode" with
+Klipper! Do not use the TMC2224 driver in "stealthchop standalone
+mode" with Klipper!
 
-It is believed this may be due to "over current" detection within the
-TMC driver. Trinamic has indicated that this could occur if the driver
-is in "stealthChop mode" and an abrupt velocity change occurs. If you
-experience this problem during homing, consider using a slower homing
-speed. If you experience this problem in the middle of a print,
-consider using a lower square_corner_velocity setting.
+Long answer: Klipper implements very precise timing.
+
+![tmc2208](img/tmc2208.svg.png)
+
+In the above picture, if Klipper is requested to move along the red
+line and if each black line represents the nominal location to step a
+stepper, then in the middle of that movement Klipper will arrange to
+take a step, change the step direction, and then step back. Klipper
+can perform this step, direction change, and step back in a very small
+amount of time.
+
+It is our current understanding that the TMC2208 and TMC2224 will
+react poorly to this when they are in "stealthchop" mode. (It is not
+believed any other TMC drivers are impacted.) It is believed that when
+the driver sees the two step requests in a small time frame that it
+dramatically increases current in anticipation of high acceleration.
+That high current can trip the driver's internal "over current"
+detection which causes the driver to disable itself.
+
+This pattern of steps can occur on all stepper motors and on all
+robot kinematics.
+
+The TMC2208 and TMC2224 do work well with Klipper when run-time
+configuration mode is used (that is, when a wire is routed from the
+micro-controller to the PDN-UART pin and the printer config file has a
+corresponding [tmc2208] config section). When using run-time
+configuration, either configure the drivers to use "spreadcycle mode"
+or configure them to use "stealthchop mode" with a reasonable
+"stealthchop threshold". If one wishes to exclusively use
+"stealthchop" mode with run-time UART configuration then make sure the
+stealthchop_threshold is no more than about 10% greater than the
+maximum velocity of the given axis. It is speculated that with a
+reasonable stealthchop threshold, then if Klipper sends a "step,
+direction change, step back" sequence, the driver will briefly
+transition from stealthchop mode, to spreadcycle mode, and back to
+stealthchop mode, which should be harmless.
 
 ### I keep getting random "Lost communication with MCU" errors
 

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