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I'm going to use TMC2208 stepstick in a printer and the firmware can control via UART the current during printing and during holding (static, no movement): the firmware has an explicit setting for running current and for holding current.

In this case, should I care about the physical trimmer which controls the Vref?

I tried to understand the TMC2208 datasheet (page 50) when it's talking about current control but I don't understand whether the Vref becomes superfluous or it still acts as a maximum value which the UART must obey.

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The potmeter does not work in UART operation, see e.g. this quote:

You don't have to fiddle with jumpers to set your micro-steps, just change it in the firmware, and you can dynamically change the amount of current going to each stepper motor (no more adjusting that small potentiometer on the driver board) just by sending a GCode command (M906).

So, you cannot change the Vref of a UART operated stepper driver, setting the current directly is the way to go on UART operated stepper drivers; in fact that is actually what you are doing on non-UART operated stepper drivers, you change the Vref to change the current through the stepper motor, the higher the voltage the higher the current through the stepper the more torque the stepper has.

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    $\begingroup$ You are right. I found now in the datasheet on page 5: "STEP/DIR Driver with Full Diagnostics and Control" ... "This mode allows replacing all control lines like ENN, DIAG, INDEX, MS1, MS2, and analog current setting VREF by a single interface line. This way, only three signals are required for full control: STEP, DIR and PDN_UART. Even motion without external STEP pulses is provided by an internal programmable step pulse generator: Just set the desired motor velocity." This means that I may be able to use only three data lines per TMC! I have to check Klipper $\endgroup$
    – FarO
    Jul 8 '20 at 12:35

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