Correct calculation for TMC2100 vref

Just a quick question, what is the correct formula for working out the vref?

I have 2 stepper motors wired in parallel and I am having trouble with them (posted here [reprap.org]).

I see a few different formulas for calculating the vref.

Found one saying vref = current * 8 * 0.11 (0.11 being the sense resistance).
Then another which was completly different.

My motors are Moons rated at 0.39Amps.

Without going into great details. I will show you what my stepper settings are.

X, Y, E0 and E1 are all 0.695 on my 12 volt 3D Printer
X, Y, E0 and E1 are all 0.395 on my 24 volt 3D Printer

Now the dual Z axis is 1.396 on my 12 volt 3D Printer this is almost exactly double, being only 0.006 volt differences.
And the dual Z axis is 0.710 on my 24 volt 3D Printer this is almost exactly double, being only 0.080 volt differences.

Just double the Vref two steppers twice the vref current.

vref = (current * 8 * 0.11) * two steppers
vref = (current * 8 * 0.11) * 2

This current can vary slightly.

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Computer Programmer / Electronics Technician

ok I will go with that.

What is the 8 in the formula?

Quoted from
Texas Instruments SLVA443


"In order to setup Microsteps, full steps must be divided. Each full step is then divided into eight smaller steps.
It is an industry-standard practice to use a sine waveform shape, an internal 5-bit DAC was optimized for each corresponding combination.

[www.ti.com]

Edited 6 time(s). Last edit at 06/29/2018 09:40AM by Roberts_Clif.

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Computer Programmer / Electronics Technician

Thanks for that