Utilizing a fixed voltage divider may work for limiting start and stall current on DC motors, but may result constrictive when dealing with most stepper applications in which a current level is required during run time and a different value during position/torque holding. A simple topology often utilized to derive a dual current level into the VREF pin is to use a conventional GPIO microcontroller terminal to drive a resistor into the VREF voltage divider.

Figure 2-2 A MCU Controllable Voltage Divider

Figure 2-2 shows how the GPIO resistor is placed in parallel with the high side or the low side depending on whether the digital output is made HI or LO, respectively. It is also possible to make the GPIO an input, in which case the GPIO resistor becomes a high impedance path, offering a third current level setting. If the GPIO resource is configured as an input, RGPIO disappears from the system, and the equation is the same as depicted on Figure 2-1(b).