Just received a DRV8825. Trying to measure/set the VREF using the pololu video. Hooked it up to the raspberry pi using the minimal wiring diagram and the 12V power supply for VMOT. VREF will not give any reading (0 V). I measured it from the little circle and the top of the potentiometer. I checked it to both ground pins. I tried disconnecting everything except the 12V power supply but that did not change anything.